Idle speed is computer-controlled. If it's too high, suspect a vacuum leak. If you're also losing fuel mileage, you might look at the intake air temperature sensor. Failure is pretty rare because there's just one component in them, but if it's saying the air temperature is significantly colder than it really is, the computer will add fuel, like a choke used to do, and raise idle speed.
Thursday, July 10th, 2014 AT 1:18 PM