She's acting up - idle problem

[Black97Cobra]

Like every other girl i know my car is in a mood:cuss: Shes a 97 Cobra with 50,000 on her, no mods but a K&N. If the heat is on Mix or Defrost the car idle surges 200 rpm, any gear, any speed. So while you a driving you can feel the car surging in a rocking motion. If you put the heat on floor it stops. I think its a vacuum leak but i don't have a clue were to begin to hunt it down. But if you have any suggestions as to what it could be or where to start i'm all ears.

 

[GR8WHITE]

It is probably the compressor cutting off and on. The AC compressor works on the defroster also to keep it from "freezing" up in the winter.