air in fuel pump

[Green3R]

Something the car has always done, so may not be a problem-

when I turn on ignition and the pump primes, it sounds like it's sucking in tiny air bubbles.
(and when it first starts).

This does go away after a few seconds of running.

It doesn't leak a drip of fuel, or smell in any way.
Doesn't matter how full/empty the tank is.

So, any thoughts? If it was air, where's it likely to be getting in?

Thanks

[andygtt]

its simply building the fuel pressure back up... if you had a gauge you will see when the car is turned off the pressure won't maintain the 3 bar needed while running.... when you prime its building this back up before starting hence the changes in tone of the pump.

Personally I let the pump do a full prime cycle before starting the car.

[Green3R]

Thanks Andy, yep it's always wise to let it get to full pressure.
Just sounds like it's sucking aerated fuel for the first 5-10 seconds of running.

Maybe it's just an artifact of the pump running full steam to build initial pressure (maybe cavitation even- any fluid dynamics experts out there...?). My suspicion is that the fuel pump wiring mod will help with this as it's probably during the prime cycle that max current will be drawn.

Thanks again

[anonymous-user]

Almost certainly cavitation.

When the pump is first powered up, there is no flow in the system, and without any kind of "soft start" for the pump, it just kicks into life at full speed, instantly cavitates (because the inlet pressure falls below the vapour point) and it takes a while for the fuel in the pipes to get up to speed, especially on the intake side, and for the inlet pressure to climb to a point where the cavitation stops.

[Green3R]

thanks Max, a eloquent description. I'd be quite happy to accept that's what it is- as I'd be sure there would be other evidence if air was getting in somewhere.