Desperate! - Stumble above 2200 RPM

I really need your help. This is a tough one. 1995 S10 4.3L 4X4 Pickup with CMFI. Two independent shops, the local Chevy dealer and I have all looked at my friend's truck and no one is able to fix it. My friend has spent over $3500 dollars and six months on this issue. Please HELP!!!

Symptoms: Stumbles when RPMs go over about 2200 RPMS. Even does it in park in my driveway. If you keep your foot on the gas, it eventually stalls and will not restart for 1/2 hour or so. Fuel can be smelled out the tailpipe. I see signs of fuel washing inside the intake on the passenger side near rear of engine but might be from previous problem as there doesn't appear to be any leaks.

The following parts have been replaced: ECM (twice), FPR, injector, TPS, IAT, IAC, MAP, ignition coil, both O2 Sensors, ICM, cap, rotor, plugs, fuel pump, fuel filter, wires, distributor rebuilt.

The timing has been reset and no ECM codes set. Strong spark has been confirmed as well as fuel pressure while parked and driven and even while the problem is happening. Fuel is spraying out each poppet (CMFI) when cranking engine. There are signs of fuel washing but may be old. Not sure if first mechanic fixed that problem. Vaccum test looks good also after I replaced some degraded vacuum hoses.

I tested the resistance of the crank sensor and it tested OK. Can anyone suggest a possibility? I think it is fuel related due to the rich smell and that it appears flooded after stalling and won't restart for a while but I don't see any signs of leaking around the spider assembly. Also, could the airflow control valve in the center of the intake manifold cause this?

W or Z engine code.
PCM or VCM?
Does it set any codes?
What is the fuel pressure when it stalls?
Does it lose spark or fuel when it quits?
Has it been tested for exhaust blockage?
What is engine vacuum at idle and at 2K?

W or Z engine code. - W Code
PCM or VCM? - PCM
Does it set any codes? - No
What is the fuel pressure when it stalls? - It never changes from factory specs, even when stalls. I forget the number but I did verify and so did the dealer.
Does it lose spark or fuel when it quits? - Spark is present and fuel pressure does not drop out.
Has it been tested for exhaust blockage? - I considered this but not sure how to test. I heard you can check for a temperature difference at the front and rear of the cat. Is this so and what should it be? How else to test?
What is engine vacuum at idle and at 2KI? verifed the vacuum was steady but I don't recall the number. It was in range according to the Haynes manual I have but I will recheck and let you know.?

Thanks for the help. I am stumped on this one.

Jeff

I removed the air plenum and the fuel injection spider. I completely disassembled and cleaned the the spider and reassembled everything. I saw no problems but I do see signs of fuel washing in the passenger side of the manifold.

When I reassembled it, i tested the vacuum and found it idles at 18lbs and now it when I accelerate gently, it does not stumble. If I press the accelerator quickly, it stumbles throughout the entire range. I noticed that when it stumbles when, the vacuum goes to (or near) zero.

Since the exhaust smells so bad of fuel, and lower vacuum causes the FPR to dump more fuel, the two seem to be related. The motor smells rich but the scanner does not indicate a rich problem.

Any ideas?

Forget the stumble on vacuum check.
What is the vacumm reading at idle and at 2K rpm?

Sounds like a fuel leak under the intake manifold.

Forget the stumble on vacuum check.
What is the vacumm reading at idle and at 2K rpm?

18 at both if I accelerate gently. If I punch it, it drops like a stone which I beleive is normal. I have checked inside the manifold twice and so has the dealer. No source of fuel leakage has been found. Fuel pressure is normal also.

18 at both if I accelerate gently. If I punch it, it drops like a stone which I beleive is normal. I have checked inside the manifold twice and so has the dealer. No source of fuel leakage has been found. Fuel pressure is normal also.

What engine code.
What are the fuel pressure readings?
Cold start and engine warm and on the road reading and any fast leak down when key off?

What engine code.
What are the fuel pressure readings?
Cold start and engine warm and on the road reading and any fast leak down when key off?

It is a W engine code. All the scenarios you asked for checked out in the normal range after I replaced the fuel pump. It was leaking down after shutoff but now it holds steady with the new pump. The dealer drove it last week with a guage and said the pressure was good. My guage doesn't have a long enough hose. I don't recall the number since I tested it many weeks ago but it was in the range that Haynes manual calls for. I can test it again if it is important but the fuel pressure has been verified by several people so I don't beleive the problem is there anymore.

It is a W engine code. All the scenarios you asked for checked out in the normal range after I replaced the fuel pump. It was leaking down after shutoff but now it holds steady with the new pump. The dealer drove it last week with a guage and said the pressure was good. My guage doesn't have a long enough hose. I don't recall the number since I tested it many weeks ago but it was in the range that Haynes manual calls for. I can test it again if it is important but the fuel pressure has been verified by several people so I don't beleive the problem is there anymore.

If it quits and will not restart for a 1/2 hr.
That is the time to check it.
Check for lose of spark or lose of fuel pressure or injector pulse.
When no start or no run do you have good hot blue spark?

If it quits and will not restart for a 1/2 hr.
That is the time to check it.
Check for lose of spark or lose of fuel pressure or injector pulse.
When no start or no run do you have good hot blue spark?

I have a good spark and fuel pressure when it quits. The dealer just rebuilt the distributor and reset the timing. A previous shop replaced the coil and ICM. If I keep trying to accelerate the engine when the stumbling occurs, the fuel odor in the exhaust is so strong it makes my eyes sting.

I have a good spark and fuel pressure when it quits. The dealer just rebuilt the distributor and reset the timing. A previous shop replaced the coil and ICM. If I keep trying to accelerate the engine when the stumbling occurs, the fuel odor in the exhaust is so strong it makes my eyes sting.

What is the injector pulse doing?

What is the injector pulse doing?

I don't have a way to test it. I know you can use a noid while cranking but how can I test it during driving conditions? The fuel injector was replaced by a previous mechanic and the PCM was replaced last week by the dealer (for the second time). The dealer used their scanner and said everything was withn spec. Wouldtheir scanner test the injector? Also, they said I was not running lean or rich? I did a resistance test on the injector from the connector outside the plenum and it was within spec. Doesn't appear to be a short or open circuit.

If you had it to a dealer for testing I would take it right back.
And ask for.

A good scanner and some proper testing should show the problwm.
Tip on going to the dealer
Ask that only a factory certified gasoline engine performance specialist work on your car.
Not all dealerships are concerned with how repairs are dispatched, and not all dealership techs bother to take advantage of the training offered by factory (the vast majority of which is paid training). If the dealer is uncooperative, ask for your money back and call around for a dealer that will accommodate you.

If you had it to a dealer for testing I would take it right back.
And ask for.

A good scanner and some proper testing should show the problwm.
Tip on going to the dealer
Ask that only a factory certified gasoline engine performance specialist work on your car.
Not all dealerships are concerned with how repairs are dispatched, and not all dealership techs bother to take advantage of the training offered by factory (the vast majority of which is paid training). If the dealer is uncooperative, ask for your money back and call around for a dealer that will accommodate you.

The dealer had their best 2 mechanics work on it. I met them both and saw their certifications on the wall. They also called GM engineering support and were not able to figure it out. They told me they were not able to fix the problem and charged me for diagnositc time. I believe they used their Tech2 scanner tool.

Either way, going back to the dealer is not an option now so I am looking to fix it myself. Is it posible the fuel pressure regulaotr could be leaking fuel into the plenum but the pressure would be normal? Has anyone seen this? I ask because i do see signs of fuel wash inside the plenum.