SERVICE MANUAL
GENERAL
Section 1
Figure 4. Fuel System Flow
FUEL SYSTEM FLOW
As fuel travels through the fuel system, it will be under
suction (negative pressure), intermediate pressure, high
pressure (injection pressure) and low pressure (Figure
4).
Negative Pressure
The piston type supply pump creates a suction at its inlet
and pulls fuel from the fuel tank, through the fuel supply
lines and through the primary fuel filter. No filter is used
on the fuel suction line extending into the fuel tank
because wax particles could plug the filter in cold
weather. Fuel lines and fittings between the supply
pump inlet and the fuel tank must be a minimum of
three-eighths inches inside diameter. Small diameter
fuel supply lines or fittings will reduce fuel flow and can
reduce engine power output.
Intermediate Pressure
The supply pump produces an intermediate pressure,
called supply pump pressure, at its outlet and forces fuel
through the final fuel filter and into the injection pump
housing. Fuel under intermediate pressure surrounds
each barrel assembly. Fuel pressure in the housing is
controlled by the orificed
fitting in the fuel return hose located "downstream" of the
fuel return check valve and the supply pump piston
spring calibration. A special rubber hose between the
final filter outlet and the injection pump housing dampens
pressure waves from the supply pump. Substitute hoses
can cause lower engine power even though supply pump
pressure can be within specifications.
High Pressure
The injection pump meters and delivers fuel at high
pressure, up to an instantaneous 9000 psi pressure after
nozzle valve opening, through the injection pipes or lines
to the injection nozzle assemblies. This high pressure
fuel causes the nozzle valve to open and fuel flows
through the nozzle orifices into the combustion chamber.
The amount of fuel delivered is controlled by the injection
pump governor. A small amount of low pressure fuel
returns from the nozzle assemblies to the fuel tank
through the leakoff lines.
Low Pressure
An orifice fitting in the end of fuel hose from the fuel
return check valve limits the amount of fuel returned to
the fuel tanks. After fuel passes through the orifice, it is
at low pressure.
CGES-375
Printed in United States of America
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