My gf's mustang when idling the RPMs go down and up, worse when the a/c is on. Anybody have a clue as too what that could be? Somebody told her the thermostat but I dont know. thanks

 

nah thermostat wont cause that. Give us the type of car and maybe we can help more, but to give you some of the possible causes in order of most likely to least likely:</P>


Dirty or bad IAT / IAC / ISCM (depends on what type of car)</P>


Bad or off TPS</P>


Bad or dirty EGR valve</P>


Vacuum leak</P>