Regarding the phrase "running on fumes", if a tank is empty, will the engine get any power at all from fumes left from petrol (I don't mean drive it any distance), or is it just an expression? After all petrol fumes are flammable so should ignite right?
It's just an expression, car engines require liquid fuels. Once you get down to vapors the pump cannot pressurize the fuel system, even if you had pure gasoline/petrol fumes there simply isn't enough density. In any case you'd never have pure fuel vapor anyway as the space in the tank will be mostly air.