Swatting the diesel bug

Fuel Systems
Water in diesel fuel is not a good idea: it promotes corrosion, encourages damaging organisms and reduces the efficiency of the engine. So how does water get into DERV, how do you know if it’s there and how can you get rid of it, asks Toby Clark

Diesel fuel itself is hygroscopic — that is, it attracts water vapour from the air in contact. Most sources agree that biodiesel (found in most DERV) is substantially more hygroscopic (and some processes for making biodiesel result in a higher initial water content). Water vapour also condenses on the walls of cool, mostly empty fuel tanks.

Water and diesel fuel are immiscible liquids, so when they are mixed they form an emulsion. This consists of droplets of water suspended in the fuel. The different densities of the liquids mean that eventually the water will separate out into ‘free’ water, but there are suggestions that the additives in ultra-low sulphur diesel (ULSD) make the water more difficult to remove. These surfactants (and some fuel conditioners) are designed to improve lubricity, but tend to reduce the size of water droplets, making the emulsion more stable.

If water is finely emulsified, it may not cause an immediate problem: it stays in the fuel until it reaches the cylinder, and combustion vaporises the water, so it can leave with the exhaust gases. But the water reduces the net energy content of the fuel (sapping power and affecting fuel consumption) and can reduce exhaust temperatures enough to make aftertreatment ineffective.

If the emulsified water separates out as ‘free’ water, the problems are more obvious: rough running, low power and eventually corrosion in the tank, lines, injectors and cylinders.


Worse still, water provides a habitat for micro-organisms which feed on fuel (both biodiesel and fossil diesel), leading to the so-called ‘diesel bug’ or ‘fuel fungus’. This is rarely a single type of organism: while it may include fungi such as Hormoconis resinae, pictured top right, it is likely to be a ‘consortium’, or mixed population of bacteria and fungi. These can cause a variety of problems:

  • generating organic acids which lead to corrosion
  • converting sulphur compounds into toxic hydrogen sulphide
  • producing surfactants which promote emulsification
  • fostering sludge-forming biomass which can block filters and injectors.

You can test for organic matter in diesel; most kits take some time to give a result – up to 72 hours – but some of the latest tests, such as Fuelstat from Conidia Bioscience, work in only 15 minutes.

Free water at the bottom of a fuel tank can be removed using a pump (observing fire safety rules, of course) but this is not always practical or effective. Alternatively, you can use sodium polyacrylate, a Super-Absorbing Polymer (SAP) which can absorb up to 130 times its own weight in water. Granules of the material in a mesh sack are lowered to the bottom of the fuel tank. After a few hours the SAP will have absorbed as much free water as it can. SAP crystals will not readily absorb water from an emulsion, but one supplier says that a sufficient quantity will dry diesel after 24 hours.

Water can be filtered from diesel fuel, using a relatively coarse filter element: filters such as the Piusi Water Captor (designed for use with a bulk fuel pump) use a 30µm [30 micron] paper element. For comparison, a typical in-line fuel filter uses a 5µm element. In fact, these elements act more as water absorbers than as mesh-type filters: tiny droplets of water in emulsion are agglomerated into larger droplets on the filter mesh, until they get large enough to fall out into the bottom of the filter.

In-vehicle water-removing filters are rare, but if one is fitted it should be placed before the main fuel filter, and regularly inspected to see if it is full. Some are fitted with a drain point at the bottom; Australian manufacturer Direction Plus fits a water level sensor to the bottom of its filter, which sounds an alarm when the element needs to be drained.

A variety of off-the-shelf treatments are designed to remove water from fuel or mitigate its effects. These tend to have either an emulsifying effect – suspending the water in the fuel so that it can go through the engine and out of the exhaust – or the opposite: demulsifying the water so that it precipitates out of the fuel and can be removed from the tank. But beware some of the more outlandish claims about treatments: one claims to turn water into hydrocarbons which can then be burnt…

Biocide treatments are a proven way to remove ‘diesel bug’ organisms; ideally they should be used in a bulk tank, but can be used directly in a vehicle’s tank. Exocet says that its biocide, Anti Bug Kill, should be added to the tank in appropriate quantities for a full tank (one litre treats 5,000 litres of fuel). Then the tank should be filled with clean fuel and left to stand for 24-36 hours. After this time the micro-organisms will be dead; the fuel can still be used, but you should change fuel filters after a few days.


Water is significantly denser than diesel fuel (DERV’s specific gravity is around 0.85) so free water sinks, while emulsified water is dispersed in the fuel. How can you detect whether there is water in your fuel without draining the tank? Professional testing typically uses a technique called Coulometric Karl Fischer titration. This can be done manually, but labs usually have automated testers. The sample can be tiny (5g is enough for most purposes) and the technique can measure concentrations of water as low as 100ppm (parts per million) or 0.01% — but the sample has to be representative of the whole tank, so it needs to be thoroughly mixed before a test can be done.

Portable testers pass a measured sample of fuel through a paper pad treated with sodium fluorescein. When the pad is exposed to UV light, it will glow due to fluorescence if water is present — more brightly if the water content is higher — and a sensor uses this to assess the water content. It can be used in the field, and can measure minute quantities of water (down to 1.5ppm).

Electrical sensors can detect water which has settled in a tank or a fuel line. The simplest are conduction based, working off a low-voltage DC power supply. A promising method used in experiments is electrochemical impedance spectroscopy.

One useful way of finding free water seems almost too simple: an inexpensive water-detecting paste such as Kolor Kut or Arkema SAR-GEL Blue. Smear it on the bottom of a non-contaminating stick (a piece of aluminium extrusion is perfect) or a weighted line, and lower it to the bottom of the fuel tank for a few seconds. If there’s free water, the paste at that point will change colour — from a pale brown colour to a bright red in the case of Kolor Kut, or from orange to blue in the case of SAR-GEL.

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