Ford Mustang Forum - View Single Post - 2003 Mustang V6 Very High Idle Help!
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post #9 of (permalink) Old 12-24-2010 Thread Starter
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Originally Posted by wmburns View Post
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.

Sorry for the late reply! I had some other issues with the car to deal with, not to mention break time being limited by the holiday season and lots of garage cleaning... T.T

Now that I finally have a break, I will check this tomorrow or the following day... I disconnected my battery yesterday and the car runs fine now, but like always I know it will start to high idle again sooner or later... Thanks for the info, I will first try unplugging the IAC connector to see if the RPMs change or motor dies.

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