1996 Jeep GC multiple misfires, hesitation and bucking

From reading many other posts, I see that I am not alone here, no matter the year of Jeep. Let me see if I can provide a history of my problem and get some help. I had my mechanic (non-Jeep guy) originally work this problem, but I fear he may have made it worse.

First of all, I haven't had any significant engine or mechanical problems or symptoms to deal with at all with this Jeep. It's been great. I replaced the fuel pump 3 years ago, but that's really it. I recently had my manifold welded for a crack when it threw out a CE light and the Cat Converter rattles, but nothing that kept me from driving the wheels off this thing.

However, out of the blue in December, it started shutting off on me while driving. I got my mechanic to read the codes and sure enough (Because I don't normally don't read codes, but I do now), it called out the CPS. He replaced it.

Additionally, when I had him replace the CPS I asked him to go ahead and put some new plugs and wires in for me as well as sort of "tune up" that I thought I probably needed. No symptoms, I just thought it couldn't hurt.

Well, my Jeep hasn't been the same since and the odd thing is that I think the CPS replacement fixed my original problem (Jeep shut down while driving) but I'm now wondering if he caused another problem or if the CPS part is just cheap and is intermittent.

After that CPS repair and new plugs/wires installed, the Jeep ran fine for about a day around town between 0-45 mph and then a lesser or milder and much more random hesitation occurred, causing a "Check Engine" light, but not shutting down the Jeep. I drove it back to the shop and after 3 days he determined it was the ignition coil. He replaced it.

Got it back from him again, and after a day of driving, the same problem.

So, my brother referred me to a Jeep mechanic that has been very good to him over the past few years at diagnosing problems and we did not have to go to the dealer to check it out.

He read the codes and it spit out "multiple cylinder misfires". He immediately asked me what spark plugs my mechanic put in and I told him I didn't know. We pulled them all and found that they were Autolite. He said that in ihis experience, this was possibly the worst plug you could run in a 96 Jeep 4.0 6 cyl and generally Champion seems to work the best for some reason. We put in Champion plugs. We also put on better quality wires and dist cap (MOPAR Kit SPC40604AA) that my original mechanic did not use. The "Check Engine" went off just before we changed it and has not come back on. Driving around town was fine. We drove it around (but not at high speeds) with the reader attatched and no misfires on any cylinder.

I drove it myself over the weekend with no problem and this morning, "Check Engine" light and mild bucking/hesitation again, but not enough to have it quit running, same as before. In fact, it is running well enough to drive, for the interim.

However, when I got out on the highway again, I noticed that around 70mph when I set the cruise control, it jerked and hiccupped just once. I backed off the gas to around 60mph and gradually back up to 65mph or so. I drove 10 more miles or so on the highway and then back on secondary roads (35-45mph) all the way home without any problem at all and no CE light. The total trip is about 20 miles or 30 minuted driving time. Some flat driving (highway) and some pretty steep hills as well. Probably misfiring, but hard to tell and not throwing codes.

The difference I noticed this time around was that right before the hesitation at around 70mph on the highway, the TAC fluctuated from 1800 to 2200 or so and then.."bump"...there it was. Then, all was good the rest of the way home, all at 65mph or lower.

At lower speeds like 35-50mph, still not too many problems with the TAC fluctuating or any hesitation of any kind.

I'm REALLY stumped now....

What has been changed/installed so far since initial problem:

CPS (Once)
Ignition Coil (Once)
Plugs, Wires. Dist Cap (Twice, now has a MOPAR Kit SPC40604AA installed and Champion plugs, gapped at .035)
Lucas upper head cleaner and fuel system cleaner used
Combustion Chamber Cleaner (Sprayed into throttle body)

I'm REALLY wondering whether or not I should go to NAPA and get an Echlin CPS (I have seen it mentioned here as a VERY good CPS) and putting it on before going further.

ANY ideas would be great.....I just get the feeling that this MUST be something rather simple.......and thank you in advance.

Bryan

You'll find as many opinions as there are spark plugs. I run Autolite 985 and won't run Champion from a personal experience.

When that check engine light goes on get that code and post it here.

I have only been able to get the codes from my odometer this past week. 12 (Battery), 43 (5 times) (Mult Cylinder Misfire) and 55 (End of code sequence.

I am going to take a peek at the CPS voltages tonight and I'll post them as well.

Bryan

My wife's 97' experienced similar problems on a road trip this summer. The shop we stopped at in Jackson TN. was stumped by the problem also. The mechanic went online to a tech site and through discussion with other mechanics learned that this is a common problem with jeeps with over 100k. As it turned out, it was the upper oxygen sensor. Might be the same on your 96?

The O2 Sensor is not throwing a code and from everything I've read from those that really seem to know thier stuff, swear that the O2 sensor will throw a code. But you never know, everyone's story seems to be different with this kind of failure.

Tonight, I decided to do a few things.

First, I noticed that the wire from my TPS was looking a bit rough and the insulation was cracked in several spots, so I re-taped it just for kicks. I had not suspected this before, but it was laying on bare metal and there could have been some arcing. It can't hurt.

Anyway, after I did that and gave everything a once over, I decided that it should pull the battery cables and try to reset the CE light (I thought I had read that would work) and clean the posts and terminals while I was in there. Then I was going to probe around a little. However, what I found when I began cleaning them made me go hmmmmmmmm.

My negative terminal was cracked. I measured the resistance to ground and it was a bit difficult getting a short, but it mostly bounced all over the place (and mine Fluke is digital). I scraped and scraped and finally got it down to 0.37 ohms, but it took some work. So, I went hmmmmmm again.

What if my negative terminal, when connected to the battery and running at high speeds, was vibrating just enough from the crack and corrosion to increase the resistance to GND from the negative side of the battery? I wondered if this could possibly be throwing the whole electrical system, PCS and everything out of whack.

So, being that it was too late to go buy another one until tomorrow, I figured that it wouldn't hurt to clean this one REALLY good, along with the post and get it on there as tight as I could and give it a test run. Good to go...fired right up and no CE light as I had hoped, in case I threw another code tonight.

I drove about 10 miles, up and down steep hills, hard acceleration and cruising and it ran just fine. No codes, no hiccups, no "buck".

However, I HAVE been down this road before and I could not get it up to 70mph in my area of town without risking a major ticket, so I'll have to wait until tomorrow to give it a real test on the highway.

I'm not too optimistic yet, but has anyone ever seen this kind of problem with misfires because of a bad GND, battery cable or post??? Just curious, as I head out tomorrow morning.

Thanks for the help.

Bryan

Drove my Jeep to work this AM and no problems so far, but the real test will be this afternoon on the highway. Acceleration seemed to be OK and not sluggish as I had seen with the misfiring. But again, I went no faster than 60mph this morning at about 1800rpm in OD.

I am still going to replace the negative battery cable (first and then the Pos later) tonight and go from there.

If anyone else has seen this type of problem step from "bad battery syndrome" or just bad electrical connections, please let me know.

I'll post an update tonight after the ride home.

Bryan

Well....it's a little better, but I doubt it was because of the battery. It "bucked" on me at 70 slightly on the way home, but no CE light. It drove pretty well the rest of the way at 60mph or below. I'm leaning toward the fuel filter now, maybe I'm starving it at higher RPM's...

Bryan

I have recently been running into a very similar situation. My 94 Jeep GC would hesitate but almost always during acceleration. Originally my problem started about two weeks ago just after I filled up my fuel tank. I first noticed it when I started my car up and it hesitated for a second and then it was fine. I took a road trip that week and it started to hesitate more but it always did it a little worse when I just had filled up my tank. Finally while I was driving on the highway it completely died. I took it to the closest mechanic which ended up being a Chrysler dealership and they said I had no fuel pressure and replaced my fuel pump and just in case my throttle positioning sensor (actually I broke that, but it had to be cleaned several weeks earlier and was causing the car to die). When I left the dealership it drove fine for a couple of days even at high speeds. About three days later it began hesitating again. I am not getting any codes and I have read several posts about the possibility of the O2 sensor. I have heard somebody refer to it as possibly being "lazy", so I will check that. The hesitation and stalling making it feel like I am still not getting fuel or that possible the fuel air mixture gets thrown way off somehow. I have just replaced my MAP sensor, EGR valve, and pcv valve (was very dirty). My spark plugs and wires are less than a year old as well (don't recall the brand). I replaced my crankcase breather/filter as well. Nobody tells you but at least on the V8s that should be cleaned like every 30,000 miles or so. I bought mine used and I doubt it has ever been cleaned it now has 145k miles on it. My only hope other than the O2 sensor is possibly the fuel filter. I don't think that is the problem since it doesn't hesitate on a more regular basis and mainly at higher speeds now. But it still mainly seems to happen on acceleration no problems at all when it is just idling Does anyone know of any other sensors, switches, or filters which might be causing this type of problem? I don't think I have any vacuum leaks anywhere. I might try the battery solution but that sounds a little odd to me. I know however, that one of my engine mounts is bad so the engine may be shaking enough at the higher speeds to cause some type of problem. Besides I could use a new battery anyway. There is also the possibility that the computer is bad but I am not sure where it is located and I think that will be my last resort.

After days of analysis and reading the board, I have on my list the following to check/replace:

Bad battery (I can see fluctuations in my dash meter when driving between 12-14 volts or so, but measures 12.5VDC with a Fluke Meter with the engine off).
Bad battery cable(s)
O2 Sensor(s)
Fuel Filter and/or screens
TPS
PCS
Wire Harness to the PCS
Cat Converter (Will probably get this done at a muffler shop next week regardless if it is the source)

At this point, I am going to drive it until Saturday to make sure my symptoms are consistent to my last work (refer to above for the Neg battery cable and wiring from TPS).

Right now, it drives great in the morning when I warm it up after a cold night and drive about 10-15 miles to work around town. 60mph or lower and up and down hills, start and stop, all kids of stuff. No rough idle. You'd never know it if you didn't go more than 60mph.

When I hit the highway this afternoon though and try to go above 65mph, I fully expect a TAC fluctuation and a "buck" and "maybe" a CE light, but it has been 3 days since I cleared the CE indicator and it continues to "buck" at high speed without throwing a code.

We'll see.....if anyone has any input, I'd appreciate it. I will post any successes or failures for everyone else as I plow my way through them....

Bryan

UPDATE!!!

Well, I broke down and replaced my catalytic converter Friday night at a muffler shop (216.00 and it was going VERY soon anyway) and it appears that I "may" have fixed part of the problem, but not everything. I need to drive it a bit more first, but here it what I have so far today.

I cleared the codes this morning by removing the Neg battery terminal and then took it out for a test drive. Just fine around town, as usual and when I took it out on the bypass near my home I got it WELL above 70mph for 5-10 miles up and down inclines (using the cruise control as well) without one "buck" or "hesitation". I did notice some small fluctuations in the TAC like it had done before, BUT, no "bucking" or "hesitation".

Good news!!!

Before the new catalytic converter, I could count on a "failure" like clockwork before above 65 mph, with rapid variations in the TAC around 1800-2200 and it would be less than a mile at that speed before it happened.

However, I do still feel that it is running a bit "sluggish" though I'm not positive it is "misfiring" anymore. I had one small "hiccup" pulling from a parking lot later today and a "surge" in my neighborhood. Also, had some difficulty getting response from my gas pedal on the bypass when moving up a large incline. So, I'm tending to think that I had a fuel related problem all along in either the filter or low pressure as well.

So, I am going to drive it for several days to make sure that no more codes are thrown my way and then replace the fuel filter and check the pressure.

By the way, I've never checked fule pressure before, is the info in the Haynes manual good enough for a beginner? Can I get a pressure gauge at any auto parts store?

Thanks for the help. I'll post any new info when I have it for those having the same problems. This has been a real adventure....I'm feeling a little better today about this....finally....

Bryan

Sounds almost like my problem. Here is mine. Full tank of gas. Started to lose power like the gas pedal turned to mush and then the car died. Started up again and went a half mile and then died again and could not keep running. Was bucking and acting like mis-fires. Had it towed home. In the morning it started great and I drove it about 2 miles and it started doing the same thing. I replaced the fuel filter since I don't think it was ever replaced. Filter seemed OK...but who knows. Sprayed some starter fluid in the throat of the manifold to help it start and it ran long enough to use up the starter fluid and then died. Had it towed home again. Rolled it into the driveway and hit the key and it started just fine again and then after about 2 minutes of idling it started surging and then died. I then replaced the plugs, wiring, cap and rotor just because and it didn't change a thing. I suspect the fuel pump but I do not know how to check it and if it is bad how to change it out. This is a 6 cyc engine. All the hose etc. look fine. Don't see anything pulled loose or tweaked. My friend said to maybe drain all the gas out and put in a fresh tank since it could have water. I put two bottles of water remover in the tank and that also didn't change anything. Any ideas or explainations of how to test fuel pump pressure etc.?

Before I forget what are the TPS PCS? I have worked on many cars but mostly pre computerized. Another question. When a fuel injection engine runs out of fuel is there some trick to getting gas started into the injectors again or should the electric fuel pump be able to prime then system. Just thought it might be my problem but I would think after spraying starter fuild down the throat of the manifold and the engine running for about 10 seconds it would start pulling in gas. But maybe not.
Again is replacing the fuel pump with a full tank of gas something a home mechanic can do? I understand the fuel pump is inside the tank and there might also be a "sock" type screen filter that sometimes can go bad. Any suggestions would be great. I can take it to a mechanic but funds are limited.

Yours sounds more complicated than mine because my Jeep is perfectly drivable and really has been since I replaced the CPS (Crankshaft Position Sensor). That was definately bad and you may want to, just for kicks, check this if you haven't changed it before you do anything with the fuel pump.

A bad CPS will cause the Jeep to shutdown randomly and somtimes run for a few miles or more until it quits altogether. But given the fact that you have a "mushy" pedal, that makes it a bit more foggy and don't forget that you "could" have more than one ting going on here.

I replaced my fuel pump about 3 years ago, but had my mechanic do it and it wasn't cheap. My symptom there was that I would start it and it wouldn't hold idle at all at first. Then, later, as it got worse, it would fail altogether and not allow me to even start the Jeep. So I can't really give you any direction in that department.

What I will say is that you should be able to do your fuel filter yourself and I am going to do mine as well, but my CPS is in a bad spot, so I didn't do that myself.

As far as the fuel pressure, I think it is relatively easy based on the Haynes manual and folks I've talked to that have done it before on other Jeeps. You need a gauge (make sure the fitting matches your Jeep) in order to measuere the fuel pressure (and there is a test port shown in the manual). I'm going to attempt it this week after I drive mine with the new cataytic converter to get an idea on any new or corrected symptoms.

Hope that helps....I'll pass along what I find out.....

Bryan

mine is a 93 so it is a bit different. Mine won't even stay running in the driveway. It starts and then immediatly dies. I replace all new plugs, cap rotor and wires...no change. If I disconnect the fuel line from the rail that feeds the injectors and try to start I would expect some fuel to shoot out but nothing. I then removed the vacumn line to the value that regulates the fuel going into the rail and that was suppose to increase pressure...again nothing. I really believe the fuel pump is dead or the computer running it. Either case it seems something that is out of range of the fix it at home syndrome. That is why we have credit cards...

Thanks for the help

Yes...I would say that is the fuel pump. When mine went out, it went out completely...dead....

Bryan

Well

Back to the drawing board. On the way home, I got a few "misfires" at or above 70 mph for sure but no codes have been thrown yet. I expect the CE light to come on tomorrow and also expect a 12, 43, 43 ,43, 55 sequence at the least.

I could see the TAC shifting around the 2000 rpm range just like before, so I was thinking it was imminent. Must be something about the drive home just after work, it seems to do it every time.

I'm back to square one. It's drivable around town with little problem, but I'm staying off of the highway until I can measure the fuel pressure and change the filter.

Does anyone know whether I should change the TPS or something else based on just the misfires and the fluctuating TAC (It's minor shifting, but not steady at all above 65mph)?

PLEASE help if you can...thanks....

Bryan

I dont have much experience with the 4.0 engine in the jeeps but I just bought one and have had similar problems with other fuel injected engines. I noticed today when I tried to start the Cherokee that I bought it wanted to start but just wouldnt. I looked down into the intake manifold through the throttle body and noticed it was full of gas. I have experienced this with the Blazer 4.3 engines. The fuel lines run through the intake plenum and get weakened over time. This leads to fuel leakage into the plenum and essentially starves the engine of fuel. Due to the high pressure nature of fuel injection the fuel tends to take the path of least resistance i.e. the leak. This leads to chugging, sluggish throttle response, stalling etc. depending on how bad the leak is. I am unsure of the layout of the 4.0 intake plenum but depending on where the leak is an excess of gas can enter certain cylinders causing fouled plugs thereby making the problem worse.

Like I said. I dont have much experience with the 4.0's but that is the first thing I am going to check on my new Cherokee. Just thought I'd throw another possibility out there. Good luck all. Steve

I am still in analysis mode since I replaced the catalytic converter. One thing of note, not a single "CE Light" or code of any kind in over a week since I replaced it. That's significant to me with regard to the 43 misfire codes.

However, I can still force the hesitation or "missing" above 60 mph (especially with the cruise enabled as it tries to maintain that speed). But, there have not been any codes thrown my way, as I've said. Certainly wants to point me to fuel, but I can't get past a cheap CPS either when I REALLY look at it objectively. It was the first CONFIRMED bad part and the Jeep has not been the same since.

I didn't have any other symptoms like this before the CPS completely failed on me before.....so, go figure.....what a mess...

The good news is that it is drivable and seems to run like a top a lower speeds around town. We'll see....

Bryan

That is what it was to the tune of $899 for parts and labor. Way over priced for a pump...but as they say...they gotcha. Car runs fine now. Onto another problem....master cyclinder for the brakes seem to be leaking. UGH...if I wasn't looking for a job I'd send it back to the mechanic but even at that the part is $200 at the local parts house.

I finally got another engine misfire today as I tried to accelerate around a vehicle on the highway. It really didn't "jump" to accelerate when I pressed on the gas and then it "bucked" twice as I went into the passing lane. I guess this is pointing to a fuel filter/pressure issue, but it is interesting to me still that I didn't have any of these problems before I had the CPS replaced.

Bryan

I'm having the same problem. Have you replaced your O2 sensor and / or your TPS? I've been doing some research and these seem to be commom cures for this. I'm ordering them today to see if it makes any difference. Also, my fuel pressure is good.

I just had the O2 sensor cleaned when I put on a new Cat Converter, so I feel I may be OK there. I'm going to change the TPS tomorrow and I will post my results.

Bryan

I have a '94 GC LTD w/ the 4.0. It appears to have the same problem that has progressed for some time. Occasionally the jeep shuts off, without warning, and can easily be started while rolling in neutral. Or so that was the case up until recently. Lately the jeep does the bucking and sometimes won't start after dying so I think the problem may be getting worse. These symptoms may not manifest for days but then it may happen several times in one trip. This is after replacing the coil, wires, cap, and rotor. I may look into replacing the crank position sensor next... it sounds like a darling to do. I have noticed though while perusing the other boards on this same topic that simple CLEANING of the CPS pigtail connection worked for one jeep owner.
Also you may look into, and I apologize if you'd already mentionedthis as I had forgotten, the motor and sensor on the throttlebody. Those were the other two problem children mentioned.
For clarification the CPS is the crank(case) position sensor. Not to be confused with the camshaft position sensor. Sound like the same thing to me though. I had an autozone guy ask me today about that.

Have you tested the fuel pump and the pressure? Have you replaced the fuel filter? I had the same problems of it working sometimes and then it just finally gave it up totally.

I'm going to replace the CPS tomorrow at my fire station. I picked up the one from NAPA made by Echlin. It was roughly $20 cheaper and as I recall the better sensor when compared to the Autozone/Advance/Pep... Fuel PSI was tested around 49PSI. I did notice one hell of a bad ground. Its the wire that comes from the battery ground and goes to the block. The bolt in the block and the wires running to it were rusty and corroded. It was rough so it was replaced resulting in brighter lights all around. Still the same problem with the jeep shutting off. One other quirk has been noted besides the bad ground was replaced: When removing the gas cap to fill up there is a large amount of air rushing in. It pulls one mean vaccuum I had wondered if this was the initial problem; i.e. Vapor lock. I have left the gas cap off and very loose to no avail. Jeep still dies without warning. Once I replace this sensor though I really think this will cure the problem

I took a look at my CPS cable and connector yesterday and it was a little close to the bare metal on my engine for my comfort. It made me wonder whether I could be getting some arcing from the GND side of the connector and wonder if that could be an intermittent cause.

So I re-ran the connector around the outside of some other wire-wrap and it's a safe distance now. I probably still should take a shot at cleaning it a little, just for kicks.

All in all, I doubt this is the actual "fix" but it's worth marking off the list.

Due to the bad weather here today (Snow) I haven't been able to take it on the highway. Test drove it around the neighborhood yesterday, up and down steep hill, gunning the gas and there was no immediate indication of "misfire". and it's harder to tell from the TAC if anyting "funny" is going on. Around town driving doesn't usually throw the code though, as I've said. I need to get up to 60mph or so, watch the TAC around 2K for "bouncing" and then wait....

Will test it tomorrow if possible.

Let me know about the Echlin CPS, I've heard they are really good and may go that route myself.

Bryan

Oh by the way, I measured my fuel pressure on the rail at 50 psi as well and was told that was good to go....

I just hope that it is not dropping at highway speeds, I have no idea how I will test that...

Bryan

The symptoms that have been stated as far as the "runs ok until I ask for more power" - ie - idles ok and runs ok at lower speeds until the pedal is depressed more and then the engine stumbles or dies. The engine won't start again until it sits for a few minutes, then starts ok and runs ok for a short while and then dies again sounds like what I experienced with my 97.

The fuel pump in the 96 and later models sits in a can in the fuel tank. The can, once the tank is full, keeps the pump submerged in fuel, possible for cooling, possibly so that the pump doesn't lose prime. There are two plastic strainers on the inlet to the pump. One is located on the bottom of the outside of the can, and one is located at the bottom of the can on the inside. The pump is mounted in the can vertically with the pickup at the bottom. The outlet is at the top of the pump and is connected by a short aluminum pipe to the pressure regulator and filter assy at the top of the can. Also mounted on the side of the can is the fuel quantity float arm and sender unit.

What I found on my 97 was that both of the strainers were blocked with fine rust particles and debris. The inside of the bottom of the can had both water and rust retained there, which was blocking the inside strainer. When the pump picked up enough material/water to block the fuel flow, the engine would die. Once the debris had settled down and fell away from the strainers, the engine would start and run for a while until the pump could not keep up with the demand. The outside bottom strainer was just plain blocked all the time. I replaced both stainers and the pump precautionary and cleaned and wiped out the tank and I have not had any more problems. The new pump and strainers cost about $105 at Pep Boys. If you replace (or have replaced) the pump, make sure the tank is cleaned.

When I kept getting those symptoms, I suspected the pump, but to check it, I got a fuel pressure gage and hooked it up to the fuel rail. When I turned on the key, the pressure charged up to 47 psi, and remained about that at idle. I hooked up a long section of hose and ran it down and back underneath and out next to the driver's door. I taped it to the mirror and took a drive. When it started to cut out the pressure dropped to 10 psi and fluctuated back and forth until I let up on the pedal, so I knew I had a supply problem. I dropped the tank and found what I stated earlier.

Without reading everyones post (just reading the orignial post), I would replace the CPS again.

If the mechanic did not disconnect the battery when he replaced the original one then that would be the problem, replace the CPS with a good known brand like echlin brand (napa) or MOPAR brand (dealership). and remeber to disconnect the battery, it is very important to disconnect the battery before replacing any electrical components.

That's what I'm leaning toward doing tomorrow morning. I just feel that the CPS must be the problem now.

I will pass along this, because I've read just abot everything that you can read about this topic and more, based on my and other's symptoms.

Of the folks that had to replace the CPS because it was the known failed part (mainly internittent complee shutdown, which was my own failure point) or the CODE was thrown to indicate failure AND having had the symptoms I have had "post-repair', it has always been a poor brand or poorly installed CPS that was the problem.

Again, I had no problems pre-CPS failure.

My Jeep also initially ran fine after the CPS was changed until I hit the highway and it felt like the engine would buck out of the Jeep for a split second, now and again.

My guess is that is the "Hall Effect" working there. It wants to completely shut down for a split second, but the part is not "broken" just not communicatiig to the computer properly. So it only happens "briefly".

No matter what I've changed or "troubleshot" in the meantime just giving the Jeep a good once over, it keeps pointing back to the original repair....CPS all the way...

I'll pass along what I find this week.....

Bryan

Unreal!!!! That explains a lot and I hope it ends up being the solution for me as well.

It would also explain why sometimes I get a FLASHING CE Light (especially on steep hills or on inclines with CRUISE enabled), but it always comes up 43 (mult cyl misfire) and never coded an exhaust or emission code.

I definately drive less than 3K RPM most of the time, but tried something interesting recently. I started driving with the Jeep NOT in OD and driving at higher RPM. I still eventually got misfires, but not until after I took the OD off for a while.

I've tried a few things with the wiring, but now this is starting to make sense a little bit....I'll pass along what I find...

Bryan

The jeep shake or shudder, misfire may occur when the vehicle is operated between 50 - 70 MPH


I have a 2000 Grand Cherokee Laredo with 84,000 miles. have notice that my Jeep "sputters" when I accelerate heavily (i.e. not when I gradually accelerate but really lay on the pedal to get into traffic).


I tried 2 jeep dealers and they could not fix this, one said it was the coil, replaced it no help, and tune up, injectors cleaned too no help. The other on said it was the torque converter and pump it was under warranty, no help. So I think they just want to sell you something or they don’t know any more then we do, or maybe they throw the service bulletins away. So I did some research on the internet and this is what I found the trouble. So far
I have paid a lot of money at the dealers.

Late closure of an exhaust valve may be the result of no valve rotation and associated build up of carbon on the exhaust valve stem.

So I bought a 16 oz. bottle of Marvel Mystery Oil, put half in the gas tank and the other half in the oil. No more jeep shake or shudder or misfires.

Mabe I drove a couple of round trips to work 120 mi or so, now My power is back saved $500. I guess we should add a little just befor every oil change, just to clean up carbon build up.

By the way the $500 at the dealer for decarburizing



4.0L MULTIPLE CYLINDER MISFIRE
Date: 08/01/03
Model Year(s): 1999-2004

Description: NOTE: THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 4.0L ENGINE. This bulletin involves inspection of all engine exhaust valves and a decarbonizing procedure if necessary.

Details: The customer may experience an incident of engine misfire during certain vehicle operating conditions. The misfire may occur when the vehicle is operated between 50 - 70 MPH and under light loading conditions, e.g. slight uphill road grades. This condition may occur at all ambient conditions, but is more noticeable when ambient conditions are less than 0 C (32 F).

If the vehicle is equipped with On-Board Diagnostic (OBD), a MIL illumination may also have occurred due to Diagnostic Trouble Code (DTC) P0300 - Multiple Cylinder Misfire. Various single cylinder misfire DTC?s may also be present. If the frequency of misfire is high the Powertrain Control Module (PCM) may place the engine in ?Limp-In? mode.

The misfire condition may be caused by one or more engine exhaust valves that are slow to close. Late closure of an exhaust valve may be the result of no valve rotation and associated build up of carbon on the exhaust valve stem.

This condition may occur when the engine is not allowed to run at engine RPM?s that are greater than 3,200 RPM. At 3,200 RPM or higher the engine exhaust valves will rotate if not impeded by high carbon deposits. Low engine RPM?s and high carbon deposits are associated with short trip driving where the vehicle engine is not allowed to fully warm to normal engine operating temperatures. Cold ambient temperatures will increase engine warm-up time and add to the opportunity of carbon deposit build-up on the stem of the engine exhaust valve.

Alright...
So I replaced the crankcase position sensor yesterday at the fire station. While it was blowing and snowing up to 2" yesterday here in Chicagoland, yours truely put taxpaying dollars to work. It's a bee-yootifull thing to work in relative comfort :grinyes:. Once I unbolted the NEG off the battery I began. With the help of 2-10" extensions together and a swivel the bolt came out without complaint from the underside. Looked like the best way to do it. Putting the new part in from the top wasn't too hard. I did have to uncouple 1 vaccuum hose to get better position. Starting the bolt back through the CPS was a bit of a challenge with my ham hock hands. The new part seemed very similar to the OEM part with the sensor wires even being protected by the same type of corugated plastic as the old. Put the NEG terminal back onto the battery and it fired up.As I drove around in the parking lot there was no noticable dips in RPM's. I applied the brake while leaving the gc in drive as to simulate waiting at red light. The RPM's didn't budge, staying right around 500. Even when I moved the steering wheel from lock to lock with the power steering pump working the RPM's didn't fluctuate. I feel this may have been the cure BUT without driving it for several days I won't have any confirmation. Time will tell.
As a side note this may not have anything to do with my problem but... the radio hasn't worked it seems since about the time that this problem first manifested. Your radios may be different since mine is a '94 with cassette/clock built in. Sometimes it comes on for no reason and works for a little while. Up until recently it wouldn't even come on or display the time but now it occassionally comes on. It might be a coincidence or bad ground someplace. Did anyone else have any electrical gremlins affecting the radio?

Having disco'd the battery you cleared the PCM so it's going to rebuild it's table for fuel air ratio. I think it keeps the first 50 times the vehicle gets up to operating temp and averages that info for the best mix. Give it some varied operating scenarios and a little time before drawing a conclusion but I bet you fixed it.

I've been having the same problems and to date I have done the following (in order):

- Replaced plugs, wires, cap and rotor
- Replaced Throttle Position Sensor
- Replaced Ignition Coil
- Replaced Crankshaft Position Sensor

I'm still having the same problem. My Jeep is a '93 Limited w/ the 6cyl. Would the tip from protozee help me out? The years listed on the service bulletin he posted state 1999-2004. Also, I'm not getting any code at all. I'm stumped at this point.

The weird part is that sometimes it will not start for me at all. It will crank but I'm not getting any spark or fuel (this is why I replaced the CPS). But then out of nowhere it will start and I'll have both spark and fuel. Fuel pressure was tested in all ranges and all seems to right with that. Could it possibly be the PCM? I already replaced it once in Sept or Oct because of it preventing the fuel pump from turning on. Is it possible the new one (refurbished) could be screwing up too? I'm not getting any codes which is why I've not considered this, last time I got a code everytime I tried to start it. Also, as a side note. My windshield wiper delay stopped working a day before the other symptoms popped up. Could they be related?

My trip odometer reads a little over 100miles since I changed the CPS and 5 days have passed. The jeep hasn't inadvertently shut down once. I have to assume that the sensor was the problem. To note the radio has also mysteriously started working as well. That one I cant explain but the old lady sure digs it.

I suspect that if you havent changed the wiper relay yet AND you do change the CPS I wouldnt be surprised to hear that the wipers have begun functioning. I would love to know if it happens and more importantly why.

I seem to have the same problem with my jeep as a few of you, it idles fine, but once i give it gas, it stubbles like crazy, kind of like when a chain saw gets stuck in a log and you rev it, and doesn't stop until i release the gas, and SLOWLY accelerate, then it only stumbles a bit. i have tried injector cleaner, sea foam, etc, and gasline de-icer, but nothing, seems to me like i have an issue. Just wondering what some of your guys's results are, and what you did. Mine doesn't die or anything.

After a few weeks the jeep hasn't died on her yet. CPS was the culprit. Radio however does seem to work as long as its dry outside. If the jeep gets wet it won't work util things dry off.

MTB, try with cheap stuff first, filters mostly. Replace them all. Then begin by doing a tune up. If that fails run another search on google for "jeep 4.0 stumbles" or won't run. Play a few word games and then read some threads. This little thread saved me some buku cash.

Jeep 4.0, 1996 – problem similar to other postings, with a twist: had crank angle sensor replaced to cure very occasional stalling, which it did cure; but new problem developed immediately thereafter. (don't know the mfg or if mechanic disconnected negative terminal.)

Original problem – would stall at traffic light once every couple of weeks, could not restart for about half an hour. New CPS completely eliminated that problem.

New problem – after sitting overnight, engine misfires until it's been driven for a spell. Then it runs like a top under all conditions. The misfiring (bucking) is sometimes bad and sometimes very bad, but it's always there. When warming up at idle it's usually minor, but codes are thrown for all cylinders. While driving, the frequency varies but is usually several times per minute. Some hiccups are slightly longer than others, in a random way. Problem exaggerated under acceleration.

Clearing the problem may be done in various ways. Simply letting it idle to operating temperature wont' get it. Hopping on freeway will clear it up after 8 or 10 miles. Driving for a bit and letting it sit for 10 minutes before driving again works. Just turning off and on won't work.

Jeep needs to sit for 5 or 6 hours to bring problem back, otherwise it's ok all day and night.

Things replaced:
-crank angle sensor
-plugs, wires, cap & rotor
-distributor (rebuilt) – shaft had lots of play
-coil
-catalytic converter
-computer
-tried advancing and retarding distributor one tooth
-monitored fuel pressure during malfunction

thinking about throttle position sensor and oxygen sensor, but the postings seem to favor a second CPS.

Has anyone else had experience with major misfires going away after a good warm-up?

PS, Why is the solenoid between the air filter and the fender pulsing during idle @ around 200 cycles per minute? (both in bucking mode and normal mode)

Thanks much for any help which is forthcoming!

Would like to hear how you fix this. Keep us posted.

That's almost exactly the same situation I have to the letter.

I have done everything that you have done as well except:

-computer
-tried advancing and retarding distributor one tooth

I have really come to the conclusion that the new CPS must have been installed wrong or it is a flaky part.

Was your CPS (barryec) MOPAR and done at the dealership or did you have it done elsewhere with an aftermarket? Mine was done in a reliable shop, but I have no idea to date what brand they used or whether they followed the right procedures to install it.

I have also tried the TPS (WELLS brand though) and returned to the original with no change or improvement. The O2 sensor is not throwing any codes to me at all, so I don;t see any need in trying either of those unless somebody thinks I should. They cost about the same as a new CPS. The only codes that are ever read out are misfires on each cylinder at very random patterns. Just like yours, it runs great at points, terrible at others.

The common denominator for me has been that my Jeep ran like a top until I had to replace the old CPS (just like you) because it was bad and shutting down the Jeep while driving. So, I know the original was bad.

I am probably going to have a new MOPAR part put on while I'm on vacation. I don't kow what else to try. If I get to it, I will report back in July the results when I get back.

Thanks for the post, it really confirms some of my thoughts on this problem (now going on 7 months with no solution).

Bryan

Bryan,

Possibly some good news: I solved my problem with the misfires. (Replacing with a MOPAR CPS might have worked also, I'm not sure.) My replacement which gave the well- known problems was an AC DELCO. My solution was to file a bit of a neck in the (single) mounting bolt, which allowed me to install the sensor about .010" deeper into the bell housing. (Secured the bolt in the chuck of a drill motor.)

I have the Haynes manual, which was not much help. I believe your stick shift has a two-bolt mounting, whereas my automatic uses a single bolt. Haynes claims the OEM bolts determine the depth of the 2-bolt system, while the single bolt does not. They're wrong. They say the 1-bolt sensor body has an elongated hole, and the face of the sensor is furnished with a paper spacer to be butted against the flywheel before tightening the bolt. Actually, there's no paper spacer and the hole in the sensor body has a precision metal insert which allows about .011" clearance around the bolt (.0055" on a side) - not much room for adjustment. Obviously the need for adjustment is not anticipated, hence the bad record for after-market sensors.

I went through a lot of grief coming to this conclusion. My misfiring problem was always temperature sensitive, which is something I didn't see in any other postings. Once the car was thoroughly warmed up the problem went away, and returned only after a cool-down of several hours. The idea that the flywheel would expand with heat and change the sensor gap seemed so obvious that I assumed they had engineered their way around it. The paper spacer thing showed how critical the spacing is.

So I assumed the replacement sensor was defective. On your advice I picked up a NAPA Echlin. I put it in when the car was cold (my worst condition) and was really ecstatic with the results - the answer to many prayers! (I also found out why the mechanic had charged me $220 for the first job!)

It started right up and idled beautifully. I went for a spin and accelerated and drove 75, all without a single hitch. About 15 min. into the trip I started getting occasional misfires. By the 20 min. mark I was wondering how in hell I would get home. It got so bad I had to let it cool down, and then limp home in low range. This was ten times worse than the AC DELCO had ever been.

The routine with the NAPA turned out to be 100% repeatable, just like the DELCO was, except opposite, so to speak. When the car was cold it started great and ran perfectly under all conditions - until it was thoroughly warmed up. The DELCO, on the other hand, misfired UNTIL it was thoroughly warmed up, then ran perfectly.

I began to experiment with the small amount of adjustment which the mounting bolt allowed. With the DELCO the misfiring on cold startups was noticeably worse at the shallow limit than at the deep limit. At the risk of rubbing the sensor on the flywheel, I had to try going deeper. It worked! I'm assuming the NAPA unit would work better if it were LESS deep, but I haven't tried it yet. It seems like we have to compromise between the optimum cold setting and the optimum hot setting. Bad engineering?

By mistake the counterperson at NAPA brought out the CPS for a different Jeep model. It had a gear on it, like a distributor, and a little plastic window. Obviously such a device wouldn't be effected by heat expansion. So what made me think the change in performance was caused by expansion of the flywheel rather than warming up of the sensor itself?
When I first switched back to the DELCO, the car was hot but the sensor hadn't been used for three days. The car started immediately and ran perfectly under all conditions. Could the bell housing have pre-heated the sensor while I was fishing the cable up through to the engine compartment? Maybe. The clincher was the change in cold startup performance brought about by moving the DELCO sensor that small amount (.011") allowed by the clearance in its mounting hole. Also, it's counterintuitive that between two supposedly identical units, one should consistently malfunction only when hot, the other only when cold.

Why did the NAPA and the DELCO behave so differently: too deep and too shallow? (Hopefully the MOPAR would be "just right.") I measured the perpendicular distance from the face to the mounting hole and both were identical within maybe .002". So if we used the paper spacer method of setting the face to the flywheel, the problem would still be there. (And would you do it when the flywheel was hot or cold?) Even though the sensor housings are the same, the electrical components inside may be positioned differently, or have different "potencies," requiring compensation in depth setting.

Bryan,

Possibly some good news: I solved my problem with the misfires. (Replacing with a MOPAR CPS might have worked also, I'm not sure.) My replacement which gave the well- known problems was an AC DELCO. My solution was to file a bit of a neck in the (single) mounting bolt, which allowed me to install the sensor about .010" deeper into the bell housing. (Secured the bolt in the chuck of a drill motor.)

I have the Haynes manual, which was not much help. I believe your stick shift has a two-bolt mounting, whereas my automatic uses a single bolt. Haynes claims the OEM bolts determine the depth of the 2-bolt system, while the single bolt does not. They're wrong. They say the 1-bolt sensor body has an elongated hole, and the face of the sensor is furnished with a paper spacer to be butted against the flywheel before tightening the bolt. Actually, there's no paper spacer and the hole in the sensor body has a precision metal insert which allows about .011" clearance around the bolt (.0055" on a side) - not much room for adjustment. Obviously the need for adjustment is not anticipated, hence the bad record for after-market sensors.

I went through a lot of grief coming to this conclusion. My misfiring problem was always temperature sensitive, which is something I didn't see in any other postings. Once the car was thoroughly warmed up the problem went away, and returned only after a cool-down of several hours. The idea that the flywheel would expand with heat and change the sensor gap seemed so obvious that I assumed they had engineered their way around it. The paper spacer thing showed how critical the spacing is.

So I assumed the replacement sensor was defective. On your advice I picked up a NAPA Echlin. I put it in when the car was cold (my worst condition) and was really ecstatic with the results - the answer to many prayers! (I also found out why the mechanic had charged me $220 for the first job!)

It started right up and idled beautifully. I went for a spin and accelerated and drove 75, all without a single hitch. About 15 min. into the trip I started getting occasional misfires. By the 20 min. mark I was wondering how in hell I would get home. It got so bad I had to let it cool down, and then limp home in low range. This was ten times worse than the AC DELCO had ever been.

The routine with the NAPA turned out to be 100% repeatable, just like the DELCO was, except opposite, so to speak. When the car was cold it started great and ran perfectly under all conditions - until it was thoroughly warmed up. The DELCO, on the other hand, misfired UNTIL it was thoroughly warmed up, then ran perfectly.

I began to experiment with the small amount of adjustment which the mounting bolt allowed. With the DELCO the misfiring on cold startups was noticeably worse at the shallow limit than at the deep limit. At the risk of rubbing the sensor on the flywheel, I had to try going deeper. It worked! I'm assuming the NAPA unit would work better if it were LESS deep, but I haven't tried it yet. It seems like we have to compromise between the optimum cold setting and the optimum hot setting. Bad engineering?

By mistake the counterperson at NAPA brought out the CPS for a different Jeep model. It had a gear on it, like a distributor, and a little plastic window. Obviously such a device wouldn't be effected by heat expansion. So what made me think the change in performance was caused by expansion of the flywheel rather than warming up of the sensor itself?
When I first switched back to the DELCO, the car was hot but the sensor hadn't been used for three days. The car started immediately and ran perfectly under all conditions. Could the bell housing have pre-heated the sensor while I was fishing the cable up through to the engine compartment? Maybe. The clincher was the change in cold startup performance brought about by moving the DELCO sensor that small amount (.011") allowed by the clearance in its mounting hole. Also, it's counterintuitive that between two supposedly identical units, one should consistently malfunction only when hot, the other only when cold.

Why did the NAPA and the DELCO behave so differently: too deep and too shallow? (Hopefully the MOPAR would be "just right.") I measured the perpendicular distance from the face to the mounting hole and both were identical within maybe .002". So if we used the paper spacer method of setting the face to the flywheel, the problem would still be there. (And would you do it when the flywheel was hot or cold?) Even though the sensor housings are the same, the electrical components inside may be positioned differently, or have different "potencies," requiring compensation in depth setting.


Nice job on the troubleshooting and a clear and concise description of your deductions and reasoning. Glad you solved your problem and thanks for sharing the information with us.
Would you like to take on my daughter's Ford Contour......? ( I believe the next thing is to park it on a lonely dark side street with the keys in it with a can of gas and matches next to it......) Definitely the biggest POS that Ford ever produced.

That's a great summary of what you did and it explains a lot actually. I have always been skeptical about the original CPS that was installed as a replacement and what brand it was or whether it was even done properly to begin with. Your info echos that possibility as well.

Given the fact that I have no time to do it myself and it poses a significant challenge because of the location, I am having the dealer (and a Jeep mechanic I know and trust) install a new MOPAR CPS for me next Tuesday morning. I am VERY confident that this will correct my problem, however, if not, at least I will know that the part is MOPAR and it should be good. Right now, I have no confidence in the part that is in there.

Like I have said in many posts before, my Jeep was running like a top before the last CPS shut me down and went bad. Just seems like too much of a coincidence to be anything else and the only code that is ever thrown is 43 on all cylinders.

I'll let you know what happens...good or bad results....

Bryan

So did you ever figure out what was causing the bucking? Mine is doing the exact same thing and we have replaced everything we can think of. Would appreciate info.

The long and short of it was that I put a MOPAR CPS on it and had it done at the dealership and it has been perfect ever since. My regular mechanic must have put an aftermarket one on incorrectly or it was a bad part that was "iffy" and worked when it wanted to work...

Everything is great now, but I still have a bit of an idle problem to correct. But not one "Check Engine" light or hesitation since the MOPAR CPS was installed.

Thanks for the reminder though, I need to post a recap for everyone with details.

Bryan

I have a 96 jeep grand Cherokee Laredo with the 4.0 6 cyl I got it about two weeks ago. My first jeep ever. I love the looks, and the way it handles, but the first time I filled it up it all started. First time was a minor misfire when I pulled out of a parking lot, the next day it had a bad misfire so I pulled into Advance and they ran the codes because it caused the check engine light to come on. It came up as all multiple cylinder misfires. So they said I needed a tune up. Next day I put champion plugs, wires, cap, and button. Took it out that night and same thing multiple misfires, but did not cause the engine light to come on. So I stopped at Orileys, and they checked for codes it gave them code p1391 needless to say that code wasn't in their books so I did the next best thing lol I googled it. Turns out it was the code for the cam position sensor. So with that said when I put the new button on the distributor shaft I noticed it had a lot of excessive play I was told this was normal thanks (AdvanceAuto) so I found out that the Cps was in the distributor, so I figured I'd kill two birds with one stone. So I went and got a distributor from Autozone it was a reman but the only one in stock so I got it and installed it the next day. After installing it I started it and it idled and drove like a champ during a one mile test drive. Took it out that night, and guess what more misfires. Then it hit me it's only doing it when the engine gets good and warm. Does fine most the time, but after lots of driving especially in town that's when it happens. So I was told that my coil was getting hot, and failing thanks AutoZone so I put a new coil on it and still as soon as it gets good and warm boom! More misfires. I was reading in the Haynes book that one of the o2 sensors I think it was the pre converter oxygen sensor does not function or send signals to the PCM until it hits it's normal operating temp of 600 degrees. So maybe that is why it's only misfiring when it gets hot, could it be sending faulty signals or no signals at all once it hits it's operating temp mark?

It's driving me crazy I don't know what to do next. I have been told that the fuel injectors could be getting hot and failing. I have been told that it could be the timing chain, or a vacuum leak, or a number of different electrical components failing once they get hot. I think the o2 sensor like I stated in my last post makes sense, but hey I'm just a shade tree mechanic.

My 96 gc, would sputter after hitting a certain part of rough road... always while coming in from work... that being said, it was extremely rare for it not to sputter crossing this section of road. I would get a half mile or so, from the rough patch, and felt like I was running out of gas. I would shift to neutral and Rev the engine on the decline, until I got a clean Rev, then put in gear and go. That stopped working, and my speed got so low, that I decided to shift to 2 (auto trans), shift back and forth then back to neutral to check rev-ability. Eventually, it would start doing it in other places. It got to the point, that it spits and sputters the whole time you drive it. I have been through 4 PCM's (at least), 3 cam sensors, 3 crank sensors, 2 distributors, 2 sets of wires, 3 caps and rotors... oh and 2 ignition coils. When I got it, the dash lights always self-adjusted, but it has never affected the headlights/taillights. In trying to diagnose the issue, I ran a test on the cam sensor. There are three wires: ground, power and signal... I searched for my ground first, with a voltmeter. With the key off, ground was the middle wire. With the key on, all three wires are power!! Wth... so I pull the loom off of the wires, and I look for melted wires.. none. I checked the crank sensor wires, I checked the cam sensor wires. I checked the crank and cam back to the pcm. Nothing... I forget where on which plug for the pcm it is, but one pin likes to come loose. I checked for that. Another person stated how one or more of the pcm bolts were too long, causing a short. Checked those... an hour or more at a time of constantly feeling like you are running out of gas, is exhausting, but will keep you awake, when and if you need it. Going to have someone else look at it... she runs too good to scrap... my initial crank sensor case had broken somehow... only noticed when I swapped transmissions... thanks for any advice!! She's been down nearly two years...