The 1989 5.0 Mustang relies on a combination of electronic and mechanical devices to control idle speed. A number of mechanical and electronic issues can cause the Mustang's idle to be higher or lower than desired. In such cases, the idle can be readjusted to its original rpm speed or fine-tuned according to the owner's preference. The procedure for setting the idle on the 5.0 Mustang is relatively straightforward.
Open the hood and disconnect the battery. Remove the negative terminal cable from the battery first, and then remove the positive terminal cable. Leave both cables off of the battery for at least 30 minutes.
Locate the Idle Air Control (IAC) module on the throttle body and disconnect it. Reconnect the terminal cables to the battery and start the engine.
Locate the stop screw at the bottom of the throttle body and use a screwdriver to adjust the idle rpm. Turn off the engine and reconnect the IAC module.
Start the engine again, with all accessories including the radio, A/C and lights off. Allow the engine to run for two minutes. Turn the engine off and wait for two minutes.
Start the engine once more, this time with all accessories on. Allow the vehicle to idle for two minutes and turn the engine off.
