As highlighted by Discodicky here : What happened to your D3/D4 today? there is a 0.8mm orifice in the return line which means the fuel actually returned to the tank is restricted. It's mainly an air bleed. In general the HPFP circulates the fuel around through the filter and cooler with a minimal return to the tank. That also explains why when the HPFP feed pump lunches itself the shavings mostly wind up in the filter rather than the tank.
I can't explain the pressure difference, but I see about the same running pressure you do (or I did when I last tested it in 2019).
Just for interest I used a water fluid flow calculator for an 0.8mm orifice at the ~4psi pressure differential and got ~10L/hr. It'd be less for diesel.



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