© Thomas Gade / May 2015 (updated 2026)
Diesel Pest – Prevention
Checking the Diesel Fuel
Does the engine have to fail before you notice diesel pest? No. You can detect an infestation yourself by taking a sample from the bottom of the tank.The best way is to remove a cover on the top of the tank (for example, the mounting of the fuel gauge sender or the plate with the suction pipe). Using a hose with a thin guide tube attached to the end (long enough to reach the tank bottom), suck out a small amount of diesel. You can immediately see whether the fuel is clear or contains cloudy, dark to olive-green particles that settle after a while.
The company Schülke & Mayr offers the “mikrocount® fuel” test kit for checking for microorganisms. The test is most meaningful when the diesel has already been in the tank for some time. Taking a sample immediately after refuelling with fresh diesel is of little use, as there may still be biosludge at the bottom of the tank.
For boats that are stored on land over winter with a full tank, it is advisable to carry out a test a few days before launching. This leaves time for a tank cleaning on land, and the result is not distorted by freshly added fuel.
Alternatively, Sea-Help offers a sample set. It contains a 3.5-metre hose with a weight, a syringe, a sample bottle and packaging for shipping. The price includes laboratory analysis of the sample.
Test Kits for Detecting Microorganisms in Fuel
Schülke & Mayr – mikrocount® fuel• Single test: approx. 30 €
• Pack of 5: approx. 125 €
• Single use only
Sea-Help Sample Set
• Price: approx. 119 €
• Includes hose, syringe, bottle and laboratory analysis
• Single use only
With long storage periods of the fuel, an annual check is recommended. When buying a used boat or motorhome with a diesel engine, a test should also be carried out.
Biocides
Additives with biocidal action can be mixed into the diesel when refuelling to inhibit the growth of microorganisms. Sea-Help recommends “Diesel Plus”. Among boaters, Grotamar 82 has an excellent reputation and is also used by the German Maritime Search and Rescue Service.
Grotamar 82 (successor to Grotamar 71) distributes itself more evenly in the fuel and is therefore better suited for modern diesel with a biodiesel content than the previous product.
Important Note (as of 2026):
Since 01.12.2018, due to the Biocidal Products Regulation, Grotamar 82 may **only be sold to commercial users**. Private customers can no longer purchase it directly. However, other biocidal additives are available that are freely obtainable for private individuals.
Biocides – Environmental Compatibility and Resistance
Oil companies reject the general addition of biocides to filling station diesel for several reasons. In normally used passenger cars with frequent tank changes, problems caused by microorganisms occur very rarely. The number of seasonally used vehicles and boats with long fuel storage periods is relatively small. A general addition would make the fuel more expensive and place an additional burden on the environment. In addition, there is the risk that microorganisms will develop resistance and the products will lose their effectiveness in the long term.The situation is comparable to the use of antibiotics in medicine: these should only be used in a targeted manner and when necessary in order to preserve their effectiveness.
Biocides to Prevent the Growth of Microorganisms in Diesel Fuel
The following overview shows common additives designed to inhibit the formation of biosludge. Grotamar 82• Only available to commercial users
• Dosage: 1:4000 (25 ml per 100 L)
• Shock dosage: 1:1000
ERC Diesel Plus 2.0
• Freely available to private customers
• Dosage: 1:1000 (100 ml per 100 L)
• Shock dosage: 1:200
Yachticon Diesel Plus
• Dosage: 1:1000 (100 ml per 100 L)
• Shock dosage: 1:200
Liqui Moly Anti-Bacterial Diesel Additive
• Dosage: 1:1000 (100 ml per 100 L)
• Shock dosage: 1:200
Liqui Moly Marine Diesel Additive
• Dosage: 1:1000 (100 ml per 100 L)
• Shock dosage: 1:200
**Note:** In cases of heavy infestation, a biocide is often not enough. The killed microorganisms can themselves serve as nutrients for new ones. In such cases, professional tank cleaning is necessary. Biocides do not dissolve existing biosludge – they only make the fuel an unsuitable habitat for microorganisms.
General Prevention
No rainwater should enter the tank during refuelling. The seals of the tank caps must be checked regularly and must be tight.If the tank has a drain for separated water at the bottom, it should be drained occasionally. Refilled diesel should be fresh and come from a reliable source. It is not a good idea to be among the first customers at a boat filling station after the winter break – the fuel there has often been stored for several months.
For smaller tanks that are filled from jerry cans, diesel should be obtained from busy filling stations of well-known brands. The jerry cans themselves should be cleaned regularly, as residual diesel and summer heat in the car boot promote bacterial growth. It is best to add a biocide already when filling the cans.
Avoiding Condensation in the Tank
Condensation occurs mainly in partially empty tanks that are exposed to large temperature fluctuations. During the day, the air expands and partially escapes. At night, the air contracts and draws in moist outside air, which condenses on the cold tank walls and drips into the diesel.Vehicles with high fuel consumption and short refuelling intervals are hardly affected. In contrast, sailing boats with an auxiliary engine that is only used occasionally accumulate more and more water in the tank over time.
Possible countermeasures:
- Keep the tank as full as possible (especially during long periods of inactivity)
- Thermally insulate the tank to reduce temperature fluctuations
- Install simple devices on the tank vent so that moisture condenses before the air enters the tank