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Surging Idle On 1995 Geo Prizm 1.8L

by Randy, November 2, 2009 in Pre-1997 Toyota Corolla and Geo Prizm

I am attempting to find the cause of a weird idle on my girlfriend's 1995 Prizm. When started cold it runs at about 1500 rpm (in Park or Neutral). As it warms up, it begins to surge about every 2 seconds between 900 rpm and 1500 rpm, over and over again. I am wondering whether I should be looking for a vacuum leak? Replacing the idle air control valve ($$) ??

What suggestions do you have, and where should I start looking for a leak if that is the most likely cause?

Thanks for you help.


Hello and Welcome to the forums.

Sure sounds like a vacuum leak somewhere. Could try the old-school method of spraying vacuum hoses and unions with a little water mixed with some detergent (surfactant) - any minor cracks/leaks will be temporarily sealed by the water and your idle will change dramatically.

IAC is also a good to check out. Before I'd condemn that part, try a good throttle body cleaning and see if that helps improve the idle. Double check the plugs and wires, cap and rotor - make sure it isn't something simple as a burnt plug wire, heavy carbon tracking on the cap/rotor, or a worn/fouled plug. A significant number of misfires could cause the ECM to default to "safe" mode and the car will run a bit "off".

because of the rhythmic nature of this problem, I am starting to wonder (after searching some other postings) whether this might be the ecu doing some "idle hunting" as others have called it. Whether there is a failure of some sensor, or whatever, the ecu just can't seem to figure out what speed to set the idle at and keeps trying to drop idle speed, sensing something is wrong, then bringing it back up again quickly, then starting the process all over again.

Anyone have experience in repairing one of these wacko idle speed hunters?

Oops. Meant to say, replaced the plugs and wires over the weekend and cleaned the throttle body. No difference there.

One other question. The car is probably not worth more than $400 right now (accident damage) and the timing belt has never been changed. There are now 133,000 miles on this car and I am trying to decide if it is better to risk NOT repairing it if she is planning on replacing the car in less than a year? She drives 3 miles to work, so no real risk of being stranded on a morning commute.

Most of the idle hunting cars were the earlier 9th gen Corollas and some of the 2005+ models with DBW throttles. Wasn't a known issue with the previous two generations of Corolla/Prism cars.

As far as fixing it, on the cheap, and not dump too much money into a car that will probably be replaced soon? Hard to say, you'll pretty much covered the "easy/inexpensive" fixes. Now, it sounds like it could be anything from a bad chassis ground (electrical noise will confuse sensors and ECM), bad IAC circuit, vacuum leak, any number of sensor issues, timing belt could have slipped a tooth on you, making the car run odd (if you have a timing gun, give it a quick check, if I remember correctly - you'll have to jump the diagnostic port under the hood to get the ECM out of the loop).

Since it happens when the car warms up - I would suspect O2 sensors as another likely suspect. As the gets up to operating temperature, the ECM will use the O2 sensor output for part of its idle control feedback. Vacuum leak is also likely, bad IAC valve in the throttle body is also a good culprit. Unfortunately, those are also pricey parts. All combined, you'll probably spend more for parts than what the car is ultimately worth.

Another thing to try, especially if the commutes are that short (~3 miles). I'd take the car out onto the highway and just run the snot out of it. Could be a simple case of heavy carbon deposits inside the combustion chambers. If a car hasn't been run hard from time to time, it can generate a surprising amount of carbon in there, and not have a good way to blow it out.

Thanks for your suggestions. I suppose taking carb cleaner and spraying it around suspicious "leak-prone" areas might tell me if there is a bad gasket somewhere. Car has been on one extended freeway trip (about 6 hours) in the last two months. Did not solve it. As you say, if I can't find it with inexpensive fixes, it may be best to just try to ride it out until a replacement is in order.

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