Why does the heater in my 1999 dodge 1500 go from floor to dash when I accelerate? It sounds like the fresh air door opens and then shuts when I let off the gas? I also noticed that my transmission doesn't shift up as fast since this started? I don't know where to start looking for a vacuum leak?
After you check for vacuum leaks, locate the vacuum check valve along the firewall under the hood. It's black, and about the size of a stack of three nickels. Replace it with one that has a storage canister built in. If you check enough vehicles in the salvage yards, you'll find a few in various vehicle models. This one is about 2" long and 1 1/4" in diameter.
The system is spring-loaded to go to the defrost position if there's a vacuum failure. Your feet might freeze but the windshield will be clear. It's a safety thing. Vacuum goes low during accelerating. That's why the system switches.