the fuel system at loose joints in the piping or through a
If the engine starts missing, running irregularly,
spray nozzle that does not close properly. When a
rapping, or puffing black smoke from the exhaust
vehicle runs out of fuel, air is able to enter the system.
manifold, look for trouble at the spray nozzle valves. In
Like water, air can interfere with the unbroken flow of
this event, it is almost a sure bet that dirt is responsible
fuel from the tank to the cylinder. A great amount of air
for improper fuel injection into the cylinder. A valve
in a system prevents fuel pumps from picking up fuel
held open or scratched by particles of dirt so that it
and pushing it through the piping systems.
cannot seat properly will allow fuel to pass into the
exhaust without being completely burned, causing
Anytime the fuel flow is depleted and air enters the
black smoke. Too much fuel may cause a cylinder to
fuel system, the fuel system must be primed. This
miss entirely. If dirt prevents the proper amount of fuel
process is often called bleeding the system. If air is left
from entering the cylinders by restricting spray nozzle
in the lines, the fuel system may be air-bound. This can
holes, the engine may skip, or stop entirely. In most
result in an inability of the diesel engine to start or in its
cases, injector or spray valve troubles are easily
missing in one or more cylinders. If an engine does not
identified.
start, open the main fuel line vent and crank the engine
until a flow of fuel through the vent becomes
Improper injection pump operation, however, is not
continuous and contains no air bubbles. Be sure the
so easily recognized. It is more likely caused by
vent is closed before attempting to restart the engine.
excessive wear than by an accumulation of dirt or
carbon, such as the spray nozzle is subjected to in the
If one or more cylinders are missing when the
cylinder combustion chamber. If considerable abrasive
engine is running, it may be necessary to open the
dirt gets by the filters to increase (by wear) the very
individual vents or fuel lines leading from the injection
small clearance between the injector pump plunger and
pump to the injectors to remove the remaining air
barrel, fuel will leak by the plunger instead of being
bubbles. If air continues to collect in the fuel injection
forced into the injector nozzle in the cylinder. This
system following several bleedings, the suction
gradual decrease in fuel delivery at the spray nozzle
(vacuum) side of the fuel system needs to be checked
may remain unnoticed for some time, or until the
for loose fittings or a pinhole in the lines. For complete
equipment operator complains of sluggish engine
details on the priming (bleeding) procedure, consult the
performance.
equipment MIMs.
Although worn injector pumps will result in loss of
MAINTAINING INJECTION EQUIPMENT
engine power and hard starting, worn piston rings,
cylinder liners, and valves (air intake or exhaust) can be
Diesel injection equipment (injectors or spray
responsible for the same conditions. However, with
valves and pumps) are assembled units of precision
worn cylinder parts or valves, the hard starting and loss
parts; they cannot be cleaned or adjusted adequately
of power will be accompanied by poor compression, a
except in the work center. To operate efficiently, they
smoky exhaust, and excessive blowby from the
must be cleaned, repaired, and adjusted with special
crankcase breather.
equipment.
WATER IN THE FUEL SYSTEM
When fuel injection troubles are suspected, and
before removing the injector nozzles for shop testing, it
It requires only a little water in a fuel system to
is a good practice to check the injectors to find out if
cause an engine to miss, and if present in large enough
just one of them is causing the trouble. To do so, first
quantities, the engine will stop entirely. Many fuel
operate the engine at a speed at which the defect is more
filters are designed to clog completely when exposed to
pronounced. While the engine is operating at this
water, thereby stopping all fuel flow. Water that enters a
speed, loosen the fuel line connection at each injector
tank with the fuel, or that forms by condensation in a
one at a time to "cut out" the cylinder. When you find
partially empty tank or line, usually settles to the lowest
one that makes very little or no difference in the
part of the fuel system. This water should be drained off
irregular operation of the engine, the injector for that
daily.
cylinder is probably causing the trouble and needs to be
removed and tested. It is seldom that one injector valve
AIR IN THE FUEL SYSTEM
alone is responsible for all the trouble. Therefore, you
should continue the testing until all injectors have been
Air trapped in diesel fuel systems is one of the main
tested.
reasons for difficulty in starting an engine. Air can enter
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