Acceleration problem above 35 mph...

Can anybody help with this one?

Vehicle: 1999 Buick Century
Mileage: 40K
Engine: 3100 V6
Transmission: automatic 4T60E w/vacuum modulator

The car has been checked by my regular mechanic twice, a transmission shop and a Buick dealer service shop in Richmond, VA with no success.

The problem is more noticeable when car warms up.
Once I reach wanted cruising speed of 35 mph or above (and the car changes to third or fourth gear), the car has a poor response on a slowly, steady press on gas pedal.

For a second it seems that RPM sound goes up and then it drops down with dull sound ( the car has no RPM gauge on dashboard). At that point I keep pressing pedal slowly but car does not accelerate or needs eternity to speed up.
Sometimes I do not even feel that RPM goes up even for that second.

At that point,
If I release and press pedal lightly once or twice, it will eventually “pick-up” and response.
If press faster or harder, it will downshift and speed up like a champion but I do not want to race.

Air and fuel filters, transmission fluid and PCV valve has been replaced - problem persisted.
So far I tried test-swapping used TPS, MAF, MAP – problem persisted
Fuel pressure within specs (40 psi in idle) and fuel pressure regulator holds vacuum.
Spark plugs and wires are original but they should be good at 40K, (GM says the should last up to 100K).
Poured a bottle of injectors cleaner in tank - problem persisted
Replaced vacuum modulator with aftermarket part – slightly better but problem persists.
No SES light came, no malfunction codes set.

I love the car, it has low mileage and hopefully should give me years of service but I am desperate now.

Any help would be greatly appreciated.

Roaddirt

I did not see..."I cleaned the throttle body with throttle body cleaner". It may be that your throttle body butterfly is having trouble getting past a certain point due to gunk buildup. 40k is actually a good time to clean it if you have not.

Thank you for trying to help JoeJoe. I could give it a try since I do not know what else to do, but I somehow do not think that throttle body is the problem.
Reason is, I do not fill hesitation in first let’s say 5 min until car warms up.Also, I do not have any hesitation in 1st and 2nd gear until the car passes 35 mph and go in higher gear.

Unfortunately I cannot figure out if this is engine or transmission related.
Warming up should have something to do with either electronic, vacuum/pressure or so.

Could this be any of these: PCM, TCC solenoid, transmission pressure control solenoid, injectors?

I would disconnect the TCC. Poor mileage but if it is the problem the car should run smoother.

I would disconnect the TCC. Poor mileage but if it is the problem the car should run smoother.

I did try that and could not make a hundred yards. The transsmission immediately started slipping and SES light came on. Had to stop and plug TCC wiring back so I can drive.

The car 1999, is it possible that that old trick does not wotk with newer models? Cannot believe the transmission is that bad!

BTY, I removed air duct today and cleaned throttle body as JoeJoe advized. There was no much in it, like I thought, so it cannot be cause.

Well the tranny trick works on my 93 Z-34 wich has a 4t60e. Never tried it on the GTP wich has a 4t65e. So you are correct. By any chance, I know this is silly but did you used Mercom III and has the correct level on it?

Yes, I used Dexron III/Mercon and carefully filed it up to the proper level while idling hot.
I found a post from Flatrater where he said that there is GM's TBS regarding drivebility problems with 4T60E trany caused by failing Pressure Control Solenoid. I wonder if that could be my problem but it is too expensive fix to try without being sure that that is the cause.

It would be so helpful if I could distinct if the problem is engine or trany related.

Roaddirt I feel from common sense that if the engine does not hesitate or misses when you accelerate the car it is not the engine. If the engine does drown when WOTing the car I would think the problem is leaning more to the engine. But I think it leans more to the tranny since the engine iddles correctly and accelerates correctly. Depending on the load the tranny put on the engine is how it behaves so if the tranny puts an herratic load to a good engine the engine will accelerate herratically but without hesitation of misses. If the engine rough idles or misses the car will accelerate more or less ok and you can feel the engine failing directly.

I know my description of the problem can be confusing but you got me confused now. The car does hesitate when at 35-40 mph and above and it response very poor when press accelerator slowly and steadily.
If press hard it downshift, revs high and pull but it is inacceptable driving style. It is real worse on highway when cars get hot. :banghead:

Roadirt, let's put it this way. The car looses power when accelerating past 40mph? If that is the case could be spark timing related. The module or something like that.
Does the engine stalls, dies, black smoke or something similar to those simptoms?

Roadirt, let's put it this way. The car looses power when accelerating past 40mph? If that is the case could be spark timing related. The module or something like that.
Does the engine stalls, dies, black smoke or something similar to those simptoms?

Example:
Started from the stop. I press accelerator and the car goes fine through 1st, 2ng, 3rd to 4th gear. I can reach any speed, like 50 or 70 mph with no problem.
I reached wanted speed, let's say 50 mph and cruising.

Now if I press pedal to accellerate, the car hesitate, no RPM increase, no speed increase. Sometimes you can see speed gauge moves very, very slowly indicating weak speed increase.

If I release accelerator and press it again, after a couple of tries it will catch, engine revs and car speed up like it is supposed to.

A couple seconds after,as soon cruise again at steady speed, it become inresponsive again.

No stalling, no smoke, car does not die

Ok, this looks like the TCC but not is not completly gone. That happened to me once on my 93 Z-34 4t60e. I had to replace the tranny beause other issues and that stopped happening. Try adding high friction additives to the tranny to get a little more life out of it. But looks like it will die eventually(not in a short while, but in a while). What are U experiencing is a shifting problem do to a lack of enough pressure for the clutches to engage properly. Get the tranny rebuilt or look for the fricion adding additive. Ols school would add 1 cap of Wagner dot3 to the tranny oil, this will help the clutches engage better.

Ok, this looks like the TCC but not is not completly gone. That happened to me once on my 93 Z-34 4t60e. I had to replace the tranny beause other issues and that stopped happening. Try adding high friction additives to the tranny to get a little more life out of it. But looks like it will die eventually(not in a short while, but in a while). What are U experiencing is a shifting problem do to a lack of enough pressure for the clutches to engage properly. Get the tranny rebuilt or look for the fricion adding additive. Ols school would add 1 cap of Wagner dot3 to the tranny oil, this will help the clutches engage better.

Sounds realy bad. I bought the car three months ago and I noticed the problem a couple days after I bought it.
Is there any chance that Pressure Solenoid is bad, messing with transmission pressure?

Well man, you can replace the solenoid if you want but it would be a lot better to rebuild the whole thing. Mine broke a sproket and that is why I had to replace it, after I replaced it the jerking at high speed ended. This jerking happens like the car does not decides if it has to stick on od or 3rd. Something like that. Add a capfull of Wagner DOT3 brake fluid. If the tranny shifts well after that then the clutches are gone. If not then change the tranny oil and might be oil pressure related problem. But get a reman one from Mr. Goodwrench cost you a little more thant rebuilding yours and this has 50k miles waranty

Thanks kilroypr for all your help.

Tell me what do you think about this:
I recently bought OBD-II PC adapter and software for laptop from Car Code. It is nice piece of diagnostic and can monitor some parameters like O2, TPS, FuelTerms, RPM, VSS, MAP, MAF, CTS, Spark Advance, etc.
First time when I tried it I did not know how to translate data log and get some useful info.

I gave it try yesterday again. Whenever I noticed hesitation problem, computer showed significant Spark Advance drop (retard), despite TPS was 40-50 % opened and RPM 2500-3000.

Normally, in idle and low RPM timing showed 20º which is I believe right. It would advance spark at 3000 RPM up to 32º which again should be expected but when car hesitated and had poor acceleration, it showed retard to 10º-15º at 2600-2700 RPM.
This retard spots corresponded to speeds around 35 MPH (3rd gear shift point) and 45 MPH (4th gear shift point) though I noticed such a retard at 60 MPH as well.
No such a retard at speeds from 0-35 MPH and lower gears where I do not notice the problem.
Now that was weird isn’t it?

Any thoughts anybody?

Well as the a good friend (Ripn12s) there are many things that can cause knocking. In my case even a broken exahust manifold would cause knock. If the tranny fails and make the engine jerk the knock sensor may register false knocking because insted of being from premature ignition would be related to exagerated vibration. So in this case could not be real. But nevertheless. Did you check all the air/fuel intake system that is clean and working properly? Because if you did and is supposed to be ok then this knocking could be related to exagerated engine/tranny vibration and not necessarily due to premature ingition

I did replace air and fuel filters, according my tests fuel pump and regulator works and provide pressure within specs. Removed air intake duct and cleaned throttle body, no impurities found.

Yesterday I was using my OBDII software again. This time I was able to enable some kind of "enhanced" GM diagnostics that gave me some more readings.
The most interesting was that I had random misfiring on cylinder 4 and under higher load cylinders 1, 2 and 3 would have some low misfiring counts as well but cylinder 4 was worst. I guess misfiring count was not too serious for computer to be alarmed and treat it as malfunction.

Considering this (and occasional spark retard) I wonder if my problem is ignition system or maybe dirty injectors.

I will check my Haynes Manual and somehow try to check spark plugs, wires and coils (especially cylinder four). Also I thought I could spray intake cleaner around intake manifold gaskets while the car idling and see if there is some vacuum leaks around cylinders.
Injectors and their connectors are hidden under upper intake manifold and there is no much that I could do to test them, right?

I have EXACTLY the same problem, but my car is 2002 pontiac grand prix. Funny thing it has warranty remaining... but when I took it to the dealer they did a test drive and said: "everything is fine"!!... nobody knows what am I talking about. I did the injection/throttle cleaning, changed tranny fluid, fuel filter...the car stills starts act poor at 35-40 mph. When I press the pedal harder it downshifts & accelerates really good until cruise speed is reached, then again engine sound becomes low and the car "locks". It is really annoying when driving at 40/50 mph and traffic is dense. Car is on 4th gear and RPM is jumping between 1600-2200
Sorry, It is hard for me to describe it accurately in english. I am a foreigner and it is my "learnt in school" language.

R

Thank you for post rafraf77,
EXACTLY the same problem in details. After bringing the car to my mechanic and dealership for diagnostic and getting "the car is fine", I do not know what to do. I got back to my old "trusty" Volvo untill I figure it out.
After my all diagnostic tries, I think I will have somebody check my catalytic converter and exaust. If they are partly plugged it will cause power loss, change vacuum and vacum change will affect the way transmission works.
I still hope that the transmission itself is not culprit :-(

How many miles was on your GrandPrix when you noticed the acceleration problem? Is your trany fluid level OK? Was there any change to fluid color since the problem started? Any burning odors?

roaddirt

Guys, let me tell you. Before breaking the new engine the car has now(Thanks God the waranty already sent a new one and I am installing it) when the engine was under load it would get a heavy missfire on 4 and intermitently on 2,6,3. Then when it broke was related to the car running really lean under load and the pistons that cracked were 3,5.
After the first time I got a heavy missing on 4 it would get it under almost any normal driving conditions without a SES. I even replaced the spark plug cables with MSD 8.5mm and still got the misses(less missing but still got).
Misses could be related to a clogged injector, worn spark plug cable, worn spark plug, bad ignition coil or too lean mixture that would not cause the air to ignite properly.
In my case I think could be related to the coils and AF mixture since when we took the engine apart to check what the heck happened all the exahust valves and headers where clean(No black dust what soever). So now when the new engine arrives the first thing I would make sure is that the AF mix is richer and that the ignition coils are correct. When a car is running lean could break internal engine components(Specially pistons) since the upper side of the piston is running really dry on the cylinder.
Bottom line check that the AF mix is not too lean and that the coils/cables are ok.
Roaddirt, IF the catalytic converter is partially clogged the hesitation would be constant.
As I told you before on my Z34(1993) I got the same behaviuor. When I replaced the tranny because it spraks broke the problem went away.

In another thread related to this problem, "Public" posted that his 1998 Century has the same problem and after several weeks a mechanic could not narrow down the problem. Public suspected EGR system problem and test ran the car with unplagged EGR. According to him the acceleration/hesitation problem was gone. He will now go and narrow down the problem to EGR valve, wiring, blocked passages or PCM.

I want to run every possible tests/check ups before I suspect a major component as trany or engine.

I too would like to run the same test. I assume that test running the car with unplagged EGR will not cause any damage to the car, right?

I have a 94' gp with the 3100 and it does the same thing, around 35 40 mph I have to almost put it to the floor to pass. "Roaddirt" I think my car has the same problem as yours with the spark advance. Where did you get your testing equiptment and how much was it??? I would like to get it. Thanks

NOt really, just worst milleage. Remember the EGR is to let pass not completly burned gases back to the intake. Again, I am not sure if your hesitation problem is related to the EGR. I had EGR problems on my Z-34 and never cuased hesitation. Then when the tranny was replaced it all went away. But you are correct it is a lot cheaper to check the EGR than replacing the tranny. Go ahead and let me know, there is always new stuff to learn

Yesterday I disconnected EGR and test drove the car for 15-20 min.
No change, car behaved like before. (This test set up SES light and threw a couple of codes that I all cleared with my OBDII laptop after reconnecting valve.)

Also, OBDII scan showed that EGR valve was opening properly under load. Commended EGR percentage was equal to actual (executed) which made me believe that my EGR valve was OK.

I wanted to check the air passage underneath of EGR valve for possible carbon blockage so I removed EGR and checked. No visible carbon deposits or blockage. EGR valve gasket has to be replaced if you remove valve.

Conclusion: EGR valve dose not seem to be cause of the problem.

I have a 94' gp with the 3100 and it does the same thing, around 35 40 mph I have to almost put it to the floor to pass. "Roaddirt" I think my car has the same problem as yours with the spark advance. Where did you get your testing equiptment and how much was it??? I would like to get it. Thanks

This OBDII scanner is for OBDII equipped vehicles only, generally from 1996 and later.

I bought PC based kit "Car Code OBD-II On Board Diagnostics" for my laptop from Car Code. That is a piece of hardware attached between your PC and OBDII port in the car and software dowloaded from their site installed on laptop.
This is home page http://www.obd-2.com/

I ordered VPW and ISO interface combo
ISO - for European and Asian cars, Chrysler
VPW - for General Motors

Single interface is $122, combo only $142 plus a few bucks for shipping

Do you know of any software or any diagnostic equiptment that I could use on my 94'????

Well, I have taken my Buick to another Buick dealer, again.
The car was well warmed up when Service Chancellor took it for drive with me on passenger side. As soon as we reached cruising speed around 35-40 mph, the RPM dropped down and the car exhibited famous “poor acceleration”.
He asked me if this is my problem and I confirmed. He said that was OK, the car behaved like it was supposed to. I was stunned.

He technically said “You need to keep in mind that Buick is a luxury car designed for smooth cruising, comfort and better fuel economy. Let’s say you want to cruise at 35 mph. As soon as it passes 35 MPH, first time you set up steady speed it will always switch to 4th gear and lock up torque converter, even if the current speed more suites 3rd gear. Being in 4th gear at 35-40mph will not give you any quick acceleration or passing power. What you need is to disengage “torque lock up” with any known technique like releasing and pressing pedal deeper, touching brake pedal or pressing harder to downshift. He said that PCM is not even so smart to disengage torque lock up when the car is going uphill and the car starts bogging down. You need to press harder to downshift or even choose lower gear manually. WOW.

It seemed to me as a lot of leg shifting and science for an automatic trany. According to him the car is OK, it is design. My only problem is that I own a Buick...Century.

My understanding was that other GM products like, let’s say Pontiac Grand Prix, will behave quite differently with the same engine and seemingly the same transmission due to sportier design targeted to younger people.

Comments are very welcome.

Mines some what similiar but really missfires when i floor it.

the 40mph lag is something the GP's have dealt w/ also. Remedy - DHP/CYA/INTENSE pcms

DHP has a 3-1 and 4-2 "skip shifts"

Roaddirt,
I'm going to have to aggree w/ the technition that its just the TC locking up. I can feel when the tc locks up in my car.
btw, did u get the misfire problem solved??
Check plugs and wires first before spending big bucks on other stuff

Tim

I have the same problem with my 1997 Grand Prix. Genisys says Knock sensors are picking up a knock other than spark knock and retarding the timing. The knock sensors can pick up almost any knock so I'd listen and see if you can hear abnormal engine knocks.

I am not sure about the technician. Because for mileage i do know the TCC will lock on high gear. But is supposed to dissengage rather quickly when pressing the pedal to pass other car. Is not supposed to stick on higher gear when pressing the pedal further.
You do not have to shfit manually to make the tranny dissengage and go to a lower gear, just pressing the pedal a bit harder should do it.
The problem that I had on the Z-34 was more like the tranny not deciding to stay on 3rd or 4th and would jerk rather quickly between both gears until I floored the pedal and the tranny will stay in the lower gear.
Never the less what the technician tells makes some sense what does not make sense is that the tcc will engage too much as to stay like that when pressing the pedal further enough to make the tranny normally shift to a lower gear. I would check the TCC and add an additive that would enhance the clutches performance, what is the car mileage again?

anybody find out what the actual problem was with this? my pontiac grand am is doing the exact same thing.

anybody find out what the actual problem was with this? my pontiac grand am is doing the exact same thing.

WOW....long gone thread....but anyway...What year, what engine. COme back with details....
I had two incidents of that type...one related to failed TPS, another due to dirty MAF..the little wire that senses air flow covered with a thin layer of mud/dirt.
but 3.1 do not all have MAF...but have MAP....so it depends....

it is a 2000 pontiac grand am with a 3.4l v6 it has about 167xxx miles on it and I've replaced the mass air flow sensor and the tps already. and yeah the thread is ridiculously old but it matches it pretty well

yeah the thread is ridiculously old but it matches it pretty well


I can't understand what the original thread says...sounds like something my SM would write....

IN YOUR OWN WORDS, describe what is happening....also has the car been scanned for engine/tranny codes....does your tranny have a dipstick?

I've read the original thread and can't really understand it either. It sounds like the person describing it does not know what a locking torque converter does and how it makes the car behave. But if you read it with the idea that he's describing the locking torque converter operation, it makes sense.