FPR or fuel pump?

[mikelat]

90 325i hard starting about 60% of the time hot or cold. It needs to be cranked for about 5 seconds however when it starts it idles and runs flawlessly.
Removed the fuel hose from the FPR and there is no residual fuel pressure. Fuel pump and relay make the appropriate noises on start and there don't appear to be any air leaks.. Injectors rebuilt in the last 6 months and don't appear to be leaking so I am guessing it is either the FPR itself or the check valve in the fuel pump.
Of the two what is more likely to go wrong or do you have another suggestion?

[Brianmoooore]

There shouldn't be any residual pressure in the hose connected to the FPR - that's the return to the tank. Test on the other hose.

[mikelat]

I actually removed both hoses, the in to the injector rail and the FPR return one. Sorry for the bad explaination.

[Brianmoooore]

If there's no residual pressure in the feed hose, then the possibilities are: Leaking injector(s) into the manifold, simple leak somewhere along the fuel supply pipe, failed non return valve in the pump, failed FPR allowing fuel into the vacuum line, and failed FPR leaking fuel to the return pipe.
The first FPR failure can be checked by fitting a piece of clear tubing in the vaccum line, so that you can see any liquid in it, and the second FPR fault is checked by clamping off the return hose.
Failed non return valve and simple leaking pipe are checked by clamping off the feed pipe.
All this is best done with a fuel pressure gauge connected via a tee piece near the fuel rail. You can buy a gauge good enough for this off of ebay for around £10.

[mikelat]

Connected a gauge to the fuel rail and the pressure registered 38 when it is supposed to be 43 so I clamped the return hose and it jumped to 60ish. There is no fuel running through the vacuum line. Leaving the system pressurized it bled down to 0 in a few hours so I am assuming that the FPR return line is leaking. Reasonable?

[Brianmoooore]

Connected a gauge to the fuel rail and the pressure registered 38 when it is supposed to be 43

The pressure registered on the gauge is relative to the pressure in the inlet plenum, not atmospheric pressure, which, at idle, when presumably you were taking the reading, will be a few PSI lower than outside of the manifold. If you blip the throttle wide open, you will see the gauge jump close to your expected figure.

[mikelat]

OK just to finish this off I did the tests suggested by Brianmoooore/WillNZ, of clamping the feed line, return and vacuum lines after bringing the system up to pressure and it still leaked down in an hour or so each time. What pisses me off is that the injectors were just rebuild within the last year but I can only conclude they are the cause, or one is at least.
I have another set of injectors so I will take them in and have them rebuilt and hope that solves it.

[Shaheenk]

i have the same issue on an 88 coupe, driving me insane, pressure drops with minutes of shutting down. No fuel in vacume hose and if return valve is clamped pressure rises to 5 bar.

where is the non return valve ?

[Brianmoooore]

Non return valve is part of the pump, but BMW can (could once?) supply an external valve to fit near the pump. Clamp the feed line and see if the rail pressure drops off to check.

[Shaheenk]

So feed line before it enters the gauge ?

I will try that and see what happens. At present I have the T-Piece in a formation where the feed pipe goes into one end , the other heads off to the rail. The Gauge is on the end of the T.

[Brianmoooore]

Clamp before the tee. If the pressure then doesn't drop off, then there's either a leak in the feed pipe or the non return valve is faulty.

[Shaheenk]

pity we cannot just change the non return.

[mikelat]

Just to close this off as it is resolved, here is in summary what transpired. Hard start 60% of the time and no residual pressure in the fuel rail. Did the following tests, bring the rail up to pressure and clamp the input line to the rail, pressure fell. Brought it up to pressure again and alternatively clamped the return line to tank and the vacuum line but pressure still fell to 0 in about 30 or so minutes so I concluded that it was not the FPV nor the non return check valve in the fuel pump. Finally I installed a newly rebuilt set of injectors and the problem went away.
Why didn't I replace the injectors right off? Because they had just been rebuilt less than 8 months before.