GMC 4000 Series Service Manual page 368

the rigid return line at frame rail. Clips are used
to secure the fuel lines.
Flexible high pressure line assem blies are
used to transfer fuel from supply pump to second­
ary filter, and from filter to the fuel injection pump.
LEAK-OFF HOSES
Leak-off lines are used to interconnect the
fittings on the fuel nozzle holders to the overflow
valve assembly. Leak-off line installation is shown
in figure 6.
HIGH PRESSURE LINES
The high pressure lines, which carry high
p ressure fuel from injection pump hydraulic head
to the injection nozzle holder assem blies, are of
special thick-wall tubing. High pressure lines are
carefully preformed to assure a good fit at respec­
tive locations without bending or stressing. The
fuel line duct is the same length in all lines on any
one engine.
Each line must be used in the location for
which it is designed, and none of the high pressure
lines are interchangeable. Refer to figure 27 for
fuel lines routing at engine.
The high pressure line clamps dampen vibra­
tions which would tend to cause fatigue and break­
age of lines.
COLD WEATHER OPERATION
1. Fuel oil must conform to Factory recom ­
mended specifications. Your GMC Dealer can sup­
ply you with these specifications.
2. Neglect of fuel filter service can be one of
the major contributing factors to hard starting. If
water is permitted to accumulate in the primary
filter it will freeze and make starting the engine
im possible.
3. Adding one pint of alcohol to each tank of
fuel oil during freezing weather is an excellent
safeguard against water freezing in fuel tank, lines,
and filter.
4. Make sure vent in fuel tank cap is open.
5. Always refuel at the end of a run to help
keep out moisture. Moisture may condense in a
nearly empty tank, therefore, tank should be filled
before leaving vehicle standing for an extended
period.
FUEL INJECTION SYSTEM TROUBLE
DIAGNOSIS
When trouble with engine starting or perform­
ance is experienced, it is important to follow a
definite procedure in checking possible causes of
trouble. The "Fuel System Trouble Diagnosis
Chart" provided on the following page should be
used as a guide in analyzing symptoms and cor-
G M C S E R V I C E M A N U A L
FUEL INJECTION SYSTEM— TORO-FLOW ENGINE
Figure 6-Leai\-O ff Lines and Overflow Valve (Typical)
recting difficulties which may occur in fuel injec­
tion system .
CHECKING FOR AIR LEAKS
IN FUEL SUPPLY (SUCTION) LINE
is suspected that trouble may be caused by air
leaks in line or fittings between the fuel supply
pump and the fuel tank, the following means may
be used to check for air in system .
pump and install sight glass, or plastic line so that
fuel may be observed while engine is running.
the pump discharge line. If bubbles are present,
air is being drawn into system, either through de­
fective lines, primary filter, or fittings, or p oss­
ibly through supply pump oil seal or gasket.
1 Dust Cap on Nozzle
Holder Fitting
2 Leak-off Line
3 Dust Cap on Injection
Pump Fitting
4 Plastic Plugs in
Hydraulic Head
If, when diagnosing fuel system problems, it
CAUTION: Exercise necessary care
to prevent any dirt from entering the fuel
system when fuel lines are disconnected
in making following tests:
1. Disconnect outlet line from fuel supply
2. Start engine, then look for air bubbles in
Sec. 6M
Page 357
5 Overflow Valve
Assembly
6 Fuel Return Line
7 Nozzle Assembly
(6 Used)
8 Injection Pump
Assembly -
t isis