HVAC Issue

black2002gt4.6:

Welcome to StangNet!

Yes, the ventilation doors have vacuum-powered motors. At light or steady throttle, you have plenty of vacuum so the doors stay where they are. When you step on the throttle, the vacuum drops and the doors start swinging like an old saloon when the drovers come to town.

Basically, you have a vacuum leak. You need to follow the tubing (it is tiny) from the engine bay (near the firewall) and back behind the radio (you may need to remove the radio) to find the vacuum Xmas tree. Maybe a hose has fallen off or the supply line has a crack in it.

I don't have any pics (because I don't have access to my shop manual). Your local library may have a subscription to AllData and there you can get a nice schematic of the vacuum tree.

HTH,

Chris

def vacuum leak +1

+2 on the vacuum leak. There is a check valve in the AC control vacuum line between the vacuum res and the intake manifold. The purpose of the check valve is to maintain vacuum inside the vacuum res. I suspect either the check valve is bad or the vacuum leak is after the check valve.

Note, the vacuum res is in the passenger compartment right hand side. It is well up under the dash and can be difficult to access without removing the dash.