Diesel Fuel Filters

Diesel fuel filters are very important to the operation of a diesel engine. Because of the close tolerances of the injection system components, the fuel system must be kept exceptionally clean. The system is kept so clean that diesel fuel filters are rated in microns. A micron is 0.000039" (0.001 mm). Currently, some manufacturers specify final fuel filtration down to 3 to 5 microns. To put that in perspective, the diameter of a human hair is typically about 40 to 120 microns.

Typically, a diesel engine fuel injection system has two fuel filters. A primary filter, located between the fuel tank and the supply pump, filters particles to 30 to 50 microns. A secondary fuel filter, located between the supply pump and the high-pressure fuel injection pumping element, filters particles to 10 to 12 microns. As mentioned, some secondary filters are specified to remove particles down to 3 to 5 microns.

Additionally, diesel fuel filters must remove water from the fuel system. This is accomplished with water separators or traps, sometimes called sedimenters Figure 54-13. They can be separate units or combined with an impregnated paper element filter. Separate units pass the incoming fuel over an inverted funnel. At the edge of the funnel, the fuel changes direction very quickly. Water and dirt are heavier than fuel, so they are trapped, away from the funnel edge. With gravity, they settle at the base of the trap. The lower housing is usually clear for easy inspection, and it can include a drain plug so sediment can be drained daily.

FIGURE 54-13
A fuel filter combined with a water separator.

The most common type of filter material in light diesel vehicles is resin-impregnated paper, pleated to offer a large surface area for the fuel. These filters are considered the most efficient. A fuel filter with the resin-impregnated paper element can be combined with a water separator. When this combined unit is used, the method of fuel flow is determined by the manufacturer. With some filters, fuel flows from outside to inside. In others, it flows from the base to the top, or from top to bottom.

Some of the filters use paper elements, with the fuel flow options the same as previously described. In another method, fuel first passes through the paper element to trap abrasive particles. Any water in the fuel is usually in the form of small droplets, which are forced through the paper element. Once through the filter, the small droplets combine into larger droplets and form a sediment layer in the base of the filter. The bowl may be transparent and may contain a drain plug.

Other filter types pass fuel into the separator or sedimenter first and force the larger particles out of suspension by a change in direction. The fuel is then forced through a filtering medium, ready for use. This filter usually contains a disposable element. Often, the base has a drain plug for daily draining of the water from the filter.

Additionally, a water level switch can activate a light on the dash to warn the operator that the chamber may need draining. The switch has a float that is lighter than water but heavier than fuel. In the float is a magnet. As the float rises with the water level in the fuel, the magnet closes a reed switch, which turns on a warning light in the instrument cluster. The operator can then remove the drain plug to drain the water.