During a drive a few days ago, had the ac on and noticed that air was coming in only from the vertical side vents, and nothing from the center vent. Turned fan speed to high, didn't matter (by the way, no diff in fan speed between low and medium).

Haven't driven car that much since buying it, so don't know if this problem was always there or not.

Any ideas as to what's causing this?

 

I blocked my center vent open. It never opened properly since I got the car, some problem with the vacuum I imagine.

You should be able to feel air leaking out from around it though. All that air has to be going somewhere.
(';')

 

LnRb, how did you block the center vent open??
RagJag

 

On my 86 Coupe I had a vacuum leak in engine bay right behind engine on firewall. It seem the floor, vents and defrost run on a servo type motor. The center vent uses vacuum.