Delayed Engine Shut off but not when Heaters on.

My engine delays to shut off like a vacuum leak but when I have the heater on it shuts off right away. I just bought a MityVac couple minutes ago but I just wanted to know where I should start in my situation.

[lrg]

Start at the beginning of course. Sorry couldn't resist. I'd start by double checking that the shutoff is good so start at the line under the hood that goes to the fuel shutoff, and while you're at it make sure all the connections around the oil filter are good. If they are OK since it sounds like a vent servo I'd then check the lines behind the HVAC panel above the radio. Be careful pulling this apart and only test one or two at a time because you can inadvertantly disconnect a bunch of the lines and then you'll have a tough time getting them all back together in the right places. You should test with the car off and listen for each vent to open and close. Good luck.

[kerry]

It is probably a bad vacuum servo that is activated when the heater is off and not activated when the heater is on. The one that I suspect meets this criteria is the one that lets fresh air into the system. I'm pretty sure this is the one behind the glovebox to the right. I would pull out the glovebox and check this one first. It could be as simple as the line being disconnected to that servo.
I dont know if any other vacuum servos meet this criteria.

thanks guys, I'll start checking the lines.