DO NOT HAVE THE GARAGE DOOR CLOSED WHILE IDLING A VEHICLE.
And, yes I'm yelling, ( with my index finger pounding on his forehead ). It's that important.
If idling your car in the garage is going to be a daily event, get the garage exhaust hose that is made for just such occasions and build it into the wall of the garage so that all fumes are expelled from the room. Auto repair shops use these and could guide you to a source.
( I googled 'automotive garage exhuast hose' and see lots of good souces.)