What is Diesel Bug?

This phenomenon has been known by the aviation and marine industries for many years. Diesel has inherent dormant bacteria present in very small quantities when fresh and which under the right conditions will multiply over time. In some respects this is similar to milk and other foods which if not stored properly will go off.

Why is it now affecting motorists?

With the increasing numbers of diesel cars on the road combined with Ultra Low Sulphur Diesel (ULSD) widely available since 2009 and especially the introduction of biofuel blends since 2011 this is now an emerging problem for motorists. The fuel as supplied at the pumps is safe as reputable outlets have operating procedures and controls in place to ensure the quality meets strict standards (BS EN 590). It is then down to how it is stored by the motorist and how quickly it is used that determines the potential growth of the microbes.

How does it grow?

For the bacteria to multiply it needs food and water. Its food is the hydrocarbons in diesel but without water it will remain dormant. Unfortunately diesel contains some water which is not normally a problem as it isn’t water as we know it. To understand this consider a carrot which is a relatively dry object is 87% water, the water content in diesel as delivered from the filling station is less than 0.02%. Normally this water content passes harmlessly through the engine and out the exhaust with any separated water getting caught in the water trap fitted to all diesel engines. The right conditions for Diesel Bug occur when the water evaporates from the diesel or moist air enters the tank when being filled. The moist air in the tank then condenses on the walls and the droplets sink to the bottom of the tank below the diesel. Once there is a layer of water then the bacteria has the right environment to grow between the diesel and water layers. Under normal circumstances the diesel is used and replaced before there is a problem as there is enough movement in the car to keep the water and diesel mixed. While the engine is running any neat water is trapped on route along with any particles by the fuel filter. Motorists that normally don’t fill the tank above half and frequently add say £10 - £20 at a time are more likely to increase the amount of water in the tank. The more air space the tank or container the greater the risk of condensation. It may only be a spoonful of water in a week but over a period of time it can add up to quite a puddle and provide an opportunity for the bacteria to grow. The problem is likely to occur if the diesel is in a car that is stood for over a month or stored in a can as emergency fuel, especially when the days are warm and the nights cool and there is airspace to allow condensation. The shelf life of diesel kept under optimum conditions is only 6 to 12 months before it is likely to degrade below specification.

How does it affect the engine?

Bacteria, like any living thing that eats, excretes waste. This excretion forms silt like sediment on the bottom of the tank. There is a likelihood of silt or clumps of microbes being drawn through the fuel lines until they are stopped by the fuel filter. The fuel filter will eventually become clogged and consequently restrict the flow of diesel to the engine causing a loss of power or misfiring under load due to fuel starvation. If the filter is old or damaged then particles can pass through it and into the high pressure pump and injectors causing excessive wear, damage or blockages. Bacteria in the fuel also excretes acids which can damage rubber seals causing leaks and cause corrosion in the tank and fuel lines. There is then the additional danger of particles of rubber or rust entering the fuel system.

Treatment

Diesel fuel biocides have been available from ships chandlers and yacht club shops for years. Since the introduction of biodiesel there are also products aimed at the motorist. Typically a stronger mix is advised as a kill shot and a weaker mix as an ongoing preventative. If diesel bug infestation is suspected it is advisable to repeat a kill shot when next refilling the tank and having the fuel filter replaced when the vehicle is next serviced if not before.

Maintenance

It is critical that the fuel filter is changed in accordance the vehicles maintenance schedule. If the diesel filter has been contaminated by petrol or water then it must be replaced as soon as convenient as the contamination will prematurely age the filter and reduce its effectiveness. Do not ignore the Water Trap warning light, it’s there to protect the fuel filter and the engine. If diesel bug is or has been present then failure to properly maintain the fuel filter can allow larger particles to pass through the filter causing injectors to gum up or become blocked costing hundreds to replace.

To avoid diesel bug:

• Do fill the tank completely and run the tank low before refilling • Do use and replace stored diesel regularly to ensure it is always fresh • Do store cans of diesel in a cool place if possible and protect from frost • Do ensure the engine fuel filters and water traps are serviced and maintained to the manufacturers specifications • Do Not half fill the tank and top up frequently with small amounts as that increases the likelihood of condensation occurring in the tank. • Do Not buy fuel from unauthorised sources. Cheap fuel could cost thousands in repair bills