1995 buick regal: When I turn the heat OFF, hot air will push through the heater vent (by my right foot) when I'm driving on the highway, then it stops when I come to a stop. Clearly outside air is pushing in, gets heated by the heater core, then pushes into the cabin through the heater vent. When I turn the AC on, then the air stops coming through that heater vent, and cold air starts blowing through the front vents. When I turn the heat OFF, and slide the temp control to cold, then cold air starts pushing in the heat vent, when I slide the temp control to warm, then hot air pushes into the vent.
Turn on the AC: no problem. Click the OFF button: heat flows into the car. My right foot feels like it's on fire, I can't keep my foot on the gas for longer than 5 minutes, it would be nice to drive around with the windows down.
A mechanic on justask_com says this is by design, but I don't buy it, it was not doing this a year ago.
Any ideas which parts should I replace?