my 96gt seems to run differently with the vent/heat/ac is on. the idles jumpy,will jump up a couple hundred rpms then come back to normal.

what can be the problem?

Mine does the same...Its a tad jumpy. I think it is just the extra drag.

Vacuum leak.Chances are its jumpy on defrost and mix, but fine on floor. Meaning the leak is in the defrost circuit.

hmm - mine does this too. Do you really think there's a leak? I just thought that since the a/c used the engine to work, it was the extra drag from that. It does sound like I'm revving my engine at stop lights though.

had this happened to me, only got idle surges with the a/c on and sometimes some hesitation when accelerating. What i did is disconnected the battery for 20 minutes, reconnect it and followed the procedure to set the idle correctly in my owner's manual and now I don't have this issue anymore. I suggest trying this first.

He mentioned he has this problem with the heat on also thats why i suggested a vacuum leak.

thats what mine sounds like too.

yeah defrost/vent/mix. if theres a vacuum leak where could it be coming from?

The heater controls are controlled by vacuum lines.When you adjust your setting vacuum is what opens and closes the doors.So its probably behind the dash,pull out your control panel and see if any lines have obvious holes or cracks.

isnt there vaccum lines that run to the engine? i dont remember it doing this before the pi swap and new tb/plenum.

does a vaccum leak effect anything besides the hvac? cause the a/c not to blow cold?