Diesel Fuel Polishing Systems


If you plan on doing some long-term cruising with an auxiliary diesel, and expect to be picking up fuel in out of the way places, or you find you find yourself changing dirty fuel filters frequently, then it might worth the time and expense to install a fuel polishing system. First, of course, you want to diagnose the source of the problem.

A fuel polishing system is essentially a recirculating fuel line, a pump and a pre-filter. It won’t fix whatever is causing your clogged filters—likely algae (growing in untreated fuel) or asphaltenes (microscopic tar particles in the fuel) are common. However, it will set you on your way again, and delay or, with routine use, avoid the next clogged filter.

As the fuel is polished it is circulated through coarse fuel filters with a water separator until the fuel runs clean and dry, with all the water stripped away. The severity of the contamination will determine how coarse a filter to use, but many polishing systems advise starting with 30 microns and working down to 10 micron. Ten microns is the common coarseness for a pre-filter; the final spin-on engine-mounted filter is usually between 2 and 7 microns. If your fuel is severely contaminated, these filter expenses can add up.

You could plumb some fuel-rated valves and hoses and use the ship’s own fuel lift pump and pre-filter to create a polishing system system. However, the OEM pre-filter cartridges are usually expensive, and each time you polish the tank, you are putting an extra burden on your lift pump.

One of the first skills the new owner of an old auxiliary-powered sailboat masters is changing the diesel fuel filters and bleeding the fuel lines. The next step is to address the source of the gunk that clogged the filter, but even meticulous fueling practices and a well designed fuel tank won’t guarantee that dirty diesel fuel will never haunt you again.

Our former boat, a 60-year-old ketch, had black iron tanks that proved to be one of the worlds most efficient microbe farms. Rather than replace the diesel fuel tanks (which was beyond our budget), we had the tanks professionally cleaned, changed the fuel-feed setup, and installed a 7-gallon day tank with a home-made fuel polishing system (using a continuous duty oil-transfer pump and an old Dahl 101 filter housing, which used relatively cheap replacement filters). We never had fuel problems again, but I wouldn’t recommend this arrangement for a number of reasons, including the fact that the Dahl filters were a pain to change. Although more expensive, an integrated and purpose-designed pump module and filter like the Parker Fuel Polisher is much easier to install and maintain.

Practical Sailor has looked at diesel fuel topic from many different angles and anyone dealing with fuel issues on an old boat would benefit from three articles in our archives. The first is our in-depth look at fuel tanks, focusing primarily on installation and material selection. (Aluminum is a cost effective choice, but good installation is key.)

The second article is our recent look at diesel fuel additives. This controlled test examined what worked best against bacteria and fungus in a diesel fuel. As we found from that study, trying to cure a contaminated diesel fuel tank with additives alone is not nearly as effective as a regimen that includes polishing.

If you do decide to build your own polishing system, just be sure to follow ABYC guidelines for fuel systems, at the peril of nullifying your insurance coverage—or worse. If a DIY fuel system upgrade is too intimidating, then check out our short reviews of the fuel polishing system from Parker, parent company of the well-known Racor brand fuel filters, and a similar unit, the original Filter Boss. Although the original Filter Boss is no longer available, the Keenan Filter is closely based on that system.

And if you need your fuel filtered right away, most ports have one or more mobile services that can filter your fuel for for you. The service isn’t cheap, until you consider that it costs $300 to replace the filter on the high-volume fuel polishing carts that some of these small companies use.

If you are meticulous about fuel and curious about which “miracle-cure” additives on the market actually work, we’ve got a complete ebook “Marine Fuel Additives,” addressing the care and feeding of your fuel, whether it’s gasoline for the dinghy or diesel for the mothership.


Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him by email at practicalsailor@belvoir.com.


  1. Fuel Rite additive has a very convincing webpage with video on how it’s product turns the bad stuff into burnable fuel. I first sourced it from home heating oil companies who used it to prevent their large storage tanks full of fuel from going bad. So if it is good enough for them …..? The supplier I get it from in Ontario Canada has a polishing business mostly based on back up generators for hospitals ect. Again if it’s good enough for him…? cheers RL.

  2. We use the following system to good effect:
    First stage: 2 pass 30 micron/20 micron
    Second stage: 2 pass 10 micron/5 micron
    On engine: 10 micron
    Both the 1st and second stage filters have vacuum gages with telltales.
    I’ve got a squeeze bulb plumbed in to assist in priming
    All filters are Cummins spin on. Relatively inexpensive and easy to change.