When my 2001 f-150 with a 5.4 is idling in park it is constantly revvin up and down between about 800-1000 rpms, i've noticed it only does it if i have the heater or air conditioning on, if i shut them off it gos back to normal and stays there, and ideas? thanx

 

Sounds pretty normal, PCM adjusting the idle according to the loads on the engine. It may however be the beginnings of the IAC valve failing. The IAC valve regulates air into the engine, around the throttle plate, when it is at idle. You might want to take the IAC off and clean it with throttle body cleaner. Good luck.

 

that's normal if you mean you have the defroster on when you say "heater"

it's the A/C compressor kicking off and on.