Lull 944E-42 Service Manual Download

Page: 228 / 844

Engine: Cummins 4BT 3.9, B4.5T and QSB 4.5T

7.28

Model 644E-42/944E-42

Rev. 6/04

7.8.7

Fuel Injectors

The engine uses Bosch closed nozzle hole-type injectors.
The injectors have different part numbers for different
engine horsepower ratings. The last four digits of the
Cummins part number (stamped on the bottom portion of
the injector) identify the injector.

IMPORTANT: Use only the specified injector for the
engine. Damage to engine can occur if incorrect
injector’s are used.

During the injection cycle, high pressure from the
injection pump rises to the operating (or “pop”) pressure,
which causes the needle valve in the injector to lift. Fuel
is then injected into the cylinder. A shimmed spring is
used to force the needle valve closed as the injection
pressure drops below the pop pressure to seal off the
nozzle after injection. Failure of the needle valve to lift
and to close at the correct time, or a needle valve that is
stuck open, can cause the engine to misfire and produce
low power. Fuel leaking from the open nozzle can cause
a fuel knock, poor performance, smoke, poor fuel
economy, and rough running.

a. Locating Faulty Fuel Injectors

A faulty fuel injector can cause:

• Misfiring

• Knocking

• Overheating

• Loss of power

• Smoky (black) exhaust

• Increased fuel consumption

The particular faulty fuel injector(s) may be determined by
releasing the pipe union nut on each fuel injector in-turn,
with the engine running at a fast “tick-over.” If, after
slackening a pipe union nut, the engine revolutions
remain constant, a faulty fuel injector has been identified.

To test a fuel injector:

1. Remove the fuel injector (1) from the cylinder head.

(Refer to Section 7.8.7, b. “Injector Replacement.”)

2. Invert the fuel injector with the nozzle facing out,

then retighten the unions.

3. Loosen the unions of the other fuel injector pipes (to

help avoid the possibility of the engine starting).

4. Operate the starter to turn the engine over until fuel

sprays from the nozzle. Examine the shape of the
spray. If the spray is unduly “wet” or “streaky," or
obviously sprays to one side, or if the nozzle
“dribbles” fuel, it may only be necessary to probe the
nozzle holes to remove blockage.

IMPORTANT: DO NOT attempt to adjust injection
pressure without a testing pump and pressure gauge. It
is not possible to accurately adjust the setting of fuel
injectors without the proper equipment.

A perfect fuel injector, when tested by pumping fuel
through it in the open air, gives a short “pinging” sound as
the fuel emerges from the holes. After the fuel injector has
been in service for some time, the pinging changes to a
crackling sound. It is not until the fuel injector sounds
“dead” that its condition is likely to affect the running of
the engine.

Lull 944E-42, Service Manual

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