Figure 1-21 .—Fuel filter elements.
contain more gum and abrasive particles that can cause
premature wear of injection equipment. Some diesel
fuel filters have an air valve to release any air that
accumulates in the filter during operation.
Most diesel engine designs include two filters in the
fuel supply system: a primary filter and a secondary
filter.
PRIMARY FILTER.— In most designs, the
primary filter is located between the fuel tank and the
fuel supply pump. A primary falter contains a coarse
filter material that removes the larger foreign matter.
They are metal filters and only allow fine particles to
pass through them (fig. 1-22). Solid materials larger
than 0.005 inch remain outside the metal disks, while
the larger foreign matter and most of the water settle to
the bottom of the bowl. This matter can be removed
through a drain plug. A ball relief valve in the filter cover
enables the fuel to bypass the filter element if the disks
become clogged. Most types of heavy equipment have
a fuel pressure gauge that indicates when the filters are
dirty.
NOTE: A good practice is to drain about one fourth
of a pint of fuel out of the filter into a container or onto
a rag during the prestart operations. This practice allows
you to drain out any foreign matter that has settled to the
bottom of the filter.
SECONDARY FILTER.— The secondary falter is
usually located between the fuel supply pump and the
fuel injection pump. It contains a fine filter that removes
even the most minute traces of foreign matter from the
1-17
Figure 1-22.-Primary fuel filter.
