injection of the correct amount of fuel at exactly the right
time directly into the combustion chamber.
Efficient engine operation demands that the fuel
system be maintained in first-class condition at all times.
Use only clean water-free fuel. Good maintenance of the
fuel filtering system and reasonable care in handling the
fuel are the key to a trouble-free fuel system.
Servicing the fuel system is not a difficult task.
However, because of the close tolerances of the various
fuel system components, mechanics should practice
cleanliness and strictly adhere to service instructions.
In this section, troubleshooting, testing,
disassembly, cleaning out, inspection, and reassembly
of the fuel pump and fuel injector are discussed. Before
you work on these components, refer to the
manufacturers maintenance and service manuals.
When a piece of equipment is brought into the shop
for maintenance and service, the hard card or Equipment
Repair Order (ERO) may show a fuel system problem.
You can pinpoint the problem by troubleshooting the
fuel system until you find the trouble.
Check the fuel lines for improper or faulty
connections. If any leaks occur, tighten the connection
only enough to stop the leak. Also, check the filter cover
bolt for tightness. If the fuel pump fails to function
satisfactorily, first check the level of the fuel tank; then
make sure the fuel supply valve is open. Check for a
broken pump drive shaft or drive coupling by inserting
the end of a wire through one of the pump flange drain
holes; then crank the engine and note if the wire vibrates.
Vibration will be felt if the pump shaft is turning.
The result of most fuel pump failures is that either
no fuel or an insufficient amount of fuel is delivered to
the fuel injectors. This lack of fuel will show up if the
engine runs unevenly, vibrates too much, stalls at idling
speeds, or loses power.
The most common failure of a fuel pump is a
sticking relief valve. The relief valve, due to its close fit
in the valve bore, may stick in a full-open position
because a small amount of grit or foreign material lodges
between the relief valve and its bore or seat. The fuel oil
circulates within the pump rather than being forced
through the fuel system. If the fuel pump is not
functioning properly, remove the fuel pump from the
engine. hen 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. 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. If its operation is satisfactory,
reassemble the valve in the pump. If not, replace it.
After the relief valve has been checked and the fuel
pump reinstalled on the engine, start the engine and
check the fuel flow at some point between the restricted
fitting in the fuel return manifold and the fuel tank.
If, after making the above checks, there is still a lack
of power, uneven running, excessive vibration, or
stalling at idle, you should suspect a faulty injector in
one or more cylinders. Start the engine and run it at part
load until it reaches normal operating temperature.
Remove the valve rocker cover(s) and let the engine run
at idle speed. Hold the injector follower down with a
screwdriver, which prevents operation of the injector. If
the cylinder has been misfiring, there will be no
noticeable difference in the sound or operation of the
engine. If the cylinder has been firing properly, there will
be a noticeable difference in the sound and operation
when the follower is held down. If that cylinder is firing
properly, repeat the procedure on the other cylinders
until the faulty one has been located.
At this point you can remove the fuel injector for
additional testing, provided that the injector operating
mechanism of the faulty cylinder is functioning
The General Motors injector tester gives you a
means to determine the condition of the injector to avoid
unnecessary overhauling. An injector that passes all of
the tests outlined below may be considered to be
satisfactory for service without disassembly (except for
the visual check of the plunger). If an injector fails to
pass one or more of the tests, it is unsatisfactory. Be sure
to identify each injector and record the pressure drops
and fuel output during the tests. Also remember, all tests
must be performed before the injector is disassembled.
INJECTOR CONTROL RACK AND
PLUNGER MOVEMENT TEST. To perform this
test, lock the injector in a test stand.
Keep your hands away from the tip of the
injector while depressing the plunger. High-
pressure fuel spray that penetrates the skin will
cause blood poisoning.