PS Advisor April 2008 Issue

Fuel Filtering Systems

Good engine health begins with a clean fuel tank.

I am the new owner of a 1979 Tayana Vancouver 42 with a Volvo D2-55B diesel engine installed in 2004. Fuel is held in two 80-gallon fiberglass tanks and filtered via a Racor 500 utilizing No. 2010 filters. I use 2-micron filters and Biobor JF Anti-Fungal additive. Despite fuel scrubbings, I continue to have to change primary fuel filters with each fueling. I have been told that I donít need as fine a filter as the 2-micron, but I donít want to risk increased maintenance of the diesel. What do you recommend?

Mike Hirko
Tayana Vancouver 42
Gig Harbor, Wash.

The advice youíve been given regarding your primary filterís micron rating is correct, however, this isnít the root of your problem.

Frequent Fuel-Filter Clogs
Photo by Steve DíAntonio

Frequent fuel-filter clogs can be symptomatic of a dirty fuel tank.
Think of fuel filters as the window to your fuel tankís soul. The standard rule of thumb is that if the filters must be changed more often than every 100 engine operating hoursóthis is an unscientific threshold, if the engine idles for 100 hours, it pumps less fuel than if itís operating at cruising rpmóthen the tanks require cleaning, period. Thereís no getting around the fact that the contamination found in the fuel filter comes from the tank(s). If the filter is clogging frequently, look to the tank rather than the filterís micron rating. All diesel fuel tanks should be equipped with inspection ports, one for each baffled chamber. (One clean-out port installed in a multi-chambered tank is of little value.) If a tank is not equipped with inspection ports, it can be retrofitted using an inspection-port kit. (See for kit choices.) These can be added to virtually any type tank other than polyethylene.

Using an antifungal agent with every fill up probably wonít do any harm; however, too much of a good thing can be bad. Unless you are using all of the fuel in the tank before refilling and shocking the fuel with a microbial agent, you could accumulate too much of the substance. This may put the fuel "off spec," meaning it doesnít meet the American Society for Testing and Materials (ASTM) specification for diesel fuel. Additives, especially those with alcohol, may be harmful to the tank itself, particularly if it is fiberglass.

Remember, if thereís no water in the tank, there can be no biological life. Periodically "stripping" water from the bottom of a tank negates the need for biocide. If the debris thatís accumulating in your filters is not biological, then no amount of biocide will help. Often the contaminant is asphaltine, a naturally occurring mineral-based contaminant that settles out of diesel fuel.

Finally, in our opinion, sequential filtration is particularly effective. That is, itís best to sequentially step down filtration, beginning with a comparatively coarse primary filter, say 10 microns, and then a finer, 2-micron filter. Typically, the on-engine or secondary filter specified by the manufacturer will be between 2 and 7 microns. This filter is always rated to prevent particles that can be harmful to a particular engineís fuel-injection system from making their way past this gate keeper. (Engine owners have no choice in which filter is used here.) So the advice you were given is correct. Clean the tanks and use sequential filtration, and your filters shouldnít clog before 100 hours of run time.

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