Exhaust fumes can contain carbon monoxide. CO is flammable and while the LEL (lower explosive limit) is higher than the UEL for petrol it's flammable over a wider range of mixtures. A good mate of mine used to swear by the exhaust fume 'method' until a tank popped while he was welding it. Fortunatly all he suffered was some soiled underwear. The tank wasn't so lucky... it's capacity grew some (bonus) but it changed shape enough that it no longer fitted the car!It looks like I'll be attempting this in the summer. The plan is to purge the tank with exhaust fume before and during welding.
Unfortunately new tanks aren't available and good used ones are very hard to find
IIRC petrol tanks usually operate above the UEL when in use (full, part full) but when empty/essentially empty the vapour concentration typically ends up within the explosive range. Diesel is very different, might be very wrong but i've got a vague recollection that it's vapour pressure means tanks typically operate below the LEL
To purge with argon etc you really want an LEL monitor to be absolutly sure. Briming with water and brazing is another way