What is the diesel bug and how can I prevent it?

'Diesel bug' is a commonly accepted term for a number of contaminants that include microbial bacteria, fungi and algae that live at the point of blend between water and diesel. The water from the condensation in the fuel tank is what the diesel bug survives on, allowing it to reproduce at a very fast rate. This is also less commonly referred to as the diesel virus or diesel fungus.

The microbes have a very short life but before they die they multiply and produce waste deposits, both of which descend to the bottom of the fuel tank. One bacteria microbe can reproduce more than 7 million microbes in 24 hours. Due to the rate of reproduction and their short lives, the build-up of dead microbes can be quite fast.

Any fuel tank with an air pocket will produce condensation when the temperature falls far enough, and is therefore likely to attract these tiny microbes. After a day out on the road, the remaining diesel is warm from the heat of the engine and as the temperature cools, cold air is drawn into the fuel tank. The cold air causes condensation on the inside of the tank, forming dense water that eventually sinks to the bottom of the fuel tank. In that water are the microbes as they live off the oxygen in the water and then eat the diesel.

The introduction of highly efficient engines such as the common-rail diesel engine – now require higher quality fuel. When bacteria, sludge or water is sucked into the fuel lines it causes filter and injector blockages. As a result of the blockages the filter, injector, fuel tank and lines may require replacement or cleaning, and if the damage affects the operation of the injector pump then watch out because this can often be a painfully expensive repair.

Water condensation creates the perfect environment for the diesel bug, so removing the water and the bacteria will eliminate the diesel bug and stop any further growth. Additives have been developed to kill the bugs and prevent more from forming. However, the additive doesn’t physically remove the dead bugs. Instead, engine combustion is used to burn them off. This doesn’t harm the engine as each bacterium molecule is encapsulated at a micron level, which is then safely sent through the combustion chamber. Depending on how much bacteria your tank is suffering from, the additive may not clean the whole tank on the first fill or dose. If there’s an excessive amount of bacteria it could take a few fills to completely rid the tank of any bacteria, and ideally the tank should be cleaned out prior to treatment in order to remove any sludge deposits. The treatment will break down this sludge over time but several treatments may be required. Many additives will treat water and condensation in the fuel the same way as the bacteria. It’s important to note that using a fuel additive will not stop condensation from forming. However, as the condensation forms and makes its way into the diesel and to the bottom of the tank that then forms bacteria, the additive will treat the water before it causes any damage to your engine.

To help prevent the diesel bug you can maintain the fuel system by regularly draining water from the fuel and water separator. Keep the fuel tank as full as possible – especially if the vehicle isn’t going to be used over a period of time – and try to use petrol stations that are busy as they’ll generally have a higher turnover rate of diesel meaning less chance of moisture contaminating the fuel.

If you suspect that you might have the bug, there’s no shortage of additive products, and there are also companies which can analyse your fuel and provide recommendations on how to treat it. If you’re ever unsure though, we’d recommend consulting a diesel specialist.

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