i have a 98 GT which started to run high revs while normal driving. additionally when i come to a halt the engine revs and the RPM is stuck at 2000 . when driving occasionally the car revs up all by itself as if it has a mind of its own. please help.
Our IAC's are famous for going bad, but usually they have problems with stalling. Cleaning the IAC and T/B would be a good start; its free. Pull the IAC off and clean it out real good and be sure to use a pick or small screwdriver to remove any large amounts of carbon. Worked for me once, but didnt last long.
Replace your IAC (don't buy it from Auto Zone) then reset your idle with this procedure:
You will probably get a million different responses on how to do this. This is how I have done it and it works for me. And I've done it a lot.
1. Disconnect the negative terminal to your battery and turn on your headlights for about 10 minutes(there's a lot of speculation as to how long. some people will even say to disconnect the positive terminal and some people will say to leave it disconnected over night.)
2. Disconnect the wiring harness connector to the IAC
3. Turn off headlights and reconnect battery. DO NOT TOUCH THE GAS PEDAL!!
4. DO NOT TOUCH THE GAS PEDAL!! Try to start the car. If the car won't start adjust the set screw on the throttle body a 1/4 - 1/2 turn and try again. Repeat until the car will stay running. It will run rough but that is what you want.
5. Let the car sit and idle (DO NOT TOUCH THE GAS PEDAL!!!) until the car reaches operating temperature.
6. DO NOT TOUCH THE GAS PEDAL!!! Step 5 allows the computer to relearn idle settings. It's pretty cool when you follow my procedure because you can actually hear and feel the car figuring out the proper idle settings. It will go from idling rough to idling smooth as it warms up. Once the car is warmed up verify that the car is idling at around 700rpm. If the car is not idling where you want it, adjust the screw until it hits ~700rpm and then let is sit maybe another 10 minutes DO NOT TOUCH THE GAS PEDAL!!!
DO NOT TOUCH THE GAS PEDAL!!!
Turn the car off. Reattach IAC wiring harness. Now remember we adjusted the set screw on the throttle body. That means that the voltage reading from the TPS sensor has changed. It should read between .96 and .99 volts. Anything outside of that range will cause all sorts of issues including misfires and rough idle.
7. You will have to back probe the TPS connector. With the connector attached to the TPS stick a paper clip into the rubber boot on the connector where the top and middle wires go into the connector. The rubber is very flexible the paper clip will slide in between the rubber and the wire.
8. Set your multimeter to volts. Turn ignition to ON. Then place your multimeter leads on the metal pins. If it comes up as a negative that is okay as long as you are -.96 to -.99. If you get this reading then great you are done and hopefully your issues are gone. If not proceed to step 9
9. This is where and extra pair of hands come in handy. Using a large screw driver you will need to loosen the bolts that hold the TPS. DO NOT USE A SCREWDRIVER THAT IS TOO SMALL BECAUSE YOU WILL STRIP THE BOLTS. They probably will be really tight so you have to really lean on the screwdriver and use some finesse.
10. Once the bolts are loose you will need to move the TPS up or down and continue to read the voltage. Once you get it to the desired setting you can retighten the bolts. What I do is I find .97 and then as carefully as possible I tighten the TPS down. what will happen is the voltage will change usually to .96 or .98 but that it okay. Once you are done with this manually open the throttle body a few times and close it then reread the voltage and make sure it is still within our desired range.
11. Depending upon how loose your set screw was you might want to cover it in RTV to hold your setting. At this point you have CORRECTLY reset you idle.
The newer cars (96+ I believe) do not require any adjustments to the idle. Just pull the negative terminal off the battery for a while to reset the ECU, and it will relearn just fine on its own.