Fuel Pump: Description and Operation

Transfer Pump Operation:

Fig 7 Injection Pump Overflow Valve:

PURPOSE
The purpose of the fuel transfer pump is to supply (transfer) a low-pressure fuel source of approximately 172 kPa (25 psi) to the injection pump and fuel filter/water separator from the fuel tank. Here, the low-pressure is raised by the fuel injection pump for operation of the high-pressure fuel injectors.

OPERATION
The transfer pump is driven by an eccentric on the engine camshaft that actuates a spring loaded piston within the pump. Check valves within the pump, control direction of fuel flow and prevent fuel bleedback during engine shut down.

The fuel transfer pump should never be operated without the pre-filter installed.

The fuel volume of the transfer pump will vary with engine RPM, but will always provide more fuel than the fuel injection pump requires. Excess fuel is returned to the fuel tank through an overflow valve. The valve is located on the side of the injection pump and is used to connect the fuel return line to the side of the injection pump. This valve opens at approximately 152 kPa (22 psi) and returns fuel to the fuel tank through the fuel return line.

Manual Operation Of Transfer Pump
The transfer pump has a primer button. This rubber primer button is located on the pump housing. The purpose of the button is to prime and bleed air from the fuel system if the vehicle has run out of fuel. To prime the system up to the fuel injector pump, continually press on the button until resistance is felt. This resistance will indicate that priming is completed and air has been removed.

If the primer button feels as if it is not pumping, rotate (crank) the engine approximately 90 degrees. Continue pumping until air is removed.