My '92 SSE */c recently began surging a little at idle when the engine is warm. It only does this occasionally, and when it happens, the RPMs surge up and down between 700 and 900 RPMs or so and the boost gage is pretty steady but is near the first tick mark rather than being about 1/8" above the lowest mark as it normally is while idling and the engine is warm.

The car starts and runs fine otherwise, and there are no error codes.

Any ideas?

UPDATE: I had the catalytic converter replaced a few weeks ago, and also removed and cleaned the MAF sensor (it looked like it had a fine dust on the sensor elements). The surging problem didn't happen again over the next several days, but it happened again today so it can't be cause by the cat or the MAF. On a side note, the guy at the muffler shop said that a catalytic converter wouldn't cause surging and that the one he removed from my car was clean and was definitely not plugged or restricted.

You have a vacuum leak somewhere. check all vacuum hoses.

It'* been my experience that about 75% (at least) of rough idle problems are caused by vacuum leaks.

Thanks for the tip. Coincidentally, this engine has had a rough idle since I bought it in '99. But prior to this last week or so, the RPMs have always remained steady at idle and I had never noticed any surging at idle either.

I thought about the possibility of a vacuum leak too, but if that were the case, wouldn't the surging problem be more frequent, if not all the time? It only happens occasionally.

I checked all vacuum lines several months ago and replaced a few that were showing signs of age, including the small *-shaped vacuum hose on the top of the supercharger (it was soft and collapsed on itself when vacuum was present) and I replaced that also but I didn't find any hoses that were broken, missing, or otherwise obvious problems. And to my surprise I didn't notice any driveability differences after replacing those lines.

While we're on the topic of vacuum lines, though, when I last checked the vacuum lines I noticed that the relay on top of the fuel vapor canister sounded like it was chattering constantly after the engine warmed up.

I'll check the vacuum lines again asap.

The biggest "vacuum leak" is the PCV valve.

Changed it recently?

my car does the same....i think

cept i notice it when i first start it up {in the morning or something} and after a few minutes it'll start to rev itself as if someone was inside lightly tappin on the gas....but after the engine is really warmed up, it seems to be fine...

i also notice a whinning noise at times when i first start it to...but that also goes away....is it normal? or do i have problems?? :?

My daughter'* 91 Grand Prix with the 3100 engine did this for a while. Replacing the cat fixed the problem - it was getting clogged.

I put some fuel system cleaner in the tank last week and this morning I noticed that the idle was ok on my morning commute. I'll continue to keep an eye on it.
Update: The problem occurred again the next day; it most often happens after 30 minutes of freeway driving, when I exit the freeway and stop at the first stop light. I was also wondering if maybe the problem could be caused by a fuel tank/sock/pump problem on my '92?
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Archon,
My catalytic converter may be overdue for replacement, but I have no idea how old it is. I already bought one but haven't had it installed yet. Are you sure the idle problem on the Grand Prix was caused by the catalytic converter, or did you replace other parts at that time also?
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Bob Dillon,
I changed the PCV valve about 6 months ago.
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The cat was the only thing replaced at the time. Until we got around to replacing it, running mid-grade gas instead of regular lessened the problem. The pack was rattling inside the cat., so knew it would need replacement soon. When it finally broke loose from the exhaust pipe, I had it replaced, and the surging problem went away. She'* back to regular gas, too.

I believe the cause of the problem was a combination of things.
I replaced the air filter. Although it wasn't very old, it had a lot of fine sand-like particles in it.

The rough idle was still present, and after checking and replacing other parts with no improvement, I replaced the supercharger base gasket and tightened the vacuum block on top of the supercharger and now the engine runs like it did when I bought it.