I'm at my wits end with this. When fully warmed up, my 1999 F150's (4.2 liter) idle will shoot up 2,200 RPMs. It will eventually go back down to 800 or so after several minutes. This makes exiting the interstate an interesting proposition. So far I have replaced the TPS, IAC valve, and rubber elbow under the throttle body. Any thoughts on what else to check or change? Would a failing DPFE sensor cause this? Thanks!

 

Sticky throttle linkage? Dirty throttle body? Dirty MAF sensor?

 

I would disconnect the throttle linkage from the throttle body. This will isolate the cables, linkage etc from the engine and eliminate a series of potential issues. From there I would check and clean everything around the throttle body and the sensors listed above.

 

I think I was able to solve the problem. There are three short vacuum lines (6 inches or so) at the rear of the truck that connect to the fuel vapor recovery system. Two of the lines are 1/2" and one is 7/16". They were very spongy and deteriorated. They were a pain to replace. I've been through several warm up cycles and so far, the fast idle problem has not returned.

If you are experiencing this same intermittent fast idle problem that I have and you're at wits end, try replacing those lines.