In my experience, the outside air does not come through the vents with the fan off and the sliding control for outside air closed. I think this is true because the few times I have run the fan, the dust plumes are huge. This tells me that unless the fan is on or the outside vent is open, no air is coming through. The only time I run the fan is in the winter. I only run the a/c when I think I'm going to die because it just kills the little bit of pickup this yellow beauty has when starting out from a light or stop sign.

Kevin
1979 240D manual
1984 190D manual