1994 Mustang 3.8 L high idle above 1800 rpm all the time

timing...or first check if all cylinders fire....

Replace the air filter and IAC idler air control valve.

Replace Idle Contrl Module.

1996 mustang You just go to the link I attached I'm sure you will get the solution of your problem.
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white smoke out of the exhaust would be a sign that it is burning coolant... if it goes away after it warms up, it could just be condensation inside the exhaust pipe. 1800 rpm is a very high idle, maybe have your IAC motor on the throttle body checked... check engine light? if so have the code read at your local auto zone.

check the plumbing from the maf to the engine first ..make sure there is no place it is "sucking " air from it must be solid not even a little hole or rip and make sure it's tight

Best take it to a repair shop. If your removing air intake ducting and starting the vehicle,you don't understand the technology.

yep, nothing wrong witht the ac, might need a charge, thats whats causing the high side sensor to go off, as far as the surging you might take off the oxygen sensor and clean it and replace, that usually helps, if not it needs a new one, the oxygen sensor controls the fuel mixture, and crankshaft sensors are kinda cheap so replacing that wont hurt... good luck!

these engines will rev high if there is an air leak around the throttle body ,tighten the throttle body down or replace the gasket.

Check the throttle cable and vacuum lines