This marine diesel engine had a contaminated fuel filter. As you can see, it was almost fully blocke

How to - Treating Diesel Bug

Regular engine maintenance is essential for reliable performance at sea but one area commonly overlooked is the condition of a boat’s marine diesel tank. Sure a preseason inspection of fuel tank fittings, seals and hoses is recommended, but the real issue lies within the tank and this, says John Perham of Diesel Clean, is water.


How to treat diesel bug

Because water is heavier than diesel it sinks to the lowest part of the tank and provides the oxygen necessary for the growth of a microbiological organism we commonly refer to as diesel bug.

Propagating quickly in warm, humid climates diesel bug generates acids that pass through the fuel filters and corrodes the highly polished metal surfaces found on the inside of fuel injection pumps and injectors.

This corrosion can over time lead to fuel leakage past the plungers and into the engine’s oil system compromising oil viscosity. That may result in crankshaft and/or main bearing failure.

Perham says it is not a question of if but when, and cautions that newer common rail diesel engines are particularly susceptible to damage from water ingress because of the high volume of fuel returned to the primary diesel tank.

“This return flow is warmed by the engine and raises the temperature of diesel in the tank. As the tank cools, condensation forms on its walls and settles on the floor near the fuel pick-up, creating the perfect environment for diesel bug to take hold and prosper.

“The problem occurs when you start your engine. Water at the pick-up can overwhelm primary and secondary filters and enter the fuel pump and injectors,” he said.

Perham adds that while fuel treatment additives go some way to preventing the growth of diesel bug, the only real way to prevent it is by removing its life source — the water.


How does diesel bug contaminate fuel?

According to our expert there are up to 30 known micro-organisms that can degrade diesel and these predominantly comprise of bacteria, yeasts and fungi. All, however, are airborne.

“They exist in the water molecules that are suspended in the air,” says Perham. “It is from the water that these organisms extract enough oxygen to thrive. And because fuel tanks must be vented this creates a pathway for air and microbes to flow in and out of a diesel tank.”

He explains even small changes in atmospheric pressure can result in air being sucked into and expelled out of a tank. Ordinarily this would not cause problems because most microbes require a lot of oxygen to multiply.

Diesel tanks provide something of a gourmet environment for microbes because, as we know, water is heavier than diesel and sinks to the bottom of a tank and settles on the bottom. This is known as ‘water bottom’.

Effectively providing an interface between the water and the fuel, water bottom allows microbes to dine at both tables — extracting the oxygen necessary to thrive from the water below and sourcing energy in the form of carbon and nutrients from the diesel above.

Perham says that when fuel in a diesel tank is warmed due to engine return flow, microbe growth can be extremely rapid. “What typically occurs now is this: as water in the fuel is gradually burnt off it leaves behind a build-up of slimy, black sludge on the tank floor which, in a seaway, mixes with the diesel.”


Fuel tank water ingress

The major cause of water ingress in diesel is from condensation and for this reason the best defence against diesel bug is to keep your tank topped up. This will go a long way to preventing condensation in the tank because air is unable to be exchanged. And that is simply because there is no air-space present.

Perham says faulty deck fillers and badly designed diesel air vents are also a common cause of water ingress into diesel tanks and should be checked annually. He advises the use of O-ring seals under the deck filler caps to prevent moisture wicking down the threads and into the tank.

“Poorly configured diesel air vents (those exposed to fresh or sea water) are a much more common cause of ingress and harder to rectify unless you know what you are looking for,” Perham continues. “Generally there is no quick fix and we’ll often have to reconfigure the vents. This is not difficult in itself if the vent lines (hoses) haven’t been built into internal joinery or liner.”


What does diesel bug look like?

Diesel bug can have many guises but will predominantly make itself apparent in the form of a slimy, black material or grit in diesel filters, tank floors and other fuel system components.

Other telltale signs include pitting and corrosion of fuel system components due to the acids produced during the metabolic process. Fuel colour (darker and more opaque) and fuel smell (hydrogen sulphide) might also indicate that problems exist.


Diesel bug: fixing it

Perham says there is really only one foolproof way to eradicate diesel bug from your tanks and that is to empty and clean them. On smaller boats it might be possible to remove the tank but on larger ones the diesel tanks are usually inbuilt.

This, says Perham, makes the job a lot more challenging unless the tank or tanks have inspection ports fitted that provide for internal cleaning.

Often, however, this is overlooked at the design stage and it then becomes necessary to cut inspection hatches into the tanks and fabricate inspection covers that are then sealed into and fastened to the tank.

“The biggest hurdle is getting access to tanks and it is not uncommon to have to cutaway internal joinery or floors to achieve this,” says Perham. “Remedial work is then required to close in what has been taken out. This can be costly but it is a necessary evil.”

Consideration also has to be given as to what to do with existing diesel in a tank and this will boil down to the grade quality of the diesel.

Perham says laboratory testing of fuel for contaminants and residues is part-and-parcel of the inspection process and where existing fuel is of sufficient quality it can be ‘polished’ and returned to the tank after cleaning.

Fuel polishing is now common practice and simply involves pumping it through a series of filters to remove water and contaminants and treating with either ultraviolet light and/or chemical biocides.


Diesel biocide

Biocides are often used to treat diesel but these are hazardous chemicals and must be treated as such. Adding biocides can, according to some experts, create problems and expedite the accumulation of sludge on tank floors as it kills off microbes.

Ideally, biocides should only be used after diesel tanks have been descaled and cleaned as a preventative tactic rather than a cure. Over time biocides can also lose their effectiveness as microbes build immunity to the chemicals.

While more costly, magnetic decontamination and ultraviolet fuel treatment systems are becoming more commonplace and can be very effective options for mitigating the risk of future contamination.

Magnetic solutions draw fuel through a magnet and water separation fuel filters and return the treated fuel back into the primary tank. A common magnetic fuel treatment system is De Bug, which has been in use for a number of years.

Perham says De Bug contains five magnets that effectively break down the cell wall of microbes reducing the spores to a size that pass through filter media and are burnt off during the combustion process.

Cells returned to the tank via fuel return lines are dormant and unable to grow. Perham says this type of system is a particularly good solution for commercial vessels that are used often, thus eliminating the chance of fuel sitting for longer periods of time.

There are new emerging solutions that incorporate a high-intensity light into standalone fuel flow systems incorporating both filters and magnets. The intensity of the light in these units kills spores before filtering and transfer back to the tanks.

Diesel bug is real and it has to be carefully managed to prevent the risk of not only breakdowns but costly engine repairs. The first step is to identify whether you’re infected and to what degree, and this will determine the remedial action required.

Whichever way you look at it, the price of inaction will invariably be a lot higher than the price of action.

The first sign of a diesel bug problem. This is contaminated fuel drained from the water-trap.

A check of the primary cartridge shows the filter to be almost blocked, potentially causing an engine shutdown.

The emptied tank reveals an extensive sludge issue.

Call an expert to save time and getting dirty.

From Trade-a-Boat Issue 429, July-Aug 2012.