FUEL SYSTEM - Continued
The fuel pump body is recessed to provide running space for the pump gears. Recesses are also provided at the
inlet and outlet positions of the gears. A small hole permits the fuel oil in the inlet side of the pump to lubricate the
relief valve at its outer end and to eliminate the possibility of a hydrostatic lock which would render the relief valve
inoperative. Pressurized fuel contacts the relief valve and provides for relief of excess discharge pressures. Fuel
re-enters the inlet side of the pump when the discharge pressure is great enough to move the relief valve back
from its seat. A cavity provides escape for the fuel oil, which is squeezed out of the gear teeth as they mesh
together on the discharge side of the pump. Otherwise, fuel trapped at the root of the teeth would tend to force the
gears apart, resulting in undue wear on the gears, shafts, body, and cover.
Figure 6. Fuel Pump.
During operation, fuel enters the pump on the suction side and fills the space between the gear teeth, which are
exposed at that instant. The gear teeth then carry the fuel oil to the discharge side of the pump and, as the gear
teeth mesh in the center of the pump, the fuel oil is forced out into the outlet cavity. Since this is a continuous
cycle and fuel is continually being forced into the outlet cavity, the fuel flows from the outlet cavity into the fuel
lines and through the engine fuel system under pressure. The pressure relief valve relieves the discharge
pressure by bypassing the fuel from the outlet side of the pump to the inlet side when the discharge pressure
reaches approximately 65 to 75 PSI (448 to 517 kPa). The fuel pump should maintain the fuel pressure at the fuel
A spin-on type fuel filter is used. The spin-on filter cartridge consists of a shell, element, and gasket combined into
a unitized replacement assembly. No separate springs or seats are required to support the filter.
The filter cover incorporates a threaded sleeve to accept the spin-on filter cartridges. A fuel/water separator is
installed to drain water residue. Water may also be drained by removing and inverting the filter. Refill the filter with
clean fuel oil before reinstalling it.