The lift pump transfers fuel from the tank to the fuel injection system. In modern vehicles, the tank is mounted below the engine, and the fuel has to be lifted up to the level of the engine. 3 types of lift pump are common on light vehicle diesel engines - the diaphragm-type pump, the plunger pump, and the vane pump.
The diaphragm-type pump can be mounted on the engine, or on the injection pump. It is fitted with inlet and outlet valves, and an eccentric on a camshaft acts on a 2-piece rocker arm connected to a diaphragm. Rotating the eccentric causes the rocker arm to pivot on its pin, and pull the diaphragm down. This compresses the diaphragm return spring, and increases the volume in the pumping chamber above the diaphragm.
Atmospheric pressure at the fuel tank forces fuel along the fuel line, to open the inlet valve. Fuel flows into the pumping chamber. The eccentric keeps rotating, and the rocker arm is released. The spring exerts force on the diaphragm, to pressurize the fuel in the chamber. This pressure closes the inlet valve, and opens the outlet valve, letting fuel be delivered to the injection system.
If the system doesn’t need all of the fuel delivered, the pressure in the outlet fuel line rises to the same level as in the pumping chamber. That holds down the diaphragm, and keeps the diaphragm return spring compressed. When this occurs, the split-linkage in the rocker arm, allows the lever to maintain contact with the eccentric, without acting on the diaphragm pull-rod.
