solids fall to the bottom of the filter housing, where they
Diesel fuel filters are referred to as full-flow filters,
can be removed through the drain plug hole. A ball
since all the fuel must pass through the filters before
relief valve in the filter cover enables the fuel to bypass
reaching the injection pumps. Filters must be inspected
the filter element if the discs becomes clogged. A diesel
regularly and cleaned or replaced, if their maximum
fuel filter usually has an air vent for releasing any air
efficiency is to be maintained. All metal-disc filters are
that might accumulate in the filter.
cleanable, but most cloth or fabric elements must be
replaced when they become dirty.
Metal Disc Filters
Fabric filters, because of their greater filtering
qualities, are used principally as the main filter for
The metal disc filter (fig. 4-32) is made of a series
protecting the fuel injection pump. Most fuel systems
of laminated discs placed within a large bowl, which
now use microporous paper fuel filters (fig. 4-33),
acts as a settling chamber for the fuel and encloses the
which are of the replaceable cartridge type.
discs or strainer assembly. Fuel enters the filter at the
top inlet connection and, flowing down, goes between
FUEL SUPPLY PUMPS
the discs, and then up a central passage to the outlet
connection at the top. Dirt and foreign matter cannot
Fuel injector pumps must be supplied with fuel
pass between the discs and are deposited at the outer
under pressure because they have insufficient suction
rim. The clearance between the discs (0.003 inch) is
ability. All diesel injection systems require a supply
small enough to prevent the passage of water. This is
pump to transfer fuel from the supply tank through the
possible because water, when present in gasoline or oil,
filters and lines to the injection pumps. There are two
forms small globules that are too large to pass between
types of supply pumps used on diesel engines
the discs. Note that the filter shown in figure 4-32 also
today--the gear type and the vane type.
has a cleaning knife. Solids larger than 0.005 inch
remain on the outside of the element, and the cleaning
knife scrapes these deposits off the filtering discs. The
Figure 4-33.--Fuel filter assembly with replaceable element.
Figure 4-32.--Metal disc fuel filter.