TM 5-3895-359-14&P
TROUBLE SHOOTING
FUEL PUMP
The most common reason for failure of a fuel pump to
The fuel pump is so constructed as to be inherently
function properly is a sticking relief valve. The relief
trouble free. By using clean, water-free fuel and
valve, due to its close fit in the valve bore, may become
maintaining the fuel filters in good condition, the fuel
stuck in a fully open or partially open position due to a
pump will provide long satisfactory service and require
small amount of grit or foreign material lodged between
very little maintenance.
the valve and its bore or seat. This permits the fuel to
circulate within the pump rather than being forced
However, if the fuel pump fails to function satisfactorily,
through the fuel system.
first check the fuel level in the fuel tank, then make sure
the fuel supply valve is open. Also check for external
Therefore, if the fuel pump is not functioning properly,
fuel leaks at the fuel line connections and filter gaskets.
remove the relief valve plug, spring and pin and check
Make certain that all fuel lines are connected in their
the movement of the valve within the valve bore. If the
proper order.
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
valve.
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
1. Disconnect the fuel return hose from the fitting at the
3. Immerse the end of the fuel hose in the fuel in the
fuel tank and hold the open end in a convenient
container. Air bubbles rising to the surface of the fuel
receptacle (Fig. 6).
will indicate air being drawn into the fuel system on the
suction side of the pump. If air is present, tighten all
2. Start and run the engine at 1200 rpm and measure
fuel line connections between the fuel tank and the fuel
the fuel flow. Refer to Page 10-9-1 for the specified
pump.
quantity per minute.
4. If the fuel flow is insufficient for satisfactory engine
performance, then:
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
10-3-57
