GTStang90

1990 GT Starts and runs fine idles great until I turn on the heater controls no matter if it is defrost, floor or mix when I stop and let it idle it jumps between 500 and 900 rpms turn off the selector and no problems.

Any ideas what would cause this to happen only when these three things are selected. I didn't notice it doing this with the A/C on just since I started to need the heat.
Thanks
Duke

mjr46

check for a vaccum leak within the hvac system/ the vaccum lines that control such

GTStang90

May sound stupid but I didn't know there was vacuum in the hvac system I thought that it was all operated by rods. I will check into it and see what I can find.
Thanks
Duke