Figure 1-21 .—Fuel filter elements.contain more gum and abrasive particles that can causepremature wear of injection equipment. Some dieselfuel filters have an air valve to release any air thataccumulates in the filter during operation.Most diesel engine designs include two filters in thefuel supply system: a primary filter and a secondaryfilter.PRIMARY FILTER.— In most designs, theprimary filter is located between the fuel tank and thefuel supply pump. A primary falter contains a coarsefilter material that removes the larger foreign matter.They are metal filters and only allow fine particles topass through them (fig. 1-22). Solid materials largerthan 0.005 inch remain outside the metal disks, whilethe larger foreign matter and most of the water settle tothe bottom of the bowl. This matter can be removedthrough a drain plug. A ball relief valve in the filter coverenables the fuel to bypass the filter element if the disksbecome clogged. Most types of heavy equipment havea fuel pressure gauge that indicates when the filters aredirty.NOTE: A good practice is to drain about one fourthof a pint of fuel out of the filter into a container or ontoa rag during the prestart operations. This practice allowsyou to drain out any foreign matter that has settled to thebottom of the filter.SECONDARY FILTER.— The secondary falter isusually located between the fuel supply pump and thefuel injection pump. It contains a fine filter that removeseven the most minute traces of foreign matter from the1-17Figure 1-22.-Primary fuel filter.
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