How to Deal with Diesel Fuel Contamination, Diesel Bug and Fuel Polishing

With the engine off, put a clear jar under the drain on the bottom of the primary filter (water-separator) bowl and open it slowly. Water is heavier than diesel and sits at the bottom as clear or cloudy bubbles or a separate layer. Drain until clean fuel runs, then close the drain. Do this before every trip in season and log what you see. Some engines also have a water drain on the engine-mounted filter; check your manual.

The filter bowl only shows what has already reached it. To see the tank, push a thin hose to the very bottom through the fuel gauge sender or an inspection port and pump a sample into a clear jar. Let it settle for an hour. Clear, bright fuel with nothing at the bottom is good. A water layer, sediment, a fuzzy or slimy layer between fuel and water, or very dark fuel means contamination.

If the tank has an inspection plate, have it opened (by you if access is easy, or by a yard). Look at the bottom and walls with a flashlight and mirror for water, black sludge or slime. Practical Sailor calls an easily accessible tank with big inspection ports one of the best defenses against fuel problems.

Before treating anything, figure out how the water got in:

• Deck fill cap: check the O-ring; a cracked or missing one lets rain and deck wash in.
• Tank vent: make sure the vent fitting on the hull isn't submerged when heeled or washed by waves, and that it has a screen.
• Bad fuel: if problems started right after a fill-up, tell the fuel dock and keep a sample.
• Condensation: a half-empty tank that sits for months breathes in moist air.

If the bowl showed water or gunk, change the primary and secondary filters (see the related guides). BoatUS says filters should be changed at the engine maker's interval, and at least once a year even with little use. Keep the old element in a zipper bag to show a mechanic: slime points to microbes. Bleed the fuel system if your engine requires it before starting.

A biocide kills bacteria and fungi; many "fuel treatment" or "algae" products don't. Practical Sailor notes that any product sold as a biocide must be registered with the EPA, and its tests found registered biocides worked while some additives that only disperse water did not kill bugs. Biobor's product page describes a higher shock dose for existing contamination, long-term storage or the first treatment, then a lower maintenance dose with later fills. Measure from the label for your tank size, and add it before fueling so the incoming fuel mixes it. More than the label says doesn't work better.

Killing the bugs doesn't remove them. Practical Sailor warns that dead biomass breaks loose from the tank walls and can clog filters quickly after a shock treatment. After treating, carry several spare filter elements, check the bowl often, and don't head offshore into rough water until the fuel has been filtered or polished.

Fuel polishing pumps fuel out of the tank, through fine water-separating filters, and back into the tank, over and over, until it comes out clean. It removes water, dirt and dead sludge, and stirs the tank bottom while doing it. Options:

• Mobile polishing service: a technician brings a pump-and-filter rig, polishes the fuel through the tank's fill or inspection ports, and often cleans the tank through the inspection plates.
• Built-in polishing system: a dedicated pump and filter that circulates fuel from the tank, run periodically. Practical Sailor's test boat used one and noted polishing makes the tank a poorer home for microbes but does not sterilize the fuel. Polishing works best when the tank has inspection ports so baffled sections can be reached.

If the tank has thick sludge, a heavy slime layer, or fuel that keeps clogging filters after polishing, the fuel usually needs to be pumped out and the tank cleaned by hand through the inspection plates. If the tank has no inspection ports, a yard may need to cut access. This is pro work.

For long lay-ups, store with the tank nearly full to leave little air space for moist air (leave room for expansion as the manual or the tank label advises). Practical Sailor says keeping tanks full plus a storage stabilizer can extend diesel storage life. Add the stabilizer and, if you use one, the biocide maintenance dose when you fill.

• Check and drain the filter bowl before each trip.
• Use fuel from busy fuel docks with high turnover.
• Some owners use a maintenance biocide dose with every fill. Practical Sailor even suggests alternating between two biocide chemistries during the year, since different bugs respond to different products.
• Keep the fill cap O-ring and vent screen in good shape.

Record the date, what you found (water, slime, sediment), what you used, and the next filter change. Patterns over time show whether the problem is coming back.