***You’ll get a small flame at the opening, that’s all. ***
If the tank is full it might, but what if the tank is low and there is air in the tank.
A man was killed near here several years ago when he tried to braze a leak on a fuel tank. Kaboom.
Two of our workers once wanted to braze a leak on a tank. They said they would start the engine, use a hose to fill the tank with exhaust before welding it. The Supervisor on duty forbade them to do it.
Many companies forbid the cutting of steel barrels unless the barrel has been steam cleaned first. Too much chance of explosion.
Brazing a tank without fully purging it will definitely do that. But then again, you are putting heat directly to the tank and the drained tank is a pure vapor situation which is likely (or at least possibly) between the UEL and LEL (upper and lower explosive limits). A flame past a fill nozzle on a car is extremely unlikely to be between UEL and LEL.
The CO idea was not a bad one. They do that on tanker ships.
I worked for a company in my younger days that rebuilt gas stations. More than once, we had dry ice-purged tanks pop the ends off when someone hit it with a torch. Scary stuff!