What would cause my car's engine to race when idling?

KMCONARD

MEMBER
SATURN

Sorry,

I think I should add a little more information to my inquiry.
The car is a Saturn SC1 year 2000. It has about 84,000 miles.

I recently went to a mechanic when I noticed the problem and he diagnosed it as a bad engine coil. But upon further investigation with a friend of the family, I learned one of the key indications of a bad engine coil is a lack of acceleration or trouble excellerating. This is not the case with my car. Granted it has a small engine so it's not speedy to begin with. But I have not noticed any change in excelleration. I'm more than a little hesitant to pay big money to replace a part that doesn't need replacing. So I'd like to have a clear answer as to whay could be causing the problem.
If this helps. The racing idle occurs when the car is both warm and cold. If I turn off the car when it is racing. And restart. The engine corrects itself and idles around 1,000 rpms as usual.( The fast idle is anywhere from 2 to 3 thousand rpms.) It is a manual transmission car and I have learned how to prevent the fast idle. If I slow the car to under 15 hundred rpms before depressing the clutch and coming to a stop. 9 times out of 10. It won't idle fast.
Any ideas?

2 Replies

KJK

MEMBER
4 POSTS

My '98 did a similar thing until I did two things, not sure which one fixed the problem though. I replaced a broken resonator that connects to the air filter box and the coolant temperature sensor. The resonator was letting air into the filter box which bypassed any sensors in to normal air intake so maybe confusing the engine controller. And while the temperature sensor seemed to be working, it was just a little bit out of spec. Either way, the engine now idles properly.

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Wednesday, December 20th, 2006 AT 7:18 AM

SERVICE WRITER

MECHANIC
9,123 POSTS

I would go along with a vacuum leak if it was consistent. While the Coolant temp sensor are very common on these, It usually causes a rough idle versus high one. Not to say that it would be very cheap to replace on a hunch. The intermittant part of the equation that makes me think it may be mechanical in nature. I would be suspicious of a throttle plate having a heavy carbon build up causing an intermittant sticking problem. With it open, it allows more air into the engine, much like a vacuum leak. The computer makes an adjustment for the extra air by sending a command for more fuel, hence the idle increase. Intermittants can be challanging.

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Saturday, December 23rd, 2006 AT 10:28 PM