90 c1500 has issues with an inconsistent misfire

Moorekennels · Jan 12, 2022

Hello everyone. I am having issues with an inconsistent misfire on my 90 c1500 with a 5.7 liter motor. The truck will run fine for 2 or 3 day. When the Miss fire starts it may last for the day or 2 and mysteriously clear up. During this time the truck also experienced idle control issues. I have replaced the entire fuel system from the tank to the relays. I have also replace the idle control, control module, map sensor, ECM, ECM prom, coolant temperature sensor, plugs and wires.
If you have any ideas where else I can look please let me know, I'm open to trying anything.

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Schurkey · Jan 12, 2022

Verify actual fuel pressure.

How old are the usual "tune-up" parts--cap, rotor, plug wires, spark plugs, PCV valve, air and fuel filter?

Have you looked at the TBI distributor, under the rotor? The pickup coil magnet assembly is commonly cracked, leading to erratic operation.

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Magnet cracked through the rivet, starting under the Copyright symbol.

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Connect a scan tool, find out what the computer is trying to tell you. Verify all sensors and computer outputs. And remember that if a sensor has problems according to "codes" or direct reading of the data stream via the scan tool, it can just as easily be the wire harness not the actual sensor. What are the MAP and O2 signals before and during the misfires? Which cylinders are misfiring? What, if any, codes are stored?

Frank Enstein · Jan 12, 2022

When it screws up knock the air cleaner off and look at the spray from the injectors. They should look like cones of fog.

My wife's Astro had a sticking injector that wouldn't fire at idle but as soon as you revved it up it would start spraying.

Gotta get a scanner and a fuel pressure gauge setup to do any in depth diagnoses.

obswill · Jan 13, 2022

I’m currently having the same issue, at idle my rpm’s drop and then Increase, I can also feel a slight misfire while in park. I’m going to test my pressure at the throttle body today. I’ve done everything you have done as well

VFJimbo · Jan 13, 2022

Nice Truck....

1993GMCC2500SLX · Jan 19, 2022

Schurkey said:
Verify actual fuel pressure.

How old are the usual "tune-up" parts--cap, rotor, plug wires, spark plugs, PCV valve, air and fuel filter?

Have you looked at the TBI distributor, under the rotor? The pickup coil magnet assembly is commonly cracked, leading to erratic operation.

You must be registered for see images attach

Magnet cracked through the rivet, starting under the Copyright symbol.

You must be registered for see images attach

Connect a scan tool, find out what the computer is trying to tell you. Verify all sensors and computer outputs. And remember that if a sensor has problems according to "codes" or direct reading of the data stream via the scan tool, it can just as easily be the wire harness not the actual sensor. What are the MAP and O2 signals before and during the misfires? Which cylinders are misfiring? What, if any, codes are stored?

My distributor when I went to put a new gm ignition control module in it was rusted worse than the 2 pics of this thread shows,so I replaced the entire thing and put the gm ignition control module in it.

Moorekennels · Feb 26, 2022

obswill said:
I’m currently having the same issue, at idle my rpm’s drop and then Increase, I can also feel a slight misfire while in park. I’m going to test my pressure at the throttle body today. I’ve done everything you have done as well

Please keep me posted.

90 c1500 has issues with an inconsistent misfire

Moorekennels

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Schurkey

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Frank Enstein

Best. Day. EVER!

obswill

Newbie

VFJimbo

OBS Enthusiast

1993GMCC2500SLX

OBS Enthusiast

Moorekennels

Newbie

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