Fuel Injectors - JLG sky trak mmv Service Manual

8.8.7

Fuel Injectors

The engine uses Bosch 17 mm closed nozzle hole-type
injectors. The injectors have different part numbers for
different engine ratings. The last four digits of the Cum-
mins 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 injec-
tion 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 slack-
ening 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 8.8.7, b. Injector Replacement ).
2. Invert the fuel injector with the nozzle facing out,
then retighten the unions.
Model MMV Rev. 12/04
Engine: Cummins 4BTA 3.9
3. Loosen the unions of the other fuel injector pipes (to
help avoid the possibility of the engine starting).
WARNING:
sure can penetrate the skin and cause serious
personal injury. Slowly loosen fittings and allow
release of residual pressure before removing.
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 pres-
sure 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 crack-
ling sound. It is not until the fuel injector sounds "dead"
that its condition is likely to affect the running of the engine.
Diesel fuel under pres-
1
MA1960
8.23