The fuel pump is so constructed as to be inherently
The most common reason for failure of a fuel pump to
By using clean, water-free fuel and
function properly is a sticking relief valve. The relief
maintaining the fuel filters in good condition, the fuel
valve, due to its close fit in the valve bore, may become
pump will provide long satisfactory service and require
stuck in a fully open or partially open position due to a
very little maintenance.
small amount of grit or foreign material lodged between
the valve and its bore or seat. This permits the fuel to
However, if the fuel pump fails to function satisfactorily,
circulate within the pump rather than being forced
first check the fuel level in the fuel tank, then make sure
through the fuel system.
the fuel supply valve is open. Also check for external
fuel leaks at the fuel line connections and filter gaskets.
Therefore, if the fuel pump is not functioning properly,
Make certain that all fuel lines are connected in their
remove the relief valve plug, spring and pin and check
the movement of the valve within the valve bore. If the
valve sticks, recondition it by using fine emery cloth to
remove any scuff marks. Otherwise, replace the valve.
Next, check for a broken pump drive shaft or drive
Clean the valve bore and the valve components. Then
coupling. Insert the end of a wire through the pump
lubricate the valve and check it for free movement
flange drain hole, then crank the engine momentarily
throughout the entire length of its travel. Reinstall the
and note whether the wire vibrates. Vibration will be felt
if the pump shaft rotates.
After the relief valve has been checked, start the engine
All fuel pump failures result in no fuel or insufficient fuel
and check the fuel flow at some point between the
being delivered to the fuel injectors and may be
restricted fitting in the fuel return manifold at the
indicated by uneven running of the engine, excessive
cylinder head and the fuel tank.
vibration, stalling at idling speeds or a loss of power.
CHECKING FUEL FLOW
3. Immerse the end of the fuel hose in the fuel in the
1. Disconnect the fuel return hose from the fitting at the
container. Air bubbles rising to the surface of the fuel
fuel tank and hold the open end in a convenient
will indicate air being drawn into the fuel system on the
receptacle (Fig. 6).
suction side of the pump. If air is present, tighten all
fuel line connections between the fuel tank and the fuel
2. Start and run the engine at 1200 rpm and measure
quantity per minute.
4. If the fuel flow is insufficient for satisfactory engine
a. Replace the fuel strainer cartridge. Then start the
engine and run it at 1200 rpm to check the fuel flow. If
the flow is still unsatisfactory, perform Step "b" below:
b. Replace the fuel filter cartridge. If the flow is still
unsatisfactory, do as instructed in Step "c".
c. Substitute another fuel pump that is known to be in
good condition and again check the fuel flow. When
changing a fuel pump, clean all of the fuel lines with
compressed air and be sure all fuel line connections are
tight. Check the fuel lines for restrictions due to bends
or other damage.
Fig. 6 Measuring Fuel Flow