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The first thing we should talk about is the engine’s idle circuit. I'm going to cover a fuel-injected engine here. Carbureted engine idle problems are handled differently. Since there are so many different configurations used depending on make and model, I'm only going to speak in general terms.

One of the prime suspects here is the idle air control valve, or IAC. This is an electronic motor that varies an opening in the intake that allows air to bypass the throttle plate when it's closed. The amount of air needed to maintain your idle is calculated by the computer. The computer dictates an IAC positioning based on inputs from the engine’s sensors, usually:

Those are just the ones I can think of off the top of my head. I have a reason for listing all of these sensors. If you have a check engine light on for any for the above sensors, repair that fault first, then recheck for your idle problem.

I've had many people come to me and say that they've cleaned their IAC valve and still have an idle problem. Personally, I haven't had much luck cleaning IAC valves. I recommend replacement instead of cleaning if you find yours to be bad. I'm not saying don't clean them, because you might just get lucky, but if cleaning doesn't work, then you might end up having to replace it.

IACV Diagram

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