It sounds like a vacuum leak to me. THe most common problem is a vacuum leak behind the throttle body (near the IAC). It is a vacuum hose that is aprox 3/8" and has a 90 degree bend in it. The hoses dry rot there and cause leaks.
Check that and let me know what you find. Also, check for other vacuum leaks (broken, dryrotted, disconnected hoses).
Sunday, March 21st, 2010 AT 12:23 AM