Fuel Pressures

[SoopaGhetto]

Today while driving the car was acting funny(318i m10). Kinda bucking back and forth and stalls. I suspected fuel and checked the pressure between the pump and fuel rail. I got a value of 43psi. The holding pressure was around 30psi...but then drops steadily to 0. No leaks in the line from what i can tell and im pretty sure the injectors arent leaking (economy was affected). Fuel filter is about a year old or less. For reference i still have the old two pump system.

Does anyone know if 43psi from high pressure pump to fuel rail is good? The test was done with the car off. When trying to start the car now it jus dies with in a few secs. I know that the loss in standing line pressure isnt a good thing as well.

[digger]

43psi is around 3b which is what the m20b25 pressure is supposed to be , so it should be fine

[SoopaGhetto]

even when the car is not running?

This q is kinda noobish but how does a FPR work? i have an idea but now im confused bc of this situation. I thought vacuum cases the retaintion of fuel pressure up to 3 bars (43psi) and anything more goes to the fuel return line . Since the car was not running there is an absence of vacuum

[digger]

even when the car is not running?

This q is kinda noobish but how does a FPR work? i have an idea but now im confused bc of this situation. I thought vacuum cases the retaintion of fuel pressure up to 3 bars (43psi) and anything more goes to the fuel return line . Since the car was not running there is an absence of vacuum

FPR allows fuel to return to the tank when the pressure reaches xbar so the rail pressure will be xbar always provided your pump can develop the pressure and the injectors don't flow more than than the pump.

The fuel pressure at idle as measured by a pressure gauge (which is relative to atmosphere) will be around 38-40psi for a 3b (43psi FPR) but the 43.5psi is actually relative to the intake manifold which is where the vacuum hose goes to. If you turn it off the pressure gauge should goto 43psi since the atmosphere and intake manifold pressures are the same when the engine isn't running.