Diesel bug can clog filters, wreck engines, corrode fuel tanks, and leave you powerless at sea. Ben Sutcliffe explains how to keep your diesel clean

How to avoid diesel bug

Diesel bug

Ben Sutcliffe-Davies is a marine surveyor and former boatbuilder who cruises the Bristol Channel from Swansea

During a balmy 1970s summer, when I was about 11 years old, I used to crew Miss Deben, Felixstowe Ferry Sailing Club’s 18ft rescue boat.

Part of the skipper’s checks before leaving the craft on her mooring involved topping up the 8hp Lister diesel’s fuel tank.

‘Damn’ said the skipper, ‘the water gauze in the filler funnel is missing.’

The funnel had probably been stepped on while we were recovering racing marks.

Skip cleaned the funnel as best he could and I held it over the fuel tank while he poured fuel from an old PVC five-gallon drum, which was about a third full.

Three weeks later, the craft lost power.

The club’s rescue officer arrived and found the fuel filter half-full of water and dirt.

We concluded that the PVC drum had evaporated and condensed enough water during the hot weather and, without gauze in the filter, we had added it to the fuel tank.

That’s when I learned Rule 1 in fuel management: respect the product.

Over forty years later the make-up of diesel has changed dramatically, making contamination even more likely, but still many yachtsman are falling foul of Rule 1.

The big difference now is diesel bug, and the sheer cost of what it can do compared to just dirty old fuel with a bit of water in it.

What is diesel bug?

Diesel bug

About 100 types of bacteria can live in fuel, feeding on condensed water and the diesel’s bio components

It’s caused by microbial bacteria that enter the fuel tank and feed on the water, nutrients and hydrocarbons that our biofuels now contain.

Temperature also plays a role in how quickly they develop.

They like it between 5-70°C but thrive around 30°C. Over 100 variants of these organisms have been identified, some airborne.

It can be expensive

Diesel bug

Replacing a filter clogged with diesel bug is the least expensive effect of contaminated fuel

Boatyards’ hourly labour rates are £45-70, depending on the engineer’s qualifications.

Cleaning tanks, replacing affected fuel lines and filters can run to hundreds of pounds.

If the injectors, or worse still the fuel pump, are affected, that can lead to bills of £400-£3,000.

I remember one incident where fuel became contaminated with microscopic rust particles from jerrycans. The owner had to pay over £4,000 for a new fuel pump, injectors, and a tank flush.

The bio content in some fuels will increase in the next few years.

The current fuel standard, BSEN590, specifies a maximum of 7% bio content but the oil giants are waiting for the Government’s say-so to introduce fuel with 12% bio content.

It will come with the caveat that, once it reaches your tank, it may last only six months.

Environmental concerns, and the fact that it’s cheaper to produce biofuels, mean that percentage will keep rising. Indeed, many UK firms process and sell biofuels with percentages from 20-100%.

What are the signs?

Diesel bug

Black sludge is visible at the bottom of the fuel line in this empty tank

The most obvious sign is that your engine stops.

Recently, YM reader Jeff Wrinch reported that diesel bug blocked a banjo connector in his filter’s fuel line, stopping the flow of fuel.

As with fuel tanks, bugs can and will damage some metals.

A sluggish response, excessive smoke from exhaust, poor starting or the odd beat missed can be an early indication of diesel bug.

I know of one incident where diesel bug prevented the injectors working properly, providing unbalanced power that damaged the engine’s crankshaft.

What can I do to prevent diesel bug?

Diesel bug

Forecourt fuel may help your engine run better, but how clean is the jerrycan?

For yachts that carry less than 10-15 gallons of fuel, sometimes it’s cheaper to dispose of fuel at the end of the season, clean tanks and start afresh in spring.

For larger craft that’s not economical, so samples can be sent for lab assessment (there are some DIY test kits) and the fuel can be treated.

Diesel bug

If the can isn’t clean, filter funnels can prevent debris and water entering your tank

Store fuel properly. Biofuel has a shelf life, so where possible try and keep its temperature around 5-10°C and stable if possible.

If in doubt at the start of the season, sometimes the only option is to dump it properly and start afresh.

Diesel bug

A filter funnel in poor condition is worse than useless

Check your deck filler

Diesel bug

Left: a good design – elevated, has a good O-ring, traps no water. Right: a poor design

When I survey boats, I often find the deck filler design traps a little water in the cap, which drops down the fuel filler pipe.

Other caps have missing or perished O-rings, which will let water in.

On some older craft, the diesel filler is in the cockpit sole and these can leak, too.

It’s often prudent to relocate it – but not to a sidedeck: it shouldn’t be submerged and spilled fuel is a slip hazard.

Should I keep the tank full?

A fuel pump being used to fill up a yacht's fuel tank

A full tank limits condensation but a diesel heater blasting away will use fuel, and heat will condense water

Yes, but it needs to stay full.

Many owners top up tanks before leaving the craft, but set their diesel heater to run occasionally while they’re away.

If the heater draws from the main tank, the fuel used creates an air space in the tank, plus the hot and cold periods can assist the bug’s growth.

Putting some heat through a boat in winter is good, but don’t overdo it. Background heat helps keep your craft dry and aired but I’ve been on some boats where going below is like arriving in the Med: far too warm.

Inspect your tank

A man changing an fuel filter in his yacht engine

Regularly check filters for contamination to avoid mechanical problems


Make sure there’s no copper, brass, bronze, lead, tin, or zinc in contact with the fuel, as these react and hasten degradation.

I prefer stainless steel but some of the new high-grade linear polythene tanks claim to be corrosion-free and less prone to condensation than other PVC or metal tanks.

Your fuel tank should allow you to drain off a small amount from its lowest point, to let you remove any water and check the fuel’s condition.

Many boats don’t allow this, so for fuel sampling I have a tube long enough to reach the bottom of the tank and a small hand vacuum pump to pull fuel into a clean bottle.

You could also check for a ‘bug mat’ of growth in the tank, using an endoscope through the inspection hatch.

A desiccant air filter in-line with the breather pipe will help keep moist air out of your tank

A desiccant air filter in-line with the breather pipe will help keep moist air out of your tank

If the tank is susceptible to engine room or cabin temperatures, try insulating it with fire-proofing to keep the fuel temperature range as low as possible.

Lastly, when fuel is drawn off, air is sucked in, but where from?

Check your breather vent has a proper swan neck to prevent water getting in.

Remember the air drawn in to your fuel tank holds at least 15-20 per cent moisture. You could fit an in-line desiccant breather for as little as £20.

I’ve got diesel bug in my tank. What do I do?

Most companies that treat diesel bug use similar biocides or enzymes – some eco-friendly, a delicious irony – but none can reverse damage.

Depending on your type of fuel, some will stop the bug’s growth but not kill it.

Most firms recommend a biocide or, in severe cases, a ‘shock’ treatment involving a big dose of chemicals that take time to work, then removing the dead bug from the tank and changing the filters.

For larger amounts of fuel, ‘diesel polishing’ removes water and impurities including the bug, but if the root cause has not been properly addressed, it’s likely to return.