[T Bird]

John
When pressure in the delivery line of the MFI fuel pump reaches 2 bar, a pressure relief valve opens in the pump and the excess fuel is dumped back to the tank through the return line. Typical operating pressure for the pump in an MFI car is 1 bar. I believe that the pressure relief valve and the return line keep pressure within operating parameters and return un-needed fuel to the tank.

By way of comparison, the pressure relief valve for the Bosch roller cell fuel pump designed for carbs operates at .3 bar and the pressure relief valve opens between .2 and .3 bar.

Years ago when I replaced the MFI fuel pump on my car with the Bosch roller cell to go with my then-new Webers, I tested the fuel pump by connecting short lengths of hose to the return, suction and delivery fittings on the pump. I placed the suction line in a coffee can full of gas and the delivery and return lines in empty cans. I energized the pump and before all the gas was sucked up and pumped into the "delivery" can, I pinched the delivery hose to restrict flow. Fuel immediately began to discharge from the return line. That's how it works.