what would cause the air to come out of the defrost vent no matter what the switch is on? would a lost vacuum line cause this? i still have the hot and cold valve working it just stays on the window vents.

 

I had the same problem in my Dodge Charger that I use to own.
It turned out that the cable that controls the doors for the venting to different spots came off.

 

Yes, a vacuum leak would cause that. That's actually a very good place to start looking.

By default, the gates in the HVAC stuff is set to blow out the defrost. It's only when a vacuum is applied that it switches to other gates.

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else