Diesel bug is insidious. It eventually blocks fuel filters which leads to fuel starvation and inevitably, engine failure.

What is '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 commonly referred 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.

It is not a question of if, but when.

The return flow of fuel 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. 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?

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. 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.

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.

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.


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

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 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. 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.

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.

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