Is the check engine light on? The computer controls the idle set point from the information it receives from various sensors. If it is getting some bad information from a faulty sensor, the computer will never be able to control the idle set point. The way Fords regulate the idle is with a Idle Air Control and the Throttle Position Sensor is critical in transmitting to the computer how fast you want to get your engine to run. Those are the likely suspects but, a scan tool would be necessary to do a proper diagnosis. Thanks, Chuck - Baum Blvd Automotive