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Q: Rough idle and RPM fluctuation

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truck has a very rough idle, today I replace both vct soilnoids and it did not help have a check engine is on runs better with overtime off, two years ago I have both timming chains replaced and plugs too. runs fine on the highway.

My car has 160000 miles.
My car has an automatic transmission.

A: This may be related to a dirty or failing i...

This may be related to a dirty or failing idle air control valve. The idle air control valve reads 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 often times will cause the engine to stall indicating a dirty or faulty idle air control valve. I would recommend having an expert from YourMechanic come to your location to diagnose and repair your idle air control valve.

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