Marine fuel polishing systems and DIY approaches

A fuel polishing system pumps diesel from the tank through a fine filter and back to the tank in a continuous or scheduled loop, keeping fuel clean between engine service intervals. Two categories: (1) STANDALONE polishing (Reverso FP-115, ESI Fuel Filtration, Alfa Laval): dedicated pump, filter housing, timer or continuous run, plumbed via separate suction and return fittings. Recommended for anything over 500 gallons of tankage or vessels that sit for months at a time. Typical spec: 100 gph flow, 2-micron final filter, plumbed to draw from the tank bottom (where water and debris settle) and return to the top. Run 1-2 hours per week when the boat isn't underway. (2) DIY / SECONDARY RACOR loop: a second Racor plumbed in a recirculation configuration, using a small 12V pump (Facet, Walbro solid-state) with a timer relay. Cheap ($400-800 vs $3000-6000 for a commercial system), but you're the one who designs the plumbing and picks the flow rate. Rule: never polish while underway if the polishing draw shares the engine's fuel line, you'll create suction anomalies at the engine's Racor and cause pressure spikes. Filter selection: 30-micron first stage, 2-micron final. When you find sludge in the filter, do a full tank clean-out (see diesel bug entry); polishing catches ongoing debris but won't dissolve an existing biofilm. Water alarm on the Racor bowl is worth adding, you know the moment the tank picks up water.