A diesel fuel lift pump is a lift pump that transfers fuel from the tank to the fuel injection system. Because in modern vehicles the tank is located below the engine, the fuel has to be lifted up from the tank to the engine, which the diesel fuel pump does.
Three types of lift pumps common in light vehicle diesel engines are the diaphragm-type pump, plunger pump and vane pump. The diaphragm-type diesel fuel pump can be mounted directly on the engine or on the injection pump. Fitted with inlet and outlet valves, the diaphragm-type pump utilizes an eccentric on a camshaft that acts on a two-piece rocker arm. When the eccentric rotates, the rocker arm pivots, and the diaphragm is pulled down. This compresses the diaphragm return spring and raises the volume in the pumping chamber above the diaphragm.
The result is atmospheric pressure at the fuel tank which forces fuel along the fuel line, opening the inlet valve. Fuel then flows into the pumping chamber. Once the inlet valve is closed, the outlet valve is opened and fuel is delivered to the injection system. If the fuel injection system doesn't need all the fuel delivered, the pressure in the outlet fuel line rises to the same level as in the pumping chamber.