BMW fpr not the type affected by vacuum. The return line isn't going to affect pressure out of the FPR so I'm the opposite of making your point I'm saying exactly the opposite. The intake into the fuel pump may change enough to affect fuel pressure also cold fuel pump may give you more pressure but the return line to the jet pumps is not going to change the output pressure it's exactly not what FPR does.

If you aren't getting stable pressure out there is 95% chance that the fuel pump is shot, 5.9% chance the FPR is the problem and 0.1% chance it's something else like backpressure from the jet pumps (which as I mentioned earlier, if that is changing the output pressure the FPR is defective).

Consider that if the pump is sending 65psi and 50 goes to the engine and 15 returns to the tank: the 0.5psi difference full to empty at the end of the jet pump is not going to make any difference.. that 0.5 is not even supplying backpressure, the Venturi nozzle is supplying the backpressure, I doubt there is any significant change at all to the back pressure since the venturi creates a vacuum it will only change how much fuel will get sucked into the surge tank but the physics don't align with affecting back pressure since the return line doesn't feed into the tank but into the venturi nozzle.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)