Common Idle Issues/Fixes
Often it is necessary to reset idle or to try and fix a hunting idle or when a new throttle body is added. The first step is to look for any vacuum leaks on the vehicle. I do not know how many times we have seen cars with vacuum leaks where the owner SWEARS there are not any - this is a weekly occurrence. You can spray around fittings and hoses with carb cleaner or use propane to try and pinpoint leaks. Inspect all hoses and fittings as well. A shop with a smoke tester will be able to pinpoint leaks as well. Be sure all sensors and mechanical problems are up to snuff as well.
Once you are 100% sure the vehicle is 100% sound, you can do the Ford recommended idle reset procedure. Do all steps in order and do them all, or it won't work.
1) Figure out where the PCM is set to idle. Often your tuner will know. If it's not tuned than 600-700 RPMS are a good bet. This will be your target idle RPM. You do not want the Idle air control (IAC) to be fighting the PCM.
2) Start it up, let it get to temp - this will take 4-5 minutes
3) Disconnect IAC. If engine stops, turn the throttle-stop screw in one full turn and start it.
4) Keep messing with this until you get an idle at the targeted RPM.
5) Shut off the car.
6) Set KEY ON/ENGINE OFF
7) Check the Throttle Position Sensor (TPS) voltage with a volt/ohm meter - it should be anywhere from .95 to .99 volts.
8) Adjust it if it's way off - anything over 1.05 v or so is not good - adjust by loosening the mounting screws, and lightly rotating TPS. If you can't get the volts in line enlarge the mounting holes with a drill.
9) The TPS wires you use are sometimes the BLACK and GREEN wire, or the GRAY/RED and GRAY/WHITE, depending the year.
10) Turn the key off.
11) Disconnect negative terminal battery cable. Hit the brake pedal a few times and turn the headlight switch on. Wait 3-5 minutes, then shut off the headlights and reconnect the battery. This clears the Keep Alive Memory where idle settings are stored.