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Q: When stopped waiting at a stop light, the engine RPMs will drop a bit and it sometimes stalls out.

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2008 Jeep Patriot 4x4:

From time to time, usually when stopped waiting at a stop light, the engine RPMs will drop a bit (from about 600/500 to 300) and sometimes even stall out. Most of the time its not a big issue, as the car will give a slight jolt back and forth and then go on as normal. I would say this happens maybe once every other week. Most of the time happens when sitting and idling at a stoplight. Do you know what is causing this?

I bought the car used with about 85,000 miles on it. Not sure about the maintenance history. Would replacing the spark plugs help with this? The car is only worth a few thousand dollars, so I don't want to put a ton of money into it.

Thanks for your help.

My car has 91300 miles.
My car has an automatic transmission.

This is a common sign of 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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