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:
- Coolant temp
- Air temp
- MAP sensor readings
- BARO sensor readings
- O2 sensor readings when in closed loop
- Air fuel sensors or AF sensors (similar to O2 sensors but a little fancier)
- MAF sensors if equipped
- TPS sensor
- Load sensors such as the type for power steering pressure
- AC clutch activation
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.