The fuel pump is so constructed as to be inherently trouble-free. By using clean, water-free fuel and
maintaining the fuel filters in good condition, the fuel pump will provide long satisfactory service and require
very little maintenance.
However, if the fuel pump fails to function satisfactorily, first check the fuel level in the fuel tank, then make
sure the fuel-supply valve is open. Also check for external fuel leaks at the fuel line connections and filter
gaskets. Make certain that all fuel lines are connected in their proper order.
Next, check for a broken pump drive shaft or drive coupling. Insert the end of a wire through the pump flange
drain hole, then crank the engine momentarily and note whether the wire vibrates. Vibration will be felt if the
pump shaft rotates.
All fuel pump failures result in no fuel or insufficient fuel being delivered to the fuel injectors and may be
indicated by uneven running of the engine, excessive vibration, stalling at idling speeds or a loss of power.
The most common reason for failure of a fuel pump to function properly is a sticking relief valve. The relief
valve, due to its close fit in the valve bore, may become stuck in a fully open or partially open position due to a
small amount of grit or foreign material lodged between the valve and its bore or seat. This permits the fuel to
circulate within the pump rather than being forced through the fuel system.
Therefore, if the fuel pump is not functioning properly, 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. Clean the valve bore and the valve components.
Then lubricate the valve and check it for free movement throughout the entire length of its travel. Reinstall the
After the relief valve has been checked, start the engine and check the fuel flow at some point between the re-
stricted fitting in the fuel-return manifold at the cylinder head and the fuel tank.
CHECKING FUEL FLOW
1. Disconnect the fuel-return hose from the fitting at the fuel tank and hold the open end in a convenient
receptacle (Fig. 1).
2. Start and run the engine at 1200 rpm and measure the fuel flow. Refer to Engine Operating Conditions for
the specified quantity per minute.
3. Immerse the end of the fuel hose in the fuel in the container. Air bubbles rising to the surface of the fuel will
indicate air being drawn into the fuel system on the suction side of the pump. If air is present, tighten all fuel
line connections between the fuel tank and the fuel pump.
4. If the fuel flow is insufficient for satisfactory engine performance, then:
a. Replace the element in the fuel strainer. Then start the engine and run it at 1200 rpm to check the fuel