Q: Erratic idle - 2013 BMW 335i xDrive

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Recently, I had my serpentine belt replaced. Since then I have had occasional idling issues. For example, sitting at a traffic light I notice that the idle can drop down to 500 rpm and then revs up to 1200 rpm.

My car has 30000 miles.
My car has a manual transmission.

This is a common sign of a dirty or failing idle air control valve. This is a unit that monitors the air intake as it is mixed with fuel prior to being injected into the motor at low speeds or at idle. This unit also assists with the deceleration process as you let off the gas pedal bringing the engine back down to an idle. When this is not working properly, you may experience an erratic idle as you describe.

I would recommend having an expert mechanic, such as one from YourMechanic, come to your location to determine why the idling is erratic and repair your idle air control valve if needed.

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