Fuel pressure

[Ait]

Hi guys,

Had a fuel pressure gauge connected to the car, I noticed that fuel pressure drops overnight. Is that normal? Isn't the pressure supposed to be constant?

[The Tool]

I don't know for sure, but from my (limited!) understanding of high school chemistry, if the temperature drops (i.e. overnight) the fuel (in vapour form - it's not always liquid) slows down thereby reducing the pressure.

Same sort of thing as tyres when they heat up - pressure rises.

Somebody correct me if I'm wrong but it seems logical to me...

[The Tool]

...btw when i said 'fuel slows down', i meant the fuel molecules/particles or whatever u call 'em slow down...

[Ait]

Hmm that sounds logical actually...

[blind_elk]

If the fuel pump runs all night. then the fuel pressure shouldn't drop. But it doesn't, so the pressure drops.

Really got nothing to do with high school chemistry. You lose pressure because the fuel leaks back past the non-return valve in the pump. I have found it normally takes less than an hour for the pressure to drop to nothing.

That's why the ECU runs the pump for about 5 secs before you start - to get fuel pressure back into the lines.

The fuel is always in liquid form. The car won't run on fuel vapour. (Except LPG, before any smarty pants attack me). If you were old enough to have owned a car with carbies, then you'd know that to be the case.