It's gonna idle high when it's cold. That's normal. What's not normal is 2200-2800. Putting the car in neutral and coasting shouldn't change anything. As long as the drive shaft is turning and the sensor on the tranny senses it, it will keep the idle high until the driveshaft stops turning. This is what keeps your car from stalling when you slam on the brakes.
What thing are you disconnecting? The sensor on your tranny or the plug going to the IAC? If it's the latter and the car runs bettter without it, that probably means you have your idle adjustment screw turned in too much.
You should unscrew it until you can fit a .010" feeler gauge between the adjustment screw and the stop plate.
Turn the screw until the gauge gives you a little (very little) resistance to pull it out.
Turn the screw another 1 1/4 turns.
Reset computer procedure
Disconnect the negative battery cable for 10 minutes and then reconnect it.
Turn off all of your accessories and start the engine. Let it idle for 10 minutes and then shut it off.
Turn on all of your accessories and start the engine. Let it idle for another 10 minutes.
Go for a drive and see how it runs.
If this doesn't work, get yourself a multimeter that has a digital tach. Hook it up and start the engine.
Disconnect the IAC.
Turn the adjustment screen in or out until you are idling at about 700 rpms.
Plug the IAC back in and then do the reset computer procedure.
Get back to us with the results.
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351W 89 Mustang GT Convertible