is there an off chance that there's a vacuum line hooked backwards? the fuel shouldn't immediately return. you should pressure test the fuel rail if at all possible. I doubt a new FPR is at fault. but whatever's metering fuel leaving the rail is surely causing some shenanigans. I wish I knew more about the M10 or whichever 4 banger they came with so I could be more useful.
Standard practice on modern fuel injection though, relay fires the pump, pump pushes x amount of fuel per hour FPR regulates how much fuel makes it to the fuel rail. fuel rail should pressurize to between 40-50psi, a fuel return valve will open when that pressure is exceeded usually when the engine was in a high load situation and no longer has the need for a high volume of fuel... enabled by the increased vacuum at high engine speed to the FPR. So, if anything's wrong here it's the check valve going to the return side of the fuel system.
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- '06 325xi 6MT
- '96 328i 5MT "drift turd"