I thought those buses had diesel engines. Diesel doesn’t flash like gasoline.
Even then, I can't really see how a spring would make it explode. Natural gas and propane tanks are rather thick. Even shooting at them repeatedly and puncturing the tank will cause a leak, not an explosion. To make an explosion, you'd have to completely destroy the tank and spark it at the same time. Propane tanks are often added to carbombs for this reason.