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  • Writer's pictureDieter Peschkes

Diesel Bug, What causes it? How to treat it.

Updated: Nov 1, 2023

The diesel bug is becoming more and more of an issue and can be costly to resolve. On top of that, OceanTrax are finding that an increasing number of yacht delivery passages are held up considerably with engine problems caused by the bug. Here is our guide to preventing the bug and what you can do if you have it.

WHAT IS THE DIESEL BUG? Diesel bug is microbial bacteria that enters the fuel tank and feeds on the water, nutrients and hydrocarbons that biofuels contain. It thrives in the interface between fuel and water just off the bottom of the tank. Here the micro-organisms form colonies and breakdown the alkanes in the fuel using oxygen from the water, precipitating a black sludge at the bottom of the tank. This sludge is formed from the microbes’ excretions and badly emulsified fuel, and consists in part of thread-like compounds a few microns (micrometers) in size that quickly agglomerate to block fuel filters and stop your engine. There are approximately 150 types of these bacteria in non biodiesel, some of which are airborne, but with the introduction of new fuels a whole host of new organisms are introduced – over 1,400 at the latest count. Biofuels may have elements of fish oil, rapeseed oil, vegetable oil and animal fat. Adding all these extra oils makes the fuel attractive to many more kinds of microbial life by offering a more varied diet. They like it between 5-70°C but thrive around 30°C.

WHAT ARE THE SYMPTOMS? Your engine will stop! But before this the early signs are poor starting, sluggish throttle response, excessive exhaust smoke, or the occasional missed beat /drop in delivery of power. If any of these symptoms are experienced traces of the bug should be visible in the prefilter in the form of sludgy deposits. OceanTrax regularly check all filters before a yacht delivery for signs of the bug before leaving the marina. If you have a polythene/PVC fuel tank and access is good, it may be visible at the bottom of the tank. At this point a filter change may be enough to get you out of trouble and back to the marina, but it is not a fix and the problem is in the tank. If left unattended the damage could spread to the fuel pump and injectors leading to big costs.

WHAT CAN I DO IF I HAVE IT? Most companies that treat diesel bug use similar Biocides and dispersants. Both rely on the bug’s reliance on water: biocides by dehydrating the cells or blocking their ability to feed, and dispersants by lifting the water into suspension, destroying the micro-organisms’ environment. They will then either starve or be burned in the engine, together with the water. In bad cases a *shock” treatment can be used which consists of a heavy dose of additives.

Dead bug in the bottom of a tank will have to be removed or it will at best block filters at worst damage engine components. If the contamination is too bad to be treated with chemicals, the other options are fuel polishing which will remove water and impurities including the bug, and in severe cases removal of the fuel tank and cleaning along with the fuel lines.

On one recent yacht delivery OceanTrax had to stop in northern Spain before a Biscay crossing to have fuel tanks removed cleaned and fuel lines cleaned. The whole process took several days and cost in both time and money. The main reason the bug had taken hold was due to the vessel left being unattended for a long time with low fuel tanks while waiting to be sold.

HOW DO I PREVENT IT? As with everything prevention is better than cure. If there is no water in the tank, diesel bug cannot survive. But in reality, in a marine environment it’s impossible to prevent water entering your tank. Even if you manage to avoid water entering through your filler, condensation will form in the tank. The air entering your tank through the breather as you use fuel will have a moisture content of at lease 15-20%. A desiccant air filter in-line with the breather pipe will help keep moist air out of your tank. Keeping your tank full when your boat is not in use will stop condensation, but in practical terms keeping your tank permanently full when the boat is in use is not possible. Also, how do you know that the fuel you are putting in is totally clear of water? Regularly draining off the water and debris from the bottom of the tank will help and if access will allow, fit a drain with a valve at the lowest point.

If filling your tank from a jerry-can, make sure the fuel in the can is fresh and the can has a gauze filter. Check your deck filler. Quite often older boats have lost the rubber “O” seal that sits inside the filler cap or it has perished and become hard. As water washes over a deck, this will allow some to enter your tank. Consider fitting a filler cap with a raised neck. Although these are not as ascetically pleasing as a flush deck fitting and certainly a possible toe breaker if in the wrong place, they are better at keeping the water out.

Keeping your boat warm during the winter when not in use is a good idea, but remember if your heater is drawing from your fuel tank it will create a gap at the top for condensation. Better is a small electric fan heater and a dehumidifier. (Your yacht delivery crew will also appreciate a warm dry boat on arrival!) If possible insulate your tank to prevent big temperature fluctuations reducing moisture build up.

Finally remember biofuel has a shelf life, so when storing fuel try to keep it at a low, constant temperature. If keeping for more than 6 months it may be worth disposing off rather than putting in your tank.

OCEANTRAX YACHT DELIVERY RECOMMENDATIONS FOR DIESEL BUG TREATMENT Grotamar 82 was developed to combat problems from low-sulphur biodiesel blends. The 92% kill rate is good and product does a good job of dispersing the water into the fuel. Formerly Soltron, Enzyme Energy works fast, scattering microbe colonies breaking up contamination to be burned in the engine. Marine 16 diesel bug treatment disperses into both the water and fuel phases in your tank and will remain sufficiently active for over a year at both high and low temperatures. Marine 16’s Diesel Bug Treatment is the fuel treatment of choice for the RNLI, The Royal Marines, Sea Start and River Canal Rescue.

If you would like to know more about preventative engine care that can save you money read OceanTrax Yacht Delivery guide to winterising your engine


Owner & Manager of Oceantrax Yacht Deliveries


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