Okay, I did a little more looking and here is what I found:
I took out the glove box and pulled out the cabin filter. I turned the key to enable the fan, but did not start the engine. Then I pressed the button to turn on the fan with Recirculate off. Through the cabin filter slot I could see the air inlet vent door (I had a flash light and mirror and also an fiber optic inspection camera). I now also know where the air inlet actuator is (on the left side of the housing).
Then I turned activated the recirculate button and saw the vent cover rotate up to cover the hole in the top of the housing to the outside air and expose the holes in housing to the cabin (it rotates a "hood" inside the housing for a vent cover). Any time I turned the fan off, the recirculate went off and the vent cover returned to the down position to block the cabin inlet and let in outside air from the air inlet pipe at the top of the housing (this makes sense).
With the fan on and the recirculate button pressed and lit, I did not see any openings to the outside air pathway. This seems to nix the rumor I had read about GM making the vents only close 90%. I was even able to reach around the side of the housing and put my fingers in the cabin inlet holes and feel the vent cover door completely covering the outside air inlet.
I still do not know why we smell other vehicle exhaust when in recirculate mode (we do not have a diesel so I know it is not us). Is it possible that running the fan too high with recirculate causes a low pressure area that draws in some outside air? The previous owner had passed power cables through the firewall for a stereo (on the driver's side). I have removed them and silicone chalked the holes.
The cabin air inlet holes are on the back (and back sides) of the housing where the cabin filter is. When the glove box is in, there is very little space to have the air drawn in from the cabin (there is even a plastic kick plate on the bottom which obstructs the airflow to the cabin air inlets). So it seems like there is a lot of negative air pressure near the firewall when recirculate is active. I think we will try leaving the bottom kick plate off, run the fan at low speed when we have recirculate active, and make sure we do not pack too many things on the passenger-side floor when traveling that could obstruct the air flow.
Am I on the right track?
Saturday, June 17th, 2017 AT 4:49 PM