I have a 1990 Pontiac 6000 LE with a 3.1L V6. It has recently developed a stumbling idle when warm with acceleration hesitation, but runs good cold. I did a major tune up, and replaced the ICM, MAP sensor, plugs, plug wires, coils, injectors and fuel pressure regulator and pump, O2 sensor, EGR (both O2 sensor and EGR were bad), and even replaced the computer and throttle body. No codes. I cannot find any vacuum leaks, and I also replaced the purge valve and evap canister. Head gasket and intake manifold are good. I am stumped. I thought maybe the catalytic converter might be bad, but that was replaced about 5 years ago. Maybe it clogged again when the O2 sensor and EGR were bad? Not sure what else to do at this point.
Well you don't have OBD2 diagnostics, just OBD1. I think 1990 is even too early for a Tech2 scanner. What you need to do next since the parts cannon approach didn't work is troubleshooting to figure out the problem.
It sounds like the kind of situation where on a carbureted car it runs OK with the choke closed at least partially, enriching the fuel mixture. In your case when the engine is cold the computer is feeding extra fuel to the injectors when cold, then when the engine warms up the extra fuel is turned off. Did the old plugs show evidence of a overly-lean mixture or other combustion problems? How did you check for vacuum leaks? Is compression good? There's plenty of opportunity for rubber hoses and other components to deteriorate over the last 34 years.
If the catalytic converter were plugged up the engine wouldn't run properly when cold.
