95 Suburban 350 breaks up on acceleration.

I developed a leaking intake manifold gasket about two weeks ago. Truck was running fine. Had a reputable mechanic replace the gaskets and while he was at it, I had him replace the plugs, distributor cap, rotor and plug wires. When I picked it up, it was idling fine, but when I accelerated it would break up and have no power when a load was placed on the engine. I replaced the ignition coil and added some injector cleaner to the fuel. No change. Took it back and he replaced all of the plugs again and said there was a lot of crud blocking the EGR valve that he cleaned out. Just drove it and it may be a little better, but is far from right. I'm leaning toward a fuel problem more than electrical. Can't figure out what might have changed. Any thoughts or ideas would be welcome! Thanks!

This is a great forum. Thanks to all of the contributors!


Ken

did the distributor get put back exactly? There is a function of the computer to look for what is called Cam Offset, or Cam Retard. You can only access this parameter by using a high quality scan tool such as the Dealer's GM Tech 2, Snap-On, Matco, or OTC. This figure has to be within +/-2 degrees. I'm surprised you haven't had a check engine light pop up yet. The scan tool gets hooked up, and then you adjust the distributor, I believe with the engine off/key on. Until this is set, the computer does not have a reference point on which to adjust the timing correctly depending on engine load and RPM. Hope this points you to a solution. -Chris

Oh and I almost forgot something very important to the information I gave above. That info only applies to the late 95 engines with the 8th digit of the VIN being "R". Those are the vortec motors. If you still have throttle body injection(two injectors can be seen when you remove the air filter), I might suggest that you check the condition of your ignition module, located under your distributor cap. And while your at it, run a continuity test on the ignition Pick Up Coil. The modules get hot and moisture causes corrosion around the module. By removing the distributor, it may have triggered something else. The ignition module is easily removed and can be tested for free at most parts stores. The pick up coil can be tested on the spot with an ohm meter(multi-meter). Don't forget to have your fuel filter changed also, our trucks need it replaced every 12K miles or every year. Again, hope this points you somewhere. -Chris

Thanks Chris,

My engine is not the vortec. I was reading this morning in my Haynes repair manual about the ignition module and the pick up coil. I was also thinking of changing the fuel filter since the line was depressurized during the manifold gasket replacement. I have gotten a "check engine soon" light on occasion when driving. Maybe the oxygen sensor is not functioning properly. I will let everyone know the outcome as I proceed.


Ken

change the spark plugs. Sometimes when you tighten them down if the wrench is not on the plug straight it can crack the ceramics but it may not be visible. Or the mechanic may have dropped a plug and cracked it. I have seen this many many times. People are pulling their hair out, replacing a lot of expensive parts and its only a plug.

If you have an insulated spark plug wire removal pliers I have someone start the car, put it in reverse with the brake on and slowly accelerate until it starts to miss. Then pull the plug wires off one by one and you can find the cylinder that is affected, replace the plug and try it again.

Like I said, I have seen this a lot.

Just a follow up. Yesterday I did change the fuel filter. It did not seem to have any impact on the problem. The mechanic did replace all of the plugs two times (platinum) and he checked the electronics with a scope and does not see anything unusual. I'm wondering if maybe there is a leak somewhere at the intake manifold gaskets. It was running fine before the intake manifold gaskets were replaced. New plugs, wires, ignition coil, rotor, distributor and fuel filter were also added and the EGR was cleaned and checked. It has no acceleration and if I hold it at 1500 RPM's, it begins to hesitatate and buck. If I give it gas, it really does nothing unless I stomp it to the floor. Then it kicks down and I do get some acceleration, but it is still sluggish and I can tell it is not right.


Ken

Did this engine have the platnum plugs in originally?? If it did not change back to original equipment plugs. In some vehicles the platnum plug will not work right. This is common, most people first think OH cool a better plug. They may not work right in an engine not designed for it.

But breaking up without any power allmost sounds like to me an injector problem.

I took the truck back to the mechanic and he worked on it all day. He gave it back to me saying he thought it was running fine. After driving it home it was obvious it was not fine. It was better, not as much hessitation or bucking on acceleration, but not fine. He replaced the plugs with the original ACDelco copper plugs. Replaced the new rotor which initially went on very tight, and ran a cleaning solution through the system that cleans the TBI and valves of build up. When the transmission shifts, the hesitation is most notable. If you press on the accelerator gradually, nothing happens. I have to stomp on it to get acceleration. It seems to bog down. The engine idles fine. I'm wondering if the oxygen sensor could be sending a bad signal to the ECM when I'm running under a load. I'm still seaching for the problem! Thanks for the help!!


Ken

Burb95,
I feel your pain! I am having exactly the same problem with my 95 K2500 350 TBI. I replaced the distributor cap, rotor, coil, plugs and wires. It now starts instantly and idles smoothly, but it hesitates and misses when accelerating.

I'll keep checking this thread to see if you come up with the answer.

Bill

Have you checked the injectors yet? You say the engine light had came on did you have it checked to see what the code was? Go down to Parts Plus they will usually plug in a diagnostic to check the codes free of charge. And no I do not work for Parts Plus heck I don't even buy my parts there. Nothing against them, but I almost always buy original equipment parts and we all know that is at the dealer.

Hi, for me it sounds like a MAP sensor failure/vacum leak, or a TBI gasket that's leaking!

About a year ago, my K2500 4x4 Suburban started running a bit rough and hesitating from a stop. I changed my spark plugs to the Bosch Platinum +4 plugs and the truck ran worse.

As I found out, the newer engines use the Denso Iridium plugs ($14 each) as the Standard OEM part. I replaced the plugs and found it ran better, but still had the miss and hesitation.

Investigating it further, I also found a clogged fuel filter... even though I replace them at less than 15K miles (usually around 10K). I guess I had gotten a bad tank of gas or something, but the fuel filter was the prime cause. The fuel coming out of the filter was black when I dumped it into a container.

OK, figured it out finally. Talked to the mechanic and after he replaced the platinum plugs with the original copper plugs, it was better, but not perfect. It still hesitated during acceleration, especially when the transmission shifted. He said that he set the timing to 0 degrees, this was the specs in the book which I confirmed. Well I read posts from other pre vortec 5.7L owners that when they set the timing to the specifications after a tune-up, it ran like a dog. So, I began turning my distrubutor counterclockwise ever so slightly to advance the spark. I noticed the performance was better. I did this three different times and then drove it and I almost have it perfect now. I borrowed a timing light so I can see exactly what my setting really is. Hope this helps someone else!

There is a wire on the back of the fire wall. (some are under the dash) I am not sure where yours is. Check the dealer or get a chiltons. Anyways to set the timing, that must be unplugged first or your timing will not be correct. My bet is he is unaware of this. Yes the timing is zero but that is without that wire plugged in. Remember older vehicles used a vacuum advance to that you took off and plugged then set the timing and hooked it back up. Newer modules use electric to advance the timing so it can be controled by the computer.

I have a similar problem with my 350 2500 van. I replaced the map sensor and the hesitation on acceleration went away. I also had the injectors cleaned which did not help. The problem is about 80% fixed. I now have a high speed miss, but it runs much better than it did.