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Fuel filters trap foreign material that may be present in the fuel and prevent it from entering the carburetor or sensitive fuel injection components.

Gasoline Fuel Filter

On a gasoline engine system there is at least one fuel filter used in the fuel system and it can be located in any accessible place along the fuel delivery line. Filters also can be located inside fuel tanks, carburetors, and fuel pumps.

The fuel filter operates by passing fuel through a porous material that blocks particles large enough to cause a problem in the system. Some filters also act as sediment bowls; water and larger particles of foreign matter settle to the bottom where they can be drained off (fig. 1-19).

Figure 1-20 shows the three types of fuel filters in common use. They are the replaceable in-line filter (view A), the in-line filter element (view B), and the glass bowl (view C).

Filter elements are made from ceramic, treated paper, sintered bronze, or metal screen, as shown in figure 1-21. A filter element that differs from the others is a series of closely spaced laminated disks. As the gasoline proses between the disks, foreign matter is blocked out.

Diesel Fuel Filters

Diesel fuel filters are called full-flow filters, because all the fuel must pass through them before reaching the injection pumps. They are very important because diesel fuels are more viscous than gasoline and

Figure 1-19.-Fuel filter operation.

Figure 1-20.-Fuel filter configurations.

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 fuel. Secondary filters (fig. 1-23) are fabric filters that have greater filtering qualities than primary filters. They arc used principally as the primary filter to protect the fuel injection pump.

Figure 1-22.-Primary fuel filter. 


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