It's crucial to check any fuel going into your tank, especially if the source may be low quality. James Devoy investigates marine diesel bug

Nearly two years of lockdown have left increasing numbers of yachts laid up and unattended around the world. The scramble to return home before travel restrictions started in 2020 meant some sailors had to abandon their boats wherever they could or risk being stranded.

Even cruisers who remained with their yachts often found themselves immobilised in marinas. Restrictions are still ongoing in many areas, and organising maintenance remotely can be challenging. This has left an increasing number of owners returning to their yachts to find a sticky and slimy problem: diesel bug.

What is diesel bug?

Diesel bug is a mixture of different microbes, bacteria and fungus that can live and thrive in the interface between water and diesel in your fuel tank. Like most bacteria it will reproduce quickly to potentially become a serious problem in a short time. It particularly likes warm weather but can strike anywhere.

Once pulled through into your engine the ‘bug’ can cause problems in almost every part of it, increasing fuel consumption, clogging filters and reducing fuel flow to a standstill.

The introduction of biofuel or FAME (Fatty Acid Methyl Ester fuels) into marine diesel has given the bug a more readily available source of food. With the source of biofuel being so diverse – literally everything from fish oil and rapeseed to the fat from a fast food fryer – there are a whole plethora of new bugs hungry for it.

Knowing how to change fuel filters is the first step to getting your engine running again when clogged with diesel bug. Photo: Tor Johnson

Biofuels are hygroscopic (ie over time they absorb moisture from the air) and it is this moisture that allows diesel bug to thrive. Over 1,500 forms of diesel bug have been identified so far. The reduction in sulphur in fuel over recent decades has been necessary to reduce marine diesel’s environmental damage, but has exacerbated the issue by creating a much less caustic environment for the bug to live in.

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Diesel bug can be brought into your tank from an infected fuel source, but also lives in the air to be absorbed into biofuel. Filter changes and regular use will usually shift low levels of infection, but long periods where the engine is unused or unserviced can create an environment where the bug can thrive.

Diesel bug can manifest itself as a thin carpet of sludge. Photo:

Diesel bug microbes appear as brown slime in your fuel tank but can also be seen as dark particles in your filter bowl. A good filter should be able to stop most of the bug getting into your engine, but none are perfect and at sea you may not have enough filters to replace all those that get clogged up.

Nichola Wright experienced a sudden engine stoppage aboard her Kelly Peterson 44 on passage from Ibiza to Mallorca due to diesel bug, despite having a dual fuel filter system. “We were surprised to have the problem as we’ve always treated the diesel we buy with conditioner,” she explains. “Perhaps it was a result of the extra long winter layup – we were eight months in a marina due to Covid. The engine had not been used in that whole time and the fuel had been left in the tank, topped up to full with a fuel treatment added.”

Modern diesel fuel is so highly refined that it begins to break down after 6-12 months. This is when the danger begins for diesel bug. As water is more dense than diesel it will sink to the bottom, allowing the bug a better hiding place from any cursory checks.

Viki Moore, a cruising sailor based in New Zealand, also fell foul of ongoing diesel bug issues on her Young 88 Wildwood. After many replaced filters the bug caused the engine to fail at a critical moment: “It was blowing about 25 knots on the nose, and we were weaving our way in between derelict marina piles towards a cradle,” she recalls.

“We were about 200m from our intended destination when the motor cut out. With the wind howling in my ears, and sails all packed away for the winter, we started getting quickly blown back towards the rocks. I panicked, madly pressing at the engine start button, but there was no response. Thankfully my crew member casually strolled up to the bow and threw the anchor over the side.

“We were safe, but stuck, with no motor, no windlass to help retrieve the anchor, confidence knocked and no appetite to unpack the sails and attempt to sail around the old marina piles that surrounded us, we had no other choice but to put the kettle on and wait for rescue.”

After being safely towed to a berth, Moore was able to assess the problem, but the tank’s location meant it eventually had to be cut out of Wildwood before being steam cleaned by a specialist and refitted.

Avoiding diesel bug

As a first precaution to avoid diesel bug, make sure you have a tight seal on a well fitted fuel cap to keep out rain and saltwater. Keeping fuel tanks full also reduces the chance of condensation inside the tank.

Peter Weide is a former marine engineer and director of Marine 16 and MarShip, companies that specialise in chemical and mechanical solutions to the diesel bug problem. We asked his advice on using fuel additives to help prevent or kill a diesel bug infestation:

“That’s a broad subject depending on how long you store your diesel. As a minimum the fuel additive should contain a lubricity improver and a detergent.

When changing filters it can pay to check the fuel line connections for sludge too. Photo: Nichola Wright

“There are some additives based on old technology that enable the free water on the bottom of the tank to be absorbed into the body of the fuel to ‘safely’ pass through the engine. That was fine a few decades ago, but if you put saturated diesel into a modern common rail engine you will certainly have very expensive engine problems.

“All tanks have to breathe and as they do the moisture comes in. With water comes diesel bug, sooner or later. So get rid of the water and dose with a ‘maintenance dose’ of biocide which will kill them before they take hold.”

Fuel polishing

Dominic Heydon and Carly James battled the bug on board their Trident Warrior 38 Cadoha (see They tried different ways to beat the problem. “We consulted forums and YouTube videos before being pointed in the direction of a ‘fuel polishing’ man who came out to ‘clean our system’ to the tune of £600!”

Nichola Wright’s clogged filter (left) cpompared to a new one after an extended layup. Photo: Nichola Wright

“I showed him where our two tanks were and he said the tanks are too tight to get his arms into to clean, but he could still polish the fuel.”

Despite having eliminated the bug, their baffled tanks were still clogged with slime so they turned to a mechanical solution to clean those areas hidden by baffles.
The Diesel Dipper is a small retrofitted filter that pulls fuel from the very bottom of the tank and can separate out any residual water, as well as diesel bug, meaning it should reduce the chance of reinfection as well as cleaning the fuel in the tank.

“We run the Dipper whenever we have our engine on and we especially have it on whenever we’re in a bumpy seaway,” Heydon explained.

“The more we use the Dipper, the smaller and smaller the ‘sludge’ is in the samples. It seems to be chipping away at anything that’s in there, managing the situation, but most importantly (for us) keeping the engine from getting blocked up and failing on us.”

Manually cleaning diesel tanks

Several biocides are on the market aimed at killing diesel bug, but once dead it still leaves a residue inside the tank. Bones Black, a marine engineer, round the world sailor and skipper of charter yacht Emily Morgan (see shared his tips for how to clean your fuel tank:

“Pump nearly all the fuel out of the tank into clean cans through a good quality filter system, they can be hired or get someone in to pump it out for you.

“Leave a centimetre of fuel in the bottom and use this to clean the rest of the tank out. Most baffles should not go to the top of the tank so you can reach each section and give it a scrub with a dish brush on a stick, or even a loo brush bought for the purpose!

“Once you’ve scrubbed all the faces and corners of the tank lay absorbent pads on the base until all the fuel residue is soaked up; we use nappies as they are cheap and do a great job. They also have a plastic outside face so can be picked up and put in a sealed bag with very little mess. Finally clean and dry with a good quality rag.”

If caught by the bug while on passage, Black suggests a short term fix which should get you back to shore safely: “If you can’t use the [fuel] tank, you can sit a can of clean fuel next to the engine, remove the feed pipe from the outlet of the fuel water filter, drop it into the can and run the engine directly from the fuel can to get you home.”

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