Special care must be exercised in
DISASSEMBLING and ASSEMBLING the parts
of a fuel injection system, since any damage to
these finely finished surfaces will necessitate
replacement of the parts. When work is being
done on any part of a fuel injection system, the
procedure outlined in the engine technical manual,
or the manufacturers fuel system technical
manual, must be followed.
Remember that the damage to a plunger and
barrel assembly of a fuel pressure pump or to the
plunger and bushing assembly of a unit injector
generally requires replacement of the parts.
A damaged part may not be replaced individually.
A plunger and its mating part (barrel, bushing,
or bore) must be installed as a complete assembly.
Trouble caused by external leakage from an
injection pump or an injector may become suffi-
ciently serious to cause an engine to misfire. It
is of extreme importance that signs of external
leakage be detected as soon as possible. Leakage
outside of the combustion space may be suffi-
ciently large not only to affect engine operation
but also to create a fire hazard. External leakage
of a unit injector can cause fuel dilution of the
engine lube oil, reduce lubrication, and increase
the possibility of a crankcase explosion.
In general, external leakage from pumps and
injectors is caused by improper assembly, loose
connections, faulty gaskets, damaged threads and
sealing surfaces, broken springs, or cracked hous-
ings or bodies. While leakage from pumps is
generally visible during engine operation, leakage
from an injector may not become apparent until
appropriate tests are performed.
You can stop the external leakage from a
pump or injector either by tightening loose con-
nections or by replacing the damaged parts.
Before the equipment is inspected for leakage,
thoroughly clean all parts. On some equipment,
you may eliminate mild roughness or discolora-
tion of the sealing surfaces by lapping.
When the cylinder of an engine fails to fire,
it is an indication that the injection pump plunger
is stuck. Misfiring may occur intermittently if the
plunger sticks and releases at intervals. Upon
disassembly, it may be difficult to remove the
plunger. Sometimes the plunger may stick when
the pump or the injector is assembled, but will
work smoothly when the unit is disassembled. At
times, the plunger will not stick until some time
after the unit has been removed from the engine.
This is particularly true when the plunger and
mating part have been stored under conditions
that cause corrosion, or when the parts have been
mishandled after removal.
A unit injector may be checked, after removal
from the engine, by performing the binding
plunger test. This test is performed by depress-
ing the plunger, either by hand or by using the
popping fixture of a test stand, and noting the
return action of the plunger. The plunger should
return with a definite snap. This test should be
performed at three successive rack settings. A
sluggish return action indicates a sticky plunger.
A sticking plunger may be caused by dirt,
gummy deposits in the unit, or distortion of the
plunger and its adjacent part.
The movement of a plunger may be restricted
or entirely prevented by small particles of dirt
which may lodge between the plunger and its
mating surface. Lacquer-like deposits, from fuel,
will also interfere with the movement of the
The greatest care must be taken when
handling the parts of a pump or injector. Because
of the extremely close clearances between plunger
and mating surfaces, a slight distortion of either
will cause binding. Distortion may result from
dropping, from striking the plunger and a mating
part, or from improper assembly.
Stuck plungers in fuel pumps or injectors
should be freed or replaced. Sometimes a little
cleaning may eliminate the need for a replace-
ment. The plunger and barrel or bushing assembly
should be soaked in an approved cleaning fluid.
The assembly should be soaked overnight, or
longer if necessary. Cleaning fluids approved for
this purpose will immediately soften and remove
any paint or enamel with which they come in con-
tact. These fluids should be used with care, since
they will damage rubber gaskets.
The specific procedures for cleaning fuel
injection equipment, although similar, vary to
some degree, depending upon the unit involved
Chapter 3ENGINE MAINTENANCE