Sounds like it's idle control valve or you have a vacuum leak. Buy a carb cleaner and spray it on the intake while the car is running, if the idle goes down when you spray it then you found the vacuum leak, if that doesn't work, take the idle control valve ( goes on the throatle body) and spray bunch of carb cleaner in it and let it seat for a half our, than put it back. It can also be tps sensor ( throttle position sensor), you can check it with the volt meter, or scan the car for any codes, tps sensor will always give you a code
90 35th anniversary sold
88 turbo coupe 14.1 at 96mph ( almost stock)
93 mustang lx sold
73 pinto station wagon ( mustang replacement), 2.3t will go in there in the future.
86 t-bird just got it