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 into your body (which is what haemoglobin does).
Carbon monoxide is odourless and colourless, and there are minimal ‘early warning’ symptoms that you are succumbing to carbon monoxide poisoning – you feel okay, possibly a bit headachy, then you fall down into a coma and die. Unfortunately, carbon-monoxide also causes mental disorientation, making self-diagnosis during early onset unlikely.
(Factoid: people who die of carbon monoxide poisoning leave behind a healthy-looking, pink-coloured corpse, as opposed to those who die from carbon dioxide poisoning, who depart this life a bluey-grey colour).
Undiluted car exhausts emit about 10 times the average lethal concentration of carbon monoxide, as well as carbon dioxide (another deadly poison). Furthermore, the car exhaust contains no appreciable oxygen, which also makes it a suffocation hazard. All very good reasons for leaving the garage door open if you’re working on the car.
The second reason to ventilation relates to any time you’re working with petrol (say, changing a fuel filter). Petrol evaporates in air quickly (this property is called ‘volatility’), forming an explosive mixture. All you need then, to make a Molotov of yourself, is a suitable spark.