I have a '99 F150 with the 4.6 that the idle goes up to almost 2000 rpm for no reason and will not come down until driving. It doesn't do it just on start up but even after driving for several miles. I replaced the IAC valve and it seemed to be ok for about 3 weeks and then started again yesterday. Please help me if you can. Thanks.

 

Have you checked for vacuum leaks? Usually they will cause a rough idle too but sometimes they just cause a high idle. A common place for a leak is the PCV hose at the back of the throttle body.
A faulty TPS can also cause a high idle.
You might want to get it scanned and see if anything shows up.

 

my 99 f150 did the same thing. I thought it was the idle sensor too, but after a week of scratching my head, I found a STUCK PCV valve. I changed that and all was fine. I ALMOST bought several new sensors, I am sure glad I took one more look at it.

 

Thanks for the tip Jeff
PCV valves often get overlooked.