my 99 4.2 has been revving higher and higher at idle lately, and when I put my truck in neutral, and am costing(any speed) it revs at like 2250 rpms, (yes my foot is not on the gas, and it is not in gear) and then as soon as i fully stop (just using the brake) it will go down to about 1500 rpm, and idle there? What the hell? The linkage is all fine, it is not that, but other then that i have no idea? this has happened for a while now, but It is steadily getting higher and higher rpms. Has anybody had this problem? I Thought is was just a dirty mass-air-flow sensor, but even after i cleaned it, it still does it. Thanks for any help.

 

Idle Air Control (IAC) valve as always.

Rule of thumb: Idle speed problems - go to the IAC first.

Steve