I bought my second f150 last June and have had a recurring problem since shortly after I bought it. It's a 2002 with the 5.4 and 4x4. When the truck is warmed up it will occasionally rev up and down repeatedly for five seconds or so when idling in neutral. The revs go from 450 - 800 back and forth. It will idle normally for a minute or so then do it again. It almost always occurs right after taking the truck out of gear and putting it in park. This has happened on and off for seven months now without throwing any codes. Also, when I am coming to a stop in drive, the engine slows way down and revs up to idle speed again. This has happened with regular gas, premium gas, ethanol added fuel, ethanol free fuel, etc. etc. etc. It has also happened before I put the performance parts on it, so I have ruled that out. It runs really well otherwise and I have no lack of power throughout the entire rpm range, so I can't imagine that it is something serious, but it is really starting to get on my nerves. Any ideas?

 

Clean your IAC valve, replace if necessary.