'97 Lumina stumbles and stalls when hot

[Luminiac]

This problem started last summer - with A/C on car ran fine until engine got real warm (heatsoaked?). After that, when coming to a stop or idle engine would suddenly drop in RPM and rumble/stumble, then die. It could be restarted, but would stay in the stumble condition, and would not respond to gas pedal. If left to cool for 30-45 minutes, engine would restart and run, until it got hot.

Since the original problem (never throws a code, except sometimes a misfire code when engine is stumbling) I have replaced plugs and wires, the ECM, IdleAirControl, and the latest is the IgnitionControlModule - all expensive "guesses" on the part of experienced (?) and well-meaning "shade-tree" mechanics (including me) - but the problem still occurs.

I have also tested the coils (cold) and all were the same, about 5% below the spec (included with the new ICM).

Lately, the problem can show up even when the sun is shining on the hood, last time on a sunny, 70 degree day, from a cold start. Obviously, whatever is the problem is getting worse.

I've had suggestions from "sugar in the gas tank" (problem ought to show up all the time, right?) to "bad grounds" (checked every one I can find, all connections were cleaned/replaced), but I still haven't found the Magic Bullet.

Jeffcoslacker, got any extra bullets?

[maxwedge]

Welcome to AF, I would start with a fuel pressure when this is happening.

[Luminiac]

Thanks - but do you know what part or parts would cause low fuel pressure when the engine compartment got hot?

[maxwedge]

Temp under the hood is not an issue with the fuel pump, running time is, just check that to eliminate it as a problem.

[Luminiac]

Thanks - but running time doesn't seem to be a factor, the problem seems to be related to engine compartment heat. Is the fuel pressure regulator heat-sensitive?
New question: if one of the coils fails when hot, could it cause the engine to stumble and run so rough it can't increase speed past an idle? or would the engine continue to run, but with less power?

[maxwedge]

Why do I have to talk you into doing basic tests here? If a coil dropped out you would loose power but you certainly would be able to drive at least 40mph. If the FP reg. was bad as you seem to suggest, then again!, a fuel pressure test would point to it or eliminate it.

[richtazz]

As Shep (maxwedge) has suggested numerous times, get a fuel pressure reading to eliminate that as a possible cause.

If you want to continue to throw parts at it, other underhood items related to heat are the crankshaft postition sensors (there are 2, one behind the harmonic balancer and one in the back of the block), cam position sensor and ECM.

[Bearwulf]

How many miles on the car? Many GM's of that vintage had a problem with fuel tank sending units after 120-150 thou. That prob. could be the cause of the problem you are having. Calls for an updated sending unit. As Maxwedge said, get your fuel system checked.