GMC 4000 Series Service Manual page 362

DESCRIPTION
FUEL INJECTION PUMP ASSEMBLY
The fuel injection pump assembly is of the
PSJ Series manufactured by American Bosch Arma
Corporation. See figure 1 for schematic of pump
system .
Injection pump assembly is mounted on ma­
chined rear face of engine front cover assembly.
Pump assembly is held in place by studs and nuts.
The high pressure pump is the single plunger
type, which distributes equal amounts of fuel to
the fuel injection nozzles, at the cylinders, in the
proper firing sequence.
Injection pump is gear driven from a pump
drive gear bolted to front face of engine camshaft
gear. Pump input shaft, on which the pump driven
gear is mounted, rotates at engine crankshaft speed.
The pump plunger is operated by a rotating
camshaft having three lobes. Thus, fuel is injected
for firing three cylinders during each crankshaft
revolution.
Included in the fuel injection pump assembly
are the following units or devices:
1. Fuel supply pump.
2. Hydraulic head assem bly.
3. Timing advance mechanism.
4. Governor and fuel control unit.
5. Excess fuel device for starting.
An overflow valve assembly is installed on the
hydraulic head housing and incorporates a valve
assembly which maintains proper fuel pressure
within the fuel chamber to assure an adequate sup­
ply of fuel to the pump plunger at all tim es.
Fuel Supply Pump
The fuel supply pump is bolted to rear face of
the governor housing and is equipped with inlet and
outlet fittings. The fuel supply pump is of the p osi­
tive displacement gear type, and pump incorpor­
ates a relief valve assembly to insure against
excessively high pressure.
Supply pump drive is from rear end of gov­
ernor drive shaft assembly.
Hydraulic Head Assembly
The hydraulic head assem bly which pumps
fuel under high pressure to the fuel injection noz­
zles, is mounted in upper front portion of the in­
jection pump assem bly, directly above the pump
camshaft lobes.
G M C S E R V I C E M A N U A L
fyuel Onjectio-n Syitem
(V6 TORO-FLOW ENGINE)
The hydraulic head bore is fitted with a pump
plunger, the lower end of which seats in the tappet
roller guide. Precision drilled fuel passages in the
head carry fuel from the plunger distributing annu-
lus to the outlet ports in upper surface of head block.
The plunger pumping action is provided by the
tappet roller which is held in contact with the ro­
tating camshaft lobes by a plunger return spring.
Rotary motion of pump plunger is supplied by
a "face gear" below the hydraulic head. Face gear
is engaged with a set of gear teeth on governor
drive gear. Two revolutions of the governor drive
gear cause one rotation of the face gear and pump
plunger.
Pump plunger has a passage drilled from upper
end down to the metering sleeve area. A hole drill­
ed radially in the plunger intersects the vertical
drilling. The drilling or "spill" port is located in
the area of fuel metering sleeve which is installed
on plunger and located by pin in the control unit
assembly.
Installed at top of hydraulic head assembly is
a fuel delivery valve assem bly, through which the
metered quantities of fuel must pass. Fuel d is­
charged through the delivery valve passes through
a duct in hydraulic head to the distributing annulus
in plunger. Refer to figure 1 for view of hydraulic
head and related mechanism.
Timing Advance Mechanism
The fuel injection timing is automatically ad­
vanced by means of the device shown in figure 2,
which is assembled in a cavity provided in the in­
jection pump drive shaft.
The advance mechanism provides the means
for varying the point at which the fuel injections
into cylinders occur.
Advance in timing is accomplished by slightly
rotating the cam in relation to the pump drive
shaft.
As the splined sleeve is moved in longitudinal
direction, the spiral splines cause a repositioning
of cam in relation to the pump drive shaft. Splines
on sleeve are engaged with mating splines on drive
shaft and in cam.
Governor and Fuel Control Unit
The gear teeth at rear end of pump drive shaft
are meshed with the governor drive gear teeth.
Governor gear drives the governor shaft on which
Sec. 6M
Page 351