Diesel MFI Pump

The Fuel Drain Circuit incorporates several sources of fuel return. Fuel travels from the fuel tank to the fuel transfer pump and is forced through the fuel filter. A portion of that fuel travels through the fuel filter and into the fuel injection pump, while the rest of the fuel flows through a passage in the fuel filter housing. It then goes to a fuel drain line and returns back to the fuel tank.

The fuel that flows to the fuel pump is pressurized and sent into a passage in the fuel pump. At this point the fuel is channeled into two passages. One passage sends fuel to the FCA (Fuel Control Actuator). The other passage sends fuel to the cascade overflow valve. The overflow valve sends some fuel to a lubrication passage. The rest of the fuel is sent to a drain passage which connects to an external fuel line. This fuel line is connected to the same fuel filter housing passage that the fuel transfer pump is connected to.

Fuel that travels through the FCA is pressurized by the fuel injection pump and sent through an external high pressure fuel line to the fuel rail. At the fuel rail, fuel is sent to the fuel injectors. If fuel pressure in the fuel rail becomes excessive, the pressure limiting valve opens and sends fuel through an external fuel line. This line is connected to the fuel filter housing at the fuel pump drain.

At the fuel injector, fuel that is not injected is used for lubrication of the fuel injectors. This fuel then travels through an internal passage in the rear of the cylinder head, an then into an external fuel line. This line is connected to the vehicles fuel return line, and returns excess fuel back to the fuel tank.


Fuel Injection Pump
A Robert Bosch high pressure fuel injection pump is used. The pump is attached to the back of the timing gear cover at the left I rear side of the engine.

The fuel injection pump supplies high pressure to the fuel rail independent of engine speed. This high pressure is then accumulated in the fuel rail. High pressure fuel is constantly supplied to the injectors by the fuel rail. The Engine Control Module (ECM) controls the fueling and timing of the engine by actuating the injectors.

Fuel enters the system from the electric fuel transfer (lift) pump, which is attached to the fuel filter assembly Fuel is forced through the fuel filter element and then enters the Fuel Pump/Gear Pump, which is attached to the rear of the fuel injection pump. The Fuel Pump/Gear Pump is a low-pressure pump and produce pressures ranging from 551.5 - 1241 kPa (80 - 180 psi). Fuel then enters the fuel injection pump. Low pressure fuel is then supplied to the FAC (Fuel Control Actuator).

The FAC is an electronically controlled solenoid valve. The ECM controls the amount of fuel that enters the high-pressure pumping chambers by opening and closing the FAC based on a demanded fuel pressure. The FPS (Fuel Pressure Sensor) on the fuel rail provides the actual fuel pressure. When the actuator is opened, the maximum amount of fuel is being supplied to the fuel injection pump. Any fuel that does not enter the injection pump is directed to the overflow valve. The overflow valve regulates how much excess fuel is used for lubrication of the pump and how much is returned to the tank through the drain manifold.

Fuel entering the injection pump is pressurized to between 300 - 1600 bar by three radial pumping chambers. The pressurized fuel is then supplied to the fuel rail.