I have a 2001 f150 with the 5.4 It will idle fine with the a/c or heater off, once you turn it to heat or a/c the engine will surge from 500rpm to 1500 rpm. I have read some of the post here and looked for a vacuum leak and nothing. It is not throwing any codes. Any ideas would be vary helpful. It does not do this all the time. It started last month and we checked for leaks, cleaned the throttle body and maf sensor, it continued to do this for about a week. Then evrything was fine for two weeks and guess what its doing it again. It is driving me crazy, the local mechanics just scratch their heads and can't find any problem?????????

 

IAC maybe??

 

Repaired short in headlight connection! Engine running fine again!?

 

electrical gremlins are always the weirdest ones to solve. Luckily you seem to have nipped it in the butt before it consumed you!

 

sorry what is a IAC

 

'idle air control' valve