Hello..i have a 99 4.6 and it is not my daily driver..i use it about once or twice a month. When i started it last night to run to home depot i could not get a constant idle...the tach would surge to about 1200 RPM and go as low as about 500. After the truck warmed up a bit it was fine and performed normally thru the 4 or 5 start cycles i did last night. After searching the site i came to the conculsion that it was most likely the IAC..no codes have been set. Just wanted to get some input from the more experienced guys and see if i am on the right path..thanks in advance.

 

Yea sounds like the iac is dirty or needs replacing. First take it off and clean it up with carb cleaner and see if that helps.