My 93 Lumina switches air from the dash vents to the floor when I accelerate, regardless of what setting is on. Could this be caused by a vaccuum leak? I checked the HVAC module under the glove box and it seems alright. Any suggestions would be appreciated.

I would bet it's a vacuum leak, or the vacuum canister under the hood is bad. Engines create less vacuum when you accelerate, so the vacuum canister is there to hold a reserve for low vacuum situations. Check the plastic lines running off the throttle body (theres a rubber piece with 3 or 4 lines coming off of it) for cracks or breaks.

Both my '97 and my wife's '98 do this. I don't hear the telltale *hiss* of a vacuum leak in either car. This canister, is it a round plastic ball? I've seen lots of them in 80's GM cars. Where is the canister located in a 3.1L Lumina?

Driver's side bumper, forward of the fenderwell. It probably broke either there, or the T connection with the check valve. You can trace the whole line starting at the top left of the plenum, facing the engine from the outside front.