Fuel Pump: Description and Operation

Mechanical Fuel Pump Operation:

The fuel pump is located on left side of the engine mounted to the block. The fuel pump is a single action mechanical type consisting of an actuating lever, a diaphragm and spring, an inlet check valve, and an outlet check valve. A camshaft eccentric operates the fuel pump lever, this lever is directly connected to the pump diaphragm. The lever pulls the diaphragm to an extended position, drawing fuel into the inlet check valve. When the lever releases the diaphragm, spring pressure forces the fuel through the outlet check valve to the fuel filter. When the pump gathers more fuel than the engine can consume, the outlet check valve is held closed. Fuel in the fuel pump chamber prevents the diaphragm from relaxing. The lever will continue to move up and down on the diaphragm shaft, but will not be actuating the diaphragm until the outlet check valve opens and the fuel in the pump chamber is expelled.

The minimum pump output pressure is 3 psi (21 kPa). The output volume should be .70 quarts (.75 liters) per 30 seconds of engine cranking.

The fuel pump has a weep hole located in the bottom of the casing. If the diaphragm ruptures, fuel will exit the weep hole instead of filling the crankcase will fuel. If fuel is coming out of the weep hole, the whole pump assembly needs to be replaced.

Mechanical Fuel Pump Manual Lever:

The pump also has a manual lever located on the side of its housing. The purpose of this lever is to prime the fuel system and remove air if the vehicle has run out of fuel. During vehicle operation the lever must be in the up position.