Fuel Pressure Regulator

[paultv]

Can someone clear this up - my cabby loses fuel pressure after being left for a few hours. Is this correct? The loss is through the FPR, the output feed back to the tank to be exact.

The marvelous Bentley states that this is correct - the FPR leaves a small residual pressure for a while:

"To avoid fuel vaporization and hard starting (vapor lock)
when the engine is hot, the system is designed to retain slight
fuel pressure for after the engine has been turned off".

This is done by the check valve in the FPR and in the fuel pump.

Is all this correct? or hogwash?

Paul

[Brianmoooore]

Sounds like a very unclear explanation to me!
If you are sure that the pressure loss is fairly rapid and via the tank return hose from the FPR (checked by clamping the hose?), then the FPR is dud. The system pressure will start to drop off very slowly as soon as the engine is stopped, but substantial pressure should be retained for days, and even some after weeks.
Pressure loss can be through a leaking pump non return valve, a FPR leaking through to the tank return hose, a FPR leaking through to the small pressure reference hose, one or more leaking injectors, a loose hose clamp somewhere on the fuel feed pipe, a perished rubber hose in the fuel feed, or a rusted fuel feed rigid pipe.

[paultv]

Blocking the return pipe from the FPR - the system retains pressure - so I guess that means the fuel rail and injectors as well as the fuel pump check valve must be good.

If I leave the system for half an hour with everything "normal" I lose about a bar - and if left overnight, there is zero residual pressure, naff all.

The car starts OK, but I feel it needs a longer cranking time than it should. Running pressure is fine as well, and the FPR appears to be doing its job, but looks like the valve inside it doesn't fully shut - assuming it is supposed to.

If I start with zero system pressure - then crank the engine, the car fires at around 25 psi, which is arrived at quickly as it rises to about 36 psi when running, which should be about right. Pulling the vacuum line from the FPR whilst running, pressure rises to 45psi or so, returning to normal 36 psi when reconnected.

Paul

[Brianmoooore]

From the above; all exactly as it should be, except for a slightly leaky FPR. Should be enough pressure for the engine to start right up after standing for 24 hours.

[paultv]

I found data in Old Haynes manual, residual pressure should not fall below 2.1 bar.

Paul

[arrisbmw]

Blocking the return pipe from the FPR - the system retains pressure - so I guess that means the fuel rail and injectors as well as the fuel pump check valve must be good.



Paul

As your clamping the return rubber pipe from the fuel pressure regulator. does that not mean the fpr is o.k too and not leaking.
I,m a bit mixed up here as Brian said your fpr is not good ??
thanks

[arrisbmw]

so i,m thinking fuel pressure loss must be from the clamped pipe back to the tank, so possibilities are rubber pipping , hard lines or any joints. is that right ?
oh i think i might have answered my own question here. i guess the fpr can leak in more than one place. diaphragm and the shut of valve, so if its leaking from the shut off valve when you clamp the pipe after the fpr that stops the leak,, oh yes. that makes sence now , sorry B

[Brianmoooore]

Pipe from the FPR back to the tank is open at the tank end, and contains zero pressure at all times (apart from pipe losses, for the pedantic.)
Wouldn't affect the running and performance of the engine if the pipe wasn't there, but fuel consumption might be a bit excessive.

[arrisbmw]

ha ha . how many gallons to the mile would i get.
that all makes sense now , thanks