Beat the [diesel] bug

Feb 21, 2019 Boat Maintenance ,Technology

It is amazing how lifeforms have evolved to flourish even in some of the most inhospitable environments, writes Norman Holtzhausen, among them the bug living in a diesel fuel tank.

Generally termed the diesel bug, it’s actually a consortium of bacteria, yeast and fungi comprising more than 120 microorganisms. Of these, about 30 have the potential to cause damage to diesel fuel systems.

The main culprit is a filamentous fungus called Hormoconis resinae. By far the most dangerous, it not only blocks filters and damages pumps and injectors, but also causes pitting corrosion in metal tanks.

Scientifically this little bug has enjoyed some interesting research because of the possibility of using it to clean up oil spills. But for a boatie this organism (and its buddies) is a complete and irredeemable villain which needs to be exterminated as soon as it is discovered.

The bug thrives in the layer between diesel and water and, unfortunately, in a marine environment it’s almost impossible to eliminate water from a fuel tank. The bug consumes the oxygen in the water and eats the carbon and alkanes from the fuel.

Condensation happens when the tank cools down after use, and even sucking in moisture-laden air through the breather pipe as fuel is burned up will eventually result in some water collecting in the bottom of a boat’s fuel tanks. Worse, because boats are often left unused for long periods, conditions are perfect for Hormoconis resinae to thrive.

This little soup of organisms causes a number of problems, including aforementioned corrosion of fuel tanks, fuel lines and fuel system component failure affecting injectors and fuel pumps. But it’s the clogging of fuel filters which is likely to cause the most grief in the short term, as your engine is starved of fuel due to the restricted flow.

When you check the filter, you may see a black or dirty brown slime all over the element. In most cases the filter element has to be replaced – it’s almost impossible to clean. And of course, you will want to prevent any long-term, and potentially expensive problems from the bug’s corrosive properties.

In our case it was a ‘water in fuel’ warning message on the engine display that alerted us to the problem. Checking the sight bowl on the external filter, I saw some water and a number of floating objects. After clearing this out I ran the engine again, and after an hour or so got the same message. This time the filter was noticeably more clogged, so back at the berth we opened up the fuel tank for a look. Not a pretty sight!

Sadly, there is no simple one-size-fits-all solution to this problem.
If you catch it early you can kill the organisms, but as the infestation gets worse the solution progresses all the way to requiring a complete drain and mechanical clean of the fuel tank.

This is a messy and time-consuming business, so some owners elect to get someone else to solve it. Companies like Diesel Clean specialise in this task and will evaluate your problem before suggesting the most appropriate solution.

The good news is that you don’t need to throw away all your fuel, even with a severe infestation, as it can be ‘polished’ by running the fuel though an external filtration system to remove the organisms – while cleaning out the tank. If you have a very large tank – and don’t feel up to dealing with lots of fuel – make a call.

But assuming you have a small to moderate issue and are keen to deal with it yourself, what are the options?

We will deal with the long-term fix in a minute, but if you cannot address the issue immediately, the first thing is to stop the organism from repeatedly clogging your filters. Research shows a very strong magnetic field disrupts the bonds between the individual organisms and breaks down the slime.

So while it doesn’t kill the bug, the magnetic field seems to put the bug into a dormant state and stops it clumping. The effectiveness of this is well-proven, with scientific research available to back up the claims. Since the organism’s only around two microns in diameter, the separated microbes pass through the filter element to be burned up safely in the engine.

There are a couple of products available locally – including Purafiner and De-Bug – and both are available from numerous retailers. The physical dimensions of the models are slightly different, so once you know what flow rate you require (based on your engine’s peak fuel consumption) the final choice may come down to which is easiest to install in the engine bay. The unit’s installed in the fuel line before your existing filter, so that the clumps break up before they can clog the filter.

Given we’d recently filled our tanks, we immediately sourced a suitable unit and installed it into our fuel line. In our case the smaller Purafiner unit was the easiest to install as it provided adequate flow rates and could be fitted in-line into the fuel hose without a permanent mounting. But for larger engines the De-Bug unit may be more appropriate – it’s attached to a bulkhead. Installation was a 10-minute job, and after replacing the filter element and re-priming the fuel pump we were good to go.

Later, we dealt to the longer-term solution. The first step is to get rid of as much of the contamination as possible. While you can (and should) add some product to the tank to kill the organisms, there’s still that slimy gunk sitting in the tank that needs to be physically removed.

This usually involves running your fuel tank down to nearly empty and opening the tank up. In most cases the fuel sender for your fuel gauge sits on the top of the tank. It’s usually accessible and can be removed to get inside the tank.

Shine a light for a look. A camera can be useful if the tank opening is in an awkward spot. If you see any large clumps of gunk, first arrange to remove these. You may need a pressure washer to loosen them before they can be sucked out with a suction pump.

In our case we could see the tank was structurally sound but there were several ‘mats’ of the bug. A hand-operated pump was used to suck out most of this, including the last puddle of water that we could see sitting in the lowest part of the tank.

Once the tank was clean, the sender unit was re-fitted. Make sure the rubber gasket seals well – if not it could become a source of water in the tank or even cause a fuel leak. I reintroduced a small amount of fuel – about 10% of the total tank capacity – and shock-dosed it with a fuel treatment product.
Fuel Treatment

Previously the treatment of choice was a biocide to kill the bugs, but some of these are extremely hazardous. Luckily several less toxic products are available. Saltaway sells a product called Fueltreat BC-250, which can be ordered from its website ( or marine chandlers.

Although also a biocide, it is non-toxic to humans and safe to use. The initial treatment dose was based on the amount of fuel we added, and the additive was left to do its stuff for a few days.

If you don’t want to use a biocide, the Fuel Right range of non-biocide treatments is another option. These don’t kill the bug directly but break up the sludge and disperse the biomass into the fuel at a low level. This then allows the organisms plus the water to be caught in the filter without clogging it. These are also available from various retailers or direct from the Diesel Clean website (

After a while I opened up and checked the tank’s insides. In most cases this step is not required, but I wanted to check the effectiveness of the treatment. I could confirm there were no visible traces of the bug left in the tank, so topped it up and used the boat as normal.

Two complete tanks of fuel later have resulted in almost zero trace of water in the filter bowl, and the filter element only shows normal levels of discolouration. It seems the problem is solved!

Of course, it’s a good idea to continue adding a maintenance dose of your chosen anti-bug additive each time you fill the tank. A lower dose is required than the initial treatment – check the instructions for the appropriate dilution.

Add the correct amount based on how much fuel you top up with each time, not the tank volume. And hopefully you’ll have beaten the bug for good.