thats why he said use a Mower or a motor bike for the exhaust - you have taken the tank out, so the car will not run
if you "purge" the tank with the exhaust, you are replacing all the oxygen (flammable with petrol fumes) with carbon monoxide (which is not flammable)
especially with a tank being welded
fire triangle.png
so by taking the oxygen out - you cannot have a fire