Fuel Return System Diagnosis

FUEL RETURN SYSTEM DIAGNOSIS






FUEL SYSTEM DESCRIPTION
Fuel is drawn by the supply pump through a pre-filter screen in the tank and to the engine through the fuel supply lines. There is no lift pump in the fuel tanks or on the frame. The fuel passes through the base plate of the fuel injector control module (FICM) to cool the module. From the FICM the fuel flows to the fuel filter/heater element housing, which combines a water separator, a prime pump, fuel heater element and a filter element. The fuel is then delivered to the high-pressure pump. An integrated hand pump is used to prime the fuel system after changing the fuel filter or servicing the fuel system. The fuel injection pump at the front of the engine valley includes the fuel supply pump and the high-pressure pump. If the fuel system is not suppling enough fuel a driveability concern may occur. If air is being drawn into the fuel injection system, a Cranks But Will Not Run or Hart Start symptom could exist.

HIGH PRESSURE SYSTEM
The much larger section of the pump assembly is the high-pressure fuel injection pump. The pump is engine-driven by the camshaft gear. From the high-pressure pump, the pressurized flows to the left common fuel rail. A balance pipe from the center of the left rail then feeds the right common fuel rail. Each common fuel rail supplies one bank of 4 fuel injectors. The fuel rail pressure (FRP) sensor is mounted in the middle of the right common fuel rail.

RETURN SYSTEM
The Fuel Return System routes fuel from the fuel injectors and the fuel injection pump. The FRP relief valve is located in the rear of the left common rail. The return fuel travels to the fuel cooler and then to the fuel tank. This fuel is used to cool and lubricate the injection pump and the injectors.

DIAGNOSTIC AIDS
Bent or pinched fuel return lines on the engine can cause a restricted fuel return flow.