Hi, I'm new here and I hope I'm doing this right because this truck is driving me nuts!! I have a 1990 C3500 with the 7.4 motor. It just all of a sudden started running like crap and then stalled, It will start back up but you have to keep your foot on the gas and after a few seconds, it quits again. I have replaced the fuel filter, ignition rotor, and MAP sensor. With the old sensor the truck would barely run and it smells, unplugged it and it ran better but would not idle. Replaced the MAP and still does the same thing. There is plenty of fuel pressure. Anyone have any ideas???? 1967 Malibu
Update; forgot to add that I hooked it up to my Snap-on scanner and it showed no codes.

 

there is a connector under the steering column to read engine codes on this car. you have to push a paperclip on double ended metal object into terminal A & B(the top 2 adjacent rows). This turns on the check engine light and flashes whatever is in the PCM memory. What you may have is a throttle position sensor issue, maybe a engine coolant issue, maybe a crank sensor, or poor spark control(doubt it-but it definitely sounds like a lack of signal from a sensor thing).

 

Thanks RacerX, I've already tries all of your suggestions, any other ideas???

 

1. you have fuel, air and spark bcause it starts.
but doesnt stay running.

2. how many seconds before it stalls?


lack of TPS signal wouldnt let it idle, but if you were goosing the pedal it still should have stayed on. lack of Idle speed control would let it idle but at odd rpms, either too high or too low. It wouldnt start with no RPM signal(the fuel pump circuit would stay open). High ECT signal would shut it off immediately to prevent engine damage. All that stuff is definitely "service engine soon"-light stuff.

well, if you start it without holding the pedal count how many seconds before it stalls. fuel pump circuit(energized by the fuel pump relay) and grounded through the PCM to continue operation when ignition switch is in RUN position...I am speculating here that your PCM PROM may be gone, but not sure yet. Give yourself a visual inspection of the engine sensors for frayed wires or broken housings.