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 Jim Grant's Tech Tips '97 Pontiac GP, Rough Idle When Cold
Q: Our ’97 Pontiac GP with a 3.8 engine suddenly had the check engine light come on and started acting strange. When first starting cold, it had a rough idle and some times would quit. After it has warmed up, it seemed to be fine. The dealer replaced the Mass Air Flow sensor at a mere $340. I was told that the intake had an air leak plus coolant leak and would be another $400.+ to repair. I elected to do the work myself, as I have been involved with auto repairs in the past. After replacing the intake gaskets, which I could not find any problems with or even a coolant leak, it still has the same problem, rough idle on start up and stalling when cold. I would appreciate any help with this matter! - Dave D. A: I’d really like to know what that computer code was that the check engine light was on about. Stuff like that is just useful in coming up with a diagnostic path. The intake manifold on your vehicle has been a problem both with causing vacuum leaks and coolant leaks. The coolant will just leak into the intake manifold and collect in the lower half. The cause for this is most often at the EGR valve passage. The plastic will corrode away allowing an adjacent coolant passage to share coolant with the wrong part of the manifold. The other problem we see is with the intake manifold’s gaskets. The rubber type seals become hard and do not seal the intake manifold. What is neat about this problem is that the heat of the engine causes the seal to become more flexible and then it will seal. Basically, when the engine is cold there is a vacuum leak and when the engine is warm the problem is gone! That one is a real bugger to figure out sometimes. If you just serviced the upper half of the intake manifold, you may not have addressed the problem that was in the lower half.
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