gas tank sodering

well I stuck a hose in my Honda car exhaust to the gas tank run it for 5 min . I used silver braze . and no there was no boom . I have done many this way and feel very safe doing so.

That is exactly like the old school pros do it. Super safe.

Well, I'm glad you guys feel safe doing it that way, and that you haven't managed to blow yourselves up. But just because you did it and didn't kill yourself doesn't mean you can't blow yourself up doing it that way.

Automobile exhaust is combustible, plain and simple. This can be especially true for older vehicles if not properly tuned. Carbon monoxide, also in exhaust fumes, has an ignition temperature of about 1,200 degrees F and has an explosive range from 12.5-74.2%. Yes, the ignition temperature is significantly higher than the gasoline vapor you're displacing, but it still has an ignition temperature, and it's still less than what you can create welding. This also assumes you're actually displacing all of the gasoline vapors in the tank, which- A. you aren't doing 100%, and B. you have no way of testing to see if you are.

Can I tell you what the odds are that you'll reach the correct concentration of exhaust gasses and enter the flammability range? No, I can't. It's probably pretty unlikely that you'll enter the flammability range while you're above the ignition temperature too. But if you did, you definitely wouldn't be the first person to have blown themselves up welding on a gas tank with a tailpipe hooked up to it. And I'm sure you wouldn't be the last, either.