Turning the garage into a gas chamber is a really, really bad idea
There are two really good reasons to ventilate your garage. They boil down to: leave the garage door open because it could save your neck.
The first reason relates to any work you’re doing with the engine running. Car exhausts emit carbon monoxide – a combustion byproduct that also happens to be a deadly poison in very small amounts. Haemoglobin in your blood likes latching onto carbon monoxide about 200 times more than it likes carrying oxygen