out under pressure. The sliding vane electric fuel pump
is similar to the roller vane pump, except vanes (blades)
are used instead of rollers.
Most rotary fuel pumps also have check valves and
relief valves. The check valves keep the fuel from
draining out of the fuel line when the pump is not in
operation. A relief valve limits the maximum output
Another type of electric fuel pump is the
reciprocating electric fuel pump. This pump has the
same basic action as a mechanical fuel pump; however,
it uses a solenoid instead of a rocker arm to produce a
plunger action. The reciprocating pump uses either
bellows (fig. 4-13) or a plunger. The solenoid turns on
and off to force the bellows or plunger up and down.
This action pushes fuel through the check valves and the
Both mechanical and electric fuel pumps can fail
after prolonged operation. Indications of fuel pump
problems are as follows:
LOW FUEL PUMP PRESSURE can be caused
by a weak diaphragm spring, ruptured diaphragm,
leaking check valves, or physical wear of moving parts.
Low fuel pressure can make the engine starve for fuel at
higher engine speeds.
HIGH FUEL PUMP PRESSURE, more frequent
with electric fuel pumps, indicates an inoperative
pressure relief valve. If the valve fails to open, both
pressure and volume can be above normal. High fuel
pump pressure can produce a rich fuel mixture or even
flood the engine.
MECHANICAL FUEL PUMP NOISE (clacking
sound from inside the pump) is commonly caused by
weak or broken rocker arm return spring or by wear of
the rocker arm pin or arm itself. This noise can be easily
confused with valve or tappet clatter. To verify
mechanical pump noise, use a stethoscope.
FUEL PUMP LEAKS are caused by physical
damage to the pump body or deterioration of the
diaphragm and gaskets. Most mechanical fuel pumps
have a small vent hole in the pump body. When the
diaphragm is ruptured, fuel will leak out of this hole.
Fuel pump testing commonly involves measuring
pump pressure and volume. Since exact procedures
vary depending on the type of fuel system, refer to the
manufacturers manual for exact testing methods.
Sometimes, fuel pump vacuum is measured as another
means of determining pump and line condition. Always
remember that there are several other problems that can
produce symptoms similar to those caused by a fuel
Figure 4-13.Bellows-type electric fuel pump.