For a high idle symptom, the most likely cause is excessive air after the throttle body. This could be from a vacuum leak or from the idle set screw being set too high (already mentioned).
Remember that the IAC is a bypass device. As such, the IAC requires that the TB butterfly block almost ALL air when the throttle is CLOSED. Then and only then can the IAC control the idle by bypassing air around the TB butterfly.
The IAC can only ADD bypass air. Never subtract.
Sooooo. if the TB idle set screw is allowing too much air to pass, the IAC can NOT slow the idle because there is no adjustment left.
As mentioned already, disconnecting the IAC electrical connector must
result in a decrease in RPM's. The idle should be very slow or the motor die outright with the IAC disconnected.
Also, if there is a vacuum leak allowing unmetered air to enter the intake, the driveability will never be correct.
Please review the following post for more information on how the IAC works. https://www.allfordmustangs.com/forum...ml#post2260649