My f150, especially when it's in gear, idles extremely rough. And it only does it when I'm sitting. As soon as I hit the gas it's fine. I took it to the ford dealer I got it from and got the typical thing about not finding the problem. I'm not at all happy with their service--but that's another post. The closest thing I got to an answer was a coworker said his truck did that and they replaced something. I just changed the air filter, oil and fuel filter. The only thing that kind of makes it a little better is if I use high octane Shell gas. Any idea's?

 

Idle Air Control valve is the first thing that comes to mind and an easy fix.

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99 F150 5.4 4x4 ORP 3.73 styleside deep wedgewood blue ..no mods (yet)

 

The EGR valve might cause something simular to what you are describing if it's not working properly...

Corey