Why do you want the garage door open? The amount of heat getting in from the outdoors will probably be more than anything the car will contribute. And if the garage ceiling is insulated, the car shouldn't make a significant difference.

Theoretically, yes, if the garage interior is warmer with its door open, then more heat will be transferred to the room above. The question is how much more heat? That all depends on factors you haven't given us.

After parking car in garage leave door open and use a fan to help blow car heat out. After engine has cooled close the door. OR leave car out of garage until the engine has cooled and then park in garage. This is what I do during the summer since the master bedroom is above the garage.

Krissie55: Have you ever measured the temp (with an accurate thermometer) and compared parking the car 'hot' vs. parking it cold (or leaving the door open) to see how much the hot engine actually heats up the garage? I'd be curious to know how much of a difference it actually makes.