Lesson 1/learning Event 1
Filters with a metal screen and a sediment bowl are in common use, but nowadays filters are often made of
pleated paper and are disposable.
The fuel pump draws fuel from the tank and delivers it to the carburetor. The pump must deliver the right
amount of fuel for engine operation for both light and heavy loads. In addition, it pressurizes fuel in the
line between the carburetor and the pump. Pressure in the line reduces the possibility of the fuel vaporizing
(boiling) because of heat from the engine.
Fuel pumps are generally classified either as positive or nonpositive.
- Positive-type pumps pump fuel continuously while the engine is running and must have some means
of returning fuel that the engine does not use. Vehicles with positive fuel pumps pass excess fuel
back to the fuel tank or to the inlet side of the fuel pump.
- Nonpositive-type pumps supply fuel only when it is needed.
The fuel pump most widely used on wheeled vehicle gasoline engines is the nonpositive, diaphragm type.
This type of pump is usually operated mechanically by the engine.
Diaphragm-Type Fuel Pumps
The operating linkage of the pump consists of a rocker arm, a link, and a spring. A flexible diaphragm is
hooked to the link by a rod. The diaphragm is clamped at its outer edges by the two halves of the pump
body. The pumping chamber is the area between the diaphragm and valve assemblies.
The fuel pump is mounted on the engine with its rocker arm in contact with an eccentric on the engine
camshaft. As the eccentric turns, it forces the rocker arm and link to pivot (turn) on its pin and pull the
diaphragm down. The downward movement of the diaphragm compresses the diaphragm spring and
enlarges the pumping chamber. This creates a low-pressure area or vacuum (suction) in the pumping
chamber. In the meantime, air entering the fuel tank vent at atmospheric pressure is pressing downward on
the fuel in the tank. This pressure forces the fuel to flow from the tank, through the line, through the pump
inlet valve, and into the low-pressure area of the pumping chamber. Remember that liquids or gases will
always flow from a high-pressure area to a low-pressure area.