After normal driving it takes a long time for the motor to return to idle. Last night it kept running at 4000 rpm after trip on the motorway. - Scared the wife, made the teenage son laugh and also made it a bit difficult to drive through smaller cities.. As far as I can figure out it is either the IAC - Idle Air Control, or the MAF sensor - Mass Airflow Sensor . I'd like nok to purchase a part that is not neccesary, so what would be the most likely part to change first?