I can't believe how many of you guys buy into this BS. In no way is that a safe practice and I don't care how many times you or your cousins brother have done it and got away with it. It will not render a tank safe for cutting or welding. Consider this as just part of it. When a vehicle is idling the efficiency is pretty low. A lot of the fuel being burned goes out the pipe. There are safe ways of doing it, the absolutely best way is to steam the tank with low pressure high volume steam for at least 4 hours and then work on it while the tank is warm.