Chevrolet experts only please! I am at my wits end!?

Question

I have a 1991 Chevrolet Caprice with a 5.0 T B I engine.The car has less than 70000 miles on it and has been well maintained. My problem is this... the car starts good and idoles smooth even in below 0 temps, but once the engine reaches full opp temp and I put a load on it /ie starting out from a stop, accelerating up hill ect. the car cuts in and out random misfire all 8 cylinders cut in and out.this only seems to happen once the computer takes over engine operation. when the car is cold it doesnt miss a beat!. so far to fix the problem I have replaced the T.P.S. sensor, the fuel filter,the coil, the ignition module, the entire distributer, all spark plugs and wires. the O-2 sensor.the main PCM computer, the ignition switch and all wiring harnesses from the steering collem to the ignition system.the compression is good and the timing chain is tight but the problem persists! I am at my wits end and I need some new ideas on how to fix the problem. the fuel pump pressure checked out to

I have cleaned both fuel injectors and they spray a good fine mist, also, this model has no cam position sensor

I have replaced the fuel pump 3 times and nothing has changed also, there are no codes showing up on my scanner

Fuel pressure is at factory specks even when the engine is at high RPMs.

I have checked the knock sensor on the engine by taking a metal pipe and placing it on the engine block and hitting the pipe with a hammer the RPM changed telling me the sensor is working

There is no smoke of any color coming out of the exhaust pipe

Answer 1

Did you check the fuel flow?
Maybe the "sock" filter in the fuel tank on the bottom of the fuel pump is plugged.

Answer 2

Sorry to hear you have been chasing a ghost and dropped so much money under the hood. Good part is once you find it everything will pretty much be new. Next time I would advise better trouble shooting.

So it runs good cold but acts up when warmed up. I would test or replace the coolant sensors. Sounds like they are not registering the proper engine temp. The PCM thinks the engine is still cold and is flooding. Are you getting black smoke out of the exhaust .. what did the plugs look like when you replaced them (tip color)?

Second check would be the knock sensor. If that is bad it will also prohibit or modify the timing the wrong way.

Check those and drop me an Email off my profile if you need further troubleshooting.

Answer 3

I had a 92 with much the same problem. I found the problem to be a bad ground after pulling out handfulls of hair trying to figure it out. Some may call me cheap but I will never replace a part that I can not prove that it is defective. In this case I checked everything except the ECU and everything came up to factory spec. so back on it went. Maybe took a bit more time but saved me a bundle of money. As the final fix involved no parts.

Answer 4

There should be a cold air intake valve somewhere in the plastic manifold that holds the air filter, or at least a sensor. Check the sensor to see if its wired up and working. If your car has a computer you really should have it plugged into a diagnostic computer and it will tell you exactly whats wrong. Thats why they started putting them in cars, because there's so much that can go wrong and it gets expensive replacing stuff until you hit the right part.

Answer 5

WOW! Since all else has failed-check around the intake,TBI,and any hoses for a vacuum leak when it's at temp(hot). The old timers would spray around the engine components with carb cleaner(its non flammable) and listen for a rise in engine rpm... Leak found.
Don't forget the charcoal canister too. Look for a loose ground somewhere too. I may seem stupid but alot of weird things can be traced to ground issues.
Good Luck!

Answer 6

I would try a second PCM or ECM. I had a Celebrity, and swapped out everything in the fuel path, still cut out intermittently.
Second control module fixed it, though. I guess whoever does the restore on electronics didn't troubleshoot it long enough.

Answer 7

You could try putting a wire directely from batt pos 12 volt to the pink feed wire on ign coil,that would eliminate any voltage drop issues. also the MAP sensor needs to be checked for proper read out.unplug hose going to EGR to see if it changes anything.

Answer 8

you have a bad grounding strap of you block when your engine gets hot the wire separates and that's when you get the misfire and drop off it will get worse as tI'me goes on im sorry that i cant tell you witch one it is , i spent a ton of money on my impala because it was doing the same thing then i went to a electrician and he was done in less than an hour

Answer 9

locate your computer (usually behind the right kick panel) let the car warm up - slap the computer a few times and see if the engine stalls or stumbles. I have been a Master Technician for 25 yrs. I have trouble shooted many of cars this way... good luck

Answer 10

As long as you're throwing parts at it, how about a new from GM delco coolant sensor. I would also try driving it with the tan timing wire disconnected, just to see if it stops the misfire bit.