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 pressure.
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 fuel system.
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 manufacturer's 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.Continue Reading