Beware Foul Fuel in Used Boats

Vigilant observation and maintenance of fuel filters against water and sediment in fuel systems can prevent expensive engine problems and tank corrosion.

When you’re buying a used boat and hire a marine surveyor to confirm the condition, one of the more difficult tasks for the surveyor is to judge the condition of the fuel system. Water in gas, particularly ethanol-boosted gas, is quite common in pleasure boats that go unused for prolonged periods. The ethanol actually attracts moisture, which is why tanks should be topped off during storage periods, leaving less room for condensation. Water can also intrude in any boat through poorly sealed fuel caps and vents.

Microbes that feed on hydrocarbons and cause sludge can only flourish if water is present. These same algae emit sulphuric acid as a waste product which further corrodes tanks, pumps, and injectors. Given that in diesel engines, 90 percent of all problems are fuel-related, fuel system inspection should be an area that gets specific attention.

Water or debris in the fuel can be an indicator of deeper problems, namely lack of maintenance, or fuel tanks that are contaminated. So, when a surveyor gets onboard for an inspection he will typically check the condition of engine hoses and fuel filters prior to running an engine.  However, some boats that are serviced at layup may show new hoses, clean bowls and filters, yet still have problems in the fuel tanks.  When running the engine look for black smoke, lack of power, or hesitation in acceleration as further indicators of fuel problems

Surveyors should check for water in tanks by using a water-indicating paste, and whenever possible visually inspect the inside of tanks.  After running an engine, he should also recheck the filters. Simply adding products like Biobor to a fouled system will only bring the problem past the filters and into the engine.  Biobor and similar products should be used as preventatives, not cures. The only real cures for fuel contamination are to completely replace the fuel or hire a company to do fuel polishing (filtration) along with a thorough tank cleaning. The presence of algae, discoloration, or sediment should not necessarily kill a sale, but you would be prudent to request that the owner pay for fuel polishing or tank cleaning from a reputable professional.

Sediment in fuel can happen to anyone, even meticulous owners. Testing fuel in a boat you are about to buy is prudent.

The worst case scenario is finding that you have bought a boat with corroded tanks that need replacing. This can be expensive, particularly if they are baffled custom tanks, or, as is the case in many pleasure boats due to the manner of construction, require that the decks be cut away to replace the damaged tanks.  Don’t be fuelish, request extensive investigation of a used boat’s fuel system from your surveyor.

For more about this topic, read Ethanol and Water in Fuel on


  1. WaterBlogged Foul fuel systems in used pleasure boats are more common than you might think and can be expensive if not dealt with.

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