Figure 1-21 .—Fuel filter elements.

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.