5.7 V8 TBI stumbles

I have a stumbling engine when my chevy shifts into over drive. I have new plugs wires dist rotor coil ign module fuel filter air filter. Checked timing with auto advance unplugged and it is not @ TDC but about 4 deg advanced. It is also not consistent in its stumble. Any help?

I’d try a full dose of Techron (or some other cleaner). Year/model/miles?

Are you sure that the motor is misfiring? Sometimes the lockup torque converter malfunction will feel like a hesitation.

Good point - if it’s fine with OD off, then it might be the trans.

Great advice from both texases and UncleTurbo. Driving with OD off is a great troubleshooting tip that costs nothing.

Did you install AC plugs #R45TS?

I took it to a tranny shop and they say there is nothing wrong with the trans. I have tried 93oct and fuel cleaner. It is not consistent and does not just miss, it stumbles until I change the rpms. When I pull the plugs (AC Delco) a few are sooty. 95 Z71 300,000

On the OBD connecter while the key is off install a paper clip between the 2 slots on the bottom row on the right corner and then turn on the ignition but don’t start the engine. In a few seconds the CEL light will blink 2 digit codes. Code 55 means end of list and the codes will repeat. Get the codes and post them. Code 12 is irrelevant.

If you leave it in 3 hi or whatever the choice possible to prevent overdrive do you still have trouble?

I’d do a compression test. You also may need to decarbon the cylinders. My '95 Suburban had significant carbon issues.

Disable the EGR valve by pulling the vacuum hose off and plugging it with a golf tee or other small item. Then drive it and see if your problem is gone.

If it is, when you pull the EGR valve to replace it, shake it around a little. That rattling noise will be the broken return spring inside the valve. Without that spring a small amount of vacuum will cause the valve to open all the way and flutter, creating the sensation of a misfire.

I’m not familiar with the method to check the ignition timing on this engine, but OP seems to imply that it measures 4 degree too far advanced. Since the timing advancement will max out in the condition the stumble occurs, is it possible that the problem is just that the timing is too far advanced for some reason?

On my 92 Corolla during a tune-up I adjust the idle rpm timing by twisting the distributor. On this 95, is it possible to adjust the timing in a similar way? Or is the timing done by the computer. If the latter, I’d be suspicious of whatever sensors the computer uses to set the timing; e.g. the crank position sensor.

+1 for @asemaster. And come to think of it the temperature controlled vacuum valve that operates the EGR might be failing.

I’m getting old asemaster and those mid 80s models were a real pain that I seem to have subconciously tried to delete from my memory. It now occurs to me that the after market EGRs for those models had metered seats (washers) based on displacement. Oh those were the Daze.

I’ll do the egr test. I have no codes showing. All compressions are great 148-150. I can prevent the stumble in 3rd gear. Thanks for all the info. I’ll keep posting until solved.

My memory is worse than I thought. The '86 5,7L pickups were carburated. The EGR is still a likely cause but when considering that the problem only occurs in OD the lockup clutch might be the cause. Having no codes is somewhat reassuring though.

? - it’s a '95. Oh, nevermind, you’re referring to the problems with the mid '80s you worked on.

How many threads are currently discussing similar complaints and models? I have somehow skipped from one thread to another and lost track. Please put me on IGNORE for now all.

I tried a few things mentioned here. I unplugged the oxy sensor, no change. I unplugged the EGR no change. I plugged them in and checked the prestart codes and a 42 came up. My Haynes manual says that is an Ignition Control Circuit (shorted circuit). Explains it this way: Check the wiring and connectors between the ignition module and the PCM. Check the ignition module (see Ch 5). Replace PCM.

Why would this cause it to stumble at this speed and rpm? I did retard the timing to 0deg TDC.

I will check the wiring tomorrow. The ignition module and coil are new. What is a PCM? Pick-up Coil?

Thank you for your help.

PCM usually mean “Power Train Control Module”. The engine computer.

Avoid replacing that until all other possibilities have been eliminated. Finding a replacement compatible with your car is often problematic.

If your vehicle has one, have you replaced the crankshaft position sensor yet?

George SanJose Quote: Or is the timing done by the computer. If the latter, I’d be suspicious of whatever sensors the computer uses to set the timing; e.g. the crank position sensor.

I just read that a 42 code is set off also if the Electronic Spark Timing (EST) system faults. I believe this is part of the ECM.