Symptom:Air all the time OR when the turbo goes into boost the air flow from the vent (selector still off) dramatically increases.
Test: No thump when switching from fresh to recerculation
Theory: All the vacuum components can fail. These failures can be either "air" or electrical or mechanical. The "air" failures can be non operation or leak.
Repair: Replace vacuum lines, preferably with silicone (NAPA) hoses. Use a ohm meter and vacuum pump with gauge to test each component.
Sooooo....after completely removing my dash (unnecessary as it turned out, Grrrrr!) I then removed the upper firewalland heater box. What I found is that the metal rod which runs from the actuator to the actual flapper door had rusted/broken off at the far end.
I actually ended up replacing the rod with a cable, which is working great