3.1 runs bad when warm?

[SHOoff]

My dad was asking me about my little brothers car. They've been having problems with it running. So far it's had a new crank sensor and new ECU and it still causes problems.

91 Grand Prix, 3.1. When it's cool the car will fire right up and run just perfectly fine like it's new. Sounds completely healthy. After the engine gets warm the engine I guess keeps stalling, has a low idle, sounds kinda like it's missing, and he has to keep his foot on the gas in order to keep it running. Which, I guess is kinda hard on it since he's riding the brake and holding the gas while trying to drive down the road.

My guess on it was since it does fine cold and bad when hot, that it's the ICM? Does it sound like I have a worthwhile guess? And if so, anyone got an extra, good ICM?

[IanSzgatti]

any check engine lights? sounds like it starts running like shit after it goes into closed loop... could be an 02 sensor problem... wouldnt discount the ICM either though.. take the coils off of it and inspect it.. or test the circuits when cold and then when hot, if it's bad you should find something then

[IsaacHayes]

Could be a bad coil pack too, once it gets hot a crack in the epoxy opens up or coil wire separates..

[SHOoff]

I almost want to say that my dad changed the O2 sensor on it (since it's only 20 bucks) but I can recheck with him and make sure.

Only reason I was counting out O2 sensor is that it still starts like hell when the engine is hot.

[Superdave]

Yep, check the coils and ICM.

Autozone will test the ICM for you for free.. just take it in and have them run the torture test like 5 times, it'll get hotter then hell, if it fails then you found your problem.

then test the coils, i think the propper ohm readings are posted on V6Z24 somewhere...

[SHOoff]

Will have to do that when we get some free time. I told my dad that if the ICM is bad he might as well just change the coils too since they really dont cost a whole lot. I figure that would probably be the best as the car is A) Old B) has a lot of miles and C) before they bought it, it had been sitting for quite some time. And just a quick look down there look like it's probably the stuff that was put on at the factory.

Q) Is the ICM for the 3400 the same?

[Superdave]

Yep, the 3400 uses the same one, same coils as well..

[LeftVentricle]

Just had this issue with my Celebrity (3.1 MFI). Would die outright when it reached operating temperature. Replaced coils and ICM ($204!), all tested bad. Test drove it, did not solve the problem. Ended up being a $24 crank sensor. I almost wish I hadn't bought the coils/ICM, but since they tested bad I would've needed them soon anyway.

EDIT: Crank sensor is a bitch, at least in an MFI A-body. You need to pull the alternator and bracket and feel around blindly for a while. Here's the thread I posted at A-body.net. I posted a diagram from Alldata and a write-up for changing the crank sensor.

[SHOoff]

Well, the car died on him once, and it turned out to be a bad crank sensor (at least that's what the mechanic said) but it still runs like ass with the new sensor.

Actually the being hot problem seems bad enough I guess. Never really did notice that it ran fine when the engine was cold until here recently when it started to get a bit cooler out. I guess only cuz it can cool off faster. You know how it is with someone who's only 16, 17 years old and just got a liscense (and probably how you were at that age, I know I was) the car barely gets to see much time sitting in park.

Thanks Dave for your answer on that. I thought they were the same but wanted to be sure. I guess if we can't get him to put the car up for a bit while we inspect the ICM issue I could just swap him for a bit off of my 3400.

[IanSzgatti]

could always let it get hot and shity running and then douche it down with some icy water and it should go back to normal with that rapid cooling... i've heard it, never tried that particular farmers diagnostic technique

[LeftVentricle]

could always let it get hot and shity running and then douche it down with some icy water and it should go back to normal with that rapid cooling... i've heard it, never tried that particular farmers diagnostic technique

Sounds like a great way to crack a block or drown some ignition coils.

[IanSzgatti]

Sounds like a great way to crack a block or drown some ignition coils.

it's good to recognize potential risks, but i think your overly concerned... while I personally havent used that technique, there are some experienced (and ASE certified) guys over on IATN who have used that technique with success.

[msdc041]

I also am having a problem with my 3.1 when warm. I have a p0300? (random misfire) mostly 1 and 5 cyl. but all are affected. It only occurs when the engine is warm (over 15 minutes of running) and only at highway speeds, and only just after shifting into overdrive with easy throttle/low rpms. Trans checked out fine, by trans shop. I started with plugs, wires, ICM, and replaced coils with known good ones. Replaced cam sensor, and fuel filter. No SES lights on. I do have an exhaust leak at the cross over pipe, which has lead to an intermittant SES light for EGR low flow, and 2nd 02 sensor issues. Any thoughts? Getting the exhaust fixed soon, but wonder why only after 15- 20 minutes of driving....sounds electrical, its after the warm up period (1st loop) by some time before it malfuncitons. Anyone think fuel pump?

Any thoughts appreciated.

[3100 MPFI]

Suprised no one has mentioned this.....

While the engine is COLD, start it up and spray around the engine with some cheap CARB CLEANER!!!!!

In my 1990 Grand Prix STE, it did the same thing. Ran fine when cold, and when warmend up woudl not run.

Turns out my plenum gasket had a crack in it.

The normally higher idle when cold was keeping it running, but once it warmed up and tried to lower the idle to normal, it would not run.

You might have to let the engine start warming up and when it gets to running bad, spray it.

If you don't know already, if you have a vacuum leak (what my problem was because of the cracked gasket) when the engine sucks in the carb cleaner, it will stumble and try to die.

Rule out the possibility of a vacuum leak first. Also spray around the vacuum line that goes down to the tranny (on the front of the transmission), as well as over near the cruise control, etc.

[msdc041]

I'll give the carb cleaner a shot to check for leaks, but I don't think its the problem, because it only has a misfire ( according to scan) at very specific times. The engine is warmed up for anywhere from 15-25 minutes BEFORE any malfunctions or misfires, and only at highway speeds with low rpms (just after shifting into OD and revs drop. Initially this problem began after driving for over 2 hours, then after and hour, now only 15- 25 minutes.