Idle Trouble, Advice Please

Reefmonster

New Member
Nov 4, 2010
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Car is a 99 GT, bone stock except flowmasters. My trouble is when I have the IAC plugged in it idles at about 2800-3000rpm, and the Service engine light is not on. But when I unplug the IAC it idles about 700rpm, and will cut off everyonce in awhile and the service engine light comes on with it unplugged I have replaced the IAC, MAF, and the TPS. Nothing seems to fix the problem. Does it need to go to the shop or do you guys have any other advice?
 
700 is too high with the IAC disconnected. Either the idle stop set screw is set too high or there's a vacuum leak (or both). With the IAC disconnected, the idle should be so low as the motor is on the verge of quitting (or it should die outright).

When was the last time the PCV valve was changed? Check valve covers, oil dip stick, PCV lines, fittings. In sort look for any source of un-metered air.

Check out the following for more informaton:
Is your mustang stumbling, dieing at idle or idling rough? - Page 14 - Ford Mustang Forums