Jimmy warms up then chokes
 

I bought a '95 GMC Jimmy SLT 4wd 4.3L knowing it had an issue (I got it dirt cheap) but not knowing the cause. I've had it for over a month and I've been unable to get it fixed as of yet. I have a very low budget so I can't go replacing parts willy-nilly.

It runs perfectly for about 10-20 minutes, until it gets good and warm, and then it starts to choke. If you step on the gas it sputters and the RPMs drop. Makes a horrible guttural noise as well, like an old dirtbike. Fuel pressure increases when it's choking. It tends to idle fine for a while, then eventually the RPMs drop to around 20-50 and it quits. TPS is fine, IAC doesn't seem to affect anything. Next to the engine oil filler cap there is something that looks like a vacuum pump, but no vacuum comes out of it even though there are two vacuum lines connected to it. I can't figure out what it is or what it's for. I also can't figure out if this is a TBI or MFI or whatever engine.

I could use any help if anyone has any to offer. I have access to a scanning computer, but not very often... next time I will be able to scan will be on Friday.

Does it have any codes? If so what are they?

You might get a can of Mass Airflow Sensor cleaner (about $6) and clean the mass air flow sensor. Cheap & easy place to start.

A 95 CPI motor does not have a MAF sensor. You could clean the EGR valve.

If no codes are present, I am going to go out on a limb and have you test the ICM and coil. Both of these components can have heat related issues. Most autoparts stores have the equipment necessary to test these components. If/when you have them tested, test them 10-15 times in rapid succession to get them good and hot.

Okay, what are the ICM and coil, and where are they located? The Haynes manual is not very descriptive.

The location of these items depends on the ignition system your vehicle has. Does your distributor cap have horizontal terminals or vertical terminals (where the plug wires connect)?

Vertical, thank God. I heard that the horizontal one is realllly expensive.

If the coil is the black box that the center wire is connected to then it's on the left side of the engine facing the back of the car.

If that is the case, then your ICM is located inside the distributor under the rotor. Your coil is what is on the other end of the center wire to your distributor.

Oh right... the ICM is that little chip that causes the spark? I thought it could be that originally, but people keep telling me that if it fails it does it all at once and it's not heat related. I'll have them both tested and let you know the outcome. Thanks!

Those people are wrong. If the ICM is not conducting heat properly into the distributor body (or heat sink on the newer distributors), it can definitely cause heat related issues. I have seen numerous ICM issues that are heat related. Many were able to be cured by cleaning the mounting surfaces and applying some dielectric grease. But while you have it off, definitely get it tested.

Coil tested fine. Took the ICM off, was told it couldn't be tested (the guy at AutoZone had the wrong part number) so I bought a new one. Put the new one on, noted that the old one did NOT have grease on it, and now my Jimmy is FIXED! Runs great now! Have to get used to driving such a heavy vehicle in these mountains though. Also have to nail down why the brake lights and turn signals don't work. Thanks, dude... I never would have replaced it if you hadn't told me it could indeed have this problem. Oh, and the old one failed the leakage test after the second round.