Very simple fuel pump question

[RobM77]

Very simple question

Why does the fuel pump in a car make a whirring noise when you turn on the ignition? What makes the whirring noise, and is it best to engage the starter motor during or after this whir?

thanks

[avocet]

It runs (on many cars but not all) just to "prime" the rest of the fuel system and make sure there's fuel pressure at the injectors for when the key is turned. I doubt it makes any real difference whether you wait or not because the pump will also run when the engine is cranking anyway. I'd be inclined to let it build up pressure before turning the key but that's just a personal perefence, I really very much doubt it matters one way or the other!

[RobM77]

Thanks. That answers both my questions

[wedg1e]

There is a third aspect: on some cars the pump runs until the fuel rail reaches a pre-determined pressure, then stops. If the car's out of fuel the pump keeps running, trying to fill the rail up. So if your car won't start and the pump runs when normally it wouldn't, maybe someone's siphoned your tank...