When my ac is off my engine idle is just fine. But when the ac is on the idle is very erratic. I have a feeling it's the iac valve because when I pulled it out it had a bunch of carbon on it. I cleaned it best I could and it's still the same.
My car has 129000 miles.
My car has an automatic transmission.
As you have said, this may be related to a dirty or failing idle air control valve. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. This valve is controlled by the vehicle's computer which will adjust idle speed based upon other measurements such as engine temperature, intake air temperature, and electrical system load or voltage.
When you accelerate, the engine RPM increases, and, as you let off the gas, the RPM slowly returns to the normal idling speed. This is all controlled by the idle air control valve. When the engine RPM drops below the normal range of 750-800 RPM (for most cars), this may indicate a dirty or faulty idle air control valve.
In your case, this may be related to the valve not maintaining the proper voltage or connection in which case it may need to be replaced. I would recommend having an expert, such as one from YourMechanic, come to your location to determine whether the idle air control valve is faulty replace it if so.
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