© Thomas Gade / May 2015 (updated 2026)
Diesel Pest
Diesel pest refers to the growth of microorganisms (bacteria, yeasts and moulds) in diesel fuel, resulting in the formation of a viscous biosludge. This sludge clogs filters, fuel lines and injectors and can ultimately cause engine failure.
Four different diesel samples. The three on the left come from pleasure boats and clearly show a layer of biosludge at the bottom. Right: sample from a motorhome.
Engine Failure Due to Diesel Pest
6 October 2015 / Thomas GadeIn the middle of the navigation channel the engine suddenly stopped and the boat drifted helplessly. Fortunately, no other vessel was nearby. After a while the engine started again, but it soon cut out once more. The cause was initially unclear.
It took several months and numerous troubleshooting attempts before it became clear that a thriving colony of microorganisms had developed in the diesel tank. A thick layer of biosludge had formed at the bottom, eventually blocking the strainer of the suction pipe.
Because the tank was built-in and difficult to access, a permanently installed refrigerator had to be removed first to reach an inspection opening. Only then was it possible to unscrew a cover with the fuel gauge sender and pump diesel from the tank bottom using a hose and pump. Instead of clean fuel, a muddy sludge came out.
The entire contents of the tank, including the biosludge, were pumped out. In the jerry cans the sludge settled at the bottom. The clearer upper diesel was filtered and treated with a biocide (Grotamar). When refuelling, the boat was switched to C.A.R.E. Diesel or – if unavailable – to Aral Ultimate Diesel.
Engine Failure Caused by Microorganisms in Diesel Fuel
A boat without engine power cannot be steered and becomes a plaything of current, waves and wind. Despite good maintenance, diesel engines can suddenly fail without warning. A frequent cause is diesel pest: the slimy biosludge first clogs the strainer at the suction pipe and then the fuel filter.
Diesel tank with biosludge on the strainer of the suction pipe.
Microorganisms in Fuel – General Information
Fuel is considered toxic, so many boat owners do not realise that microorganisms can settle in their diesel tanks. However, the phenomenon has been known since the late 19th century. Problems with contaminated kerosene were already reported in the 1950s. Biocides have been available since the 1970s to inhibit microbial growth.Microorganisms need water and hydrocarbons to thrive. Fossil diesel contains a maximum of 0.2 ml of water per litre. Part of it is dissolved, the rest settles at the bottom. Many tanks have a drain cock, and there is usually a water separator before the engine.
Microorganisms particularly thrive at the interface between water and diesel. The addition of biodiesel (FAME) has dramatically worsened the problem in recent years, as biodiesel can absorb significantly more water and form a fine emulsion.
Water in Diesel
According to EN 590, fossil diesel may contain up to 200 mg of water per kilogram, biodiesel up to 500 mg/kg. Biodiesel is hygroscopic and can bind up to 5,000 mg/kg of water without it separating. In a blend with fossil diesel, a relatively high water content can be finely distributed as an emulsion, providing microorganisms with a much larger habitat.Conventional methods such as draining water from the tank bottom or water separator are only partially effective with biodiesel blends.
Long Storage Periods Promote Diesel Pest
Seasonally used vehicles such as boats, motorhomes and agricultural machinery with long idle periods are particularly at risk. Temperature fluctuations cause moisture to enter the tank via the vent and condense on the walls. With economical engines, one tankful can last the entire summer – the fuel stands for months.
From November to April many boats stand on land with full tanks. Over time, a thicker and thicker layer of biosludge forms at the bottom.
In rough water the layers mix. The suction pipe reaches deep into the tank and eventually draws in sludge as well. The fine strainers become clogged and fuel delivery is restricted – until the engine stops.
What Can Be Done?
It is best to pump out the diesel from the bottom at the end of the winter lay-up and filter it. The clearer upper portion can be returned to the tank after adding a biocide. Ideally the tank should be completely emptied and thoroughly cleaned.
Biosludge in the bowl of the water separator.
Contaminated diesel from a boat tank. One hour after pumping, the biosludge has settled.