Figure 4-9.Fuel filter elements.
pump uses a back-and-forth motion, it is a reciprocating
pump. They are usually powered by an eccentric (egg-
shaped lobe) on the engine camshaft.
The parts of a basic mechanical fuel pump are the
rocker arm, the return spring, the diaphragm, the
diaphragm spring, and the check valves.
The ROCKER ARM, also called an actuating
lever, is a metal arm hinged in the middle. A small pin
passes through the arm and fuel pump body. The outer
end of the arm rides on the camshaft eccentric and the
inner end operates the diaphragm.
The RETURN SPRING keeps the rocker arm
pressed against the eccentric. Without a return spring,
the rocker arm would make a loud clattering sound, as
the eccentric lobe hits the rocker arm.
The DIAPHRAGM is a synthetic rubber disc
clamped between two halves of the pump body. The
core of the diaphragm is usually cloth that adds strength
and durability. A metal pull rod is mounted on the
diaphragm to connect the diaphragm with the rocker
The DIAPHRAGM SPRING, when
compressed, pushes on the diaphragm to produce fuel
pressure and flow. This springs fits against the back of
the diaphragm and against the pump body.
The CHECK VALVES are used in a mechanical
fuel pump to make the fuel flow through the pump. The
check valves are reversed. This causes the fuel to enter
one valve and exit through the other.
The basic operation of a mechanical fuel pump
operation is as follows:
INTAKE STROKE. The eccentric lobe pushes
on the rocker arm. This action pulls the diaphragm
down and compresses the diaphragm spring. Since the
area in the pumping chamber increases, a vacuum pulls
fuel through the inlet check valve.
OUTPUT STROKE. The eccentric lobe rotates
away from the pump rocker arm. This action releases
the diaphragm. The diaphragm spring then pushes on
the diaphragm and pressurizes the fuel in the pumping
chamber. The amount of spring tension controls the fuel
pressure. The fuel is then forced to flow out of the outlet
Mechanical fuel pumps are classified as positive
and nonpositive diaphragm types. The POSITIVE type