Q: Car is idling low at less than 1 RPM at a stop

asked by on October 17, 2016

When the car is at a stop light it idles at less rhan 1 rpm when it is is in drive (in neutral it doesn't do this) The car has an automatic transmission. It makes a loud hum and shakes when it does this. What might be the problem? More importantly what might be a solution? Thanks

My car has 120000 miles.
My car has an automatic transmission.

Hi there. This may be a sign of a failing idle air control valve. The idle air control valve is located on or near your intake manifold that monitors the air intake as it is mixed with fuel prior to being injected into the engine. 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. However, when the engine RPM drops below the normal range of 800 RPM (for most cars), it is more than likely that you are dealing with an IAC valve problem. In cases where this valve is clogged or is failing, your vehicle will stall instead of slowing to an idle after the release of the throttle. If you would like to have to have this done, a qualified professional, like one from YourMechanic can come to your car’s location to replace your idle control valve.

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