Diesel over winter: When fuel in the tank becomes a problem

Can diesel actually go bad? What helps against the notorious “diesel pest”? Should the tank be full or empty over winter – and are there fuels that still run smoothly even after years? SeaHelp explains what skippers need to know at the end of the season about diesel, gasoline, and fuel storage.

The boat is out of the water, the batteries are tended to, antifreeze is filled – and 200 liters of diesel are still sitting in the tank. Nothing will happen until spring. Or will it?

In fact, fuels can age. And the conditions on boats are exceptionally favorable for this: long idle periods, moisture, temperature fluctuations, and tanks that are vented to the outside air. What is consumed in a car within a few weeks may sit in a yacht’s tank for half a year or longer.

Yes, diesel can go “bad”

Diesel does not turn bad overnight. Instead, several processes run in parallel. Fuel components react with oxygen, and aging products and deposits can form. Even more problematic is water in the tank.

Standard road diesel B7 according to EN 590 may contain up to seven percent FAME – fatty acid methyl esters, commonly known as biodiesel. The engine organization CIMAC explicitly points out in its technical recommendations that FAME is more demanding regarding oxidation stability and microbial load: When biocomponents and free water meet, bacteria, yeasts, and molds can multiply. The resulting slimy residue can clog pre-filters and fine filters.

The so-called diesel pest is therefore less of a mysterious spoilage of diesel and more of a biological issue in the fuel system. Water is the decisive player here.

Neste also points out in its technical handbook that microorganisms can generally grow in diesel fuels as soon as free water is present (Neste is a Finnish energy company, formerly known primarily as an oil refining company. Today, Neste focuses heavily on renewable fuels, particularly HVO diesel). Higher temperatures further accelerate this process. Regular draining and clean tanks are therefore among the most effective countermeasures.

SeaHelp tip: Check and drain water separators, inspect the bottom of the tank for water and sludge, and check fuel filters before and after long idle periods. A filter that suddenly clogs unusually fast can be an indicator of contamination.

Yanmar Marine also explicitly calls for clean tanks and fuel systems, as well as a water separator between the tank and the injection system. For winter storage, Yanmar recommends removing any present water from the tank and filters.

Tank full or tank empty?

An old piece of “dockside wisdom” says: Fill up before winter so that as little moist air as possible remains in the tank. That is certainly not entirely wrong. For winter storage, Yanmar, for example, explicitly recommends a filled tank to reduce condensation; alternatively, the fuel system can also be drained completely.

However, the key word here is fresh. Simply topping off a tank that already contains fuel aged over months or visibly contaminated does not solve any problem.

For typical permanently installed and vented boat tanks, the sensible sequence is therefore:

1. Remove water and contamination.
2. When in doubt, do not simply continue using old or conspicuous fuel.
3. Fill up with fresh fuel approved for the engine.
4. Leave some room for expansion.
5. Check the water separator and filters.
6. If a stabilizing additive approved by the engine manufacturer is used, run the engine for a few minutes afterward so that the treated fuel also reaches the fuel lines, pump, filters, and injection system.

This “flushing” of the entire system is also recommended by Liqui Moly when using its Marine Diesel Protect. “What matters is not only what is in the tank, but also what is at the bottom of the tank,” says SeaHelp CEO Wolfgang Dauser. “Water, sludge, and an old filter can become a problem in spring much faster than the cold itself.”

Do additives help?

Yes – but you have to distinguish which problem an additive is actually supposed to address. Aging or oxidation stabilizers are designed to slow down chemical aging processes. Corrosion protection additives protect metallic components. Biocides, on the other hand, target bacteria, yeasts, and fungi.

An example from the marine trade is Liqui Moly Marine Diesel Protect. According to the manufacturer, the product combines anti-aging protection, corrosion protection, and biocidal action; for preventive use, Liqui Moly specifies a dosage of 1:1,000. For already contaminated systems, a significantly higher shock dose is intended. Note: Biocides are hazardous substances and must only be used in accordance with the product information.

An important note: Not every engine manufacturer recommends additives. Yanmar, for example, explicitly states in its current marine FAQs that it generally does not recommend fuel additives. The operating manual or the engine manufacturer’s approval therefore always remains authoritative.

Additionally: If a viscous black or brown biomass has already formed, an additive cannot replace tank cleaning. After all, killed microorganisms do not simply disappear – they can still clog filters and lines.

The diesel that lasts for years: GTL

Anyone looking to avoid long storage times or significantly reduce the aging problem will come across so-called paraffinic diesel fuels. Particularly interesting in this context is GTL – Gas-to-Liquids. In this process, natural gas is chemically converted into a very pure paraffinic fuel. GTL contains no FAME and falls under the fuel standard EN 15940.

Shell states a storage stability of more than five years for Shell GTL Fuel under suitable storage conditions. The fuel is FAME-free and, according to the company, is used in inland and recreational shipping, among other applications. Note: “Indefinitely stable” would still be the wrong phrasing here. GTL must also be stored cleanly, dryly, and professionally.

 

 

In addition: Engine approval must always be verified before using a fuel according to EN 15940. Shell itself points out, for example, that the vehicle or machine must be explicitly approved for paraffinic fuels.

In Germany, GTL is distributed by fuel dealers such as Hoyer, among others, and is occasionally available at boat fuel stations. However, availability on the water is (unfortunately) significantly lower than that of conventional diesel.

HVO100: Even more interesting for boats

Closely related to GTL is HVO100 – Hydrotreated Vegetable Oil. HVO is also a paraffinic diesel according to EN 15940, but is produced, for instance, from waste and residue materials, used cooking oils, and fats.

Chemically, HVO differs considerably from classic biodiesel or FAME. HVO is a hydrocarbon and is distinctly water-repellent. Neste therefore explains that HVO100 carries only a very low risk of fuel aging and microbial growth when stored properly.

However, the bottom line remains: Water must still not be left sitting permanently in the tank. Neste explicitly points out that microorganisms can also grow in other diesel fuels if free water is present. HVO does not automatically make a poorly maintained tank “hygienic.”

Since 2024, HVO100 may be sold regularly at public German fuel stations; at the pump, paraffinic diesel can be identified by the XTL label, among others. However, the industry association en2x also points out that manufacturer approval must always be observed before fueling. In the meantime, brands like AVIA and star carry HVO100 at selected road stations.

And HVO has also arrived “right on the water.” The Berlin-Spandau boat fuel station at Marina Lanke, for example, offered HVO100 in September 2026 for 2.64 euros per liter and bio-free B0 diesel for 2.68 euros. Marina Damp quoted an HVO100 price of 2.80 euros per liter in August 2026 (these prices are explicitly “snapshots,” however, and can fluctuate considerably by region). For boats with long idle periods, the premium can be worthwhile.

And what actually is B0?

Another alternative is FAME-free B0 diesel. Here, the more problematic biodiesel component is likewise absent. Hoyer, for example, offers its “Future Power Diesel” as B0 compliant with EN 590; this fuel is also offered at individual boat fuel stations.

However, B0 should not be confused with HVO or GTL. It is primarily diesel without added FAME. This fuel can therefore still age, and water in the tank remains undesirable.

And gasoline? It ages too

The same basic problem exists with gasoline engines – just with different focal points. Gasoline oxidizes during long storage. Volatile components alter, resinous residues can form, and injectors or carburetors can become fouled.

Particular attention should be paid to ethanol-containing fuel in this regard. Mercury Marine points out, for example, that ethanol can absorb moisture. Under unfavorable circumstances, so-called phase separation can occur: Water and ethanol settle at the bottom of the tank, while the remaining gasoline above loses part of its original properties. If the water-ethanol layer is drawn in, running problems or even severe damage can occur.

For longer idle periods, Mercury recommends fresh fuel and a marine fuel stabilizer. For older, open-vented tanks, the manufacturer advises a fill level of around 95 percent to reduce condensation while still allowing room for temperature-related expansion. With modern, largely closed tanks, a high fill level is less critical according to Mercury.

Those who can choose between approved E5 and E10 will have less ethanol in the tank with E5. The decisive factor, however, remains the fuel approval of the respective engine.

Fuel stabilizers can significantly extend storage life. Depending on the product version, Mercury cites stabilization of gasoline and ethanol blends for up to two years for its marine stabilizer Quickstor; Liqui Moly also offers anti-aging, oxidation, and corrosion protection for two-stroke and four-stroke engines with its Marine Gasoline Stabilizer.

Here, too, the treated fuel should run through the entire fuel system for a few minutes afterward. Wolfgang Dauser: “In the end, the most expensive fuel is the one with which the engine no longer starts in the spring. A clean tank, fresh fuel, and a checked filter cost considerably less than tank cleaning or a deployment due to engine failure.”

Gasoline or diesel in canisters: Do not just place them in the garage corner

Reserve fuel also ages – and here fire safety also comes into play. Fuel belongs exclusively in suitable, tightly closing, and approved containers. For spare fuel cans, appropriately approved fuel containers are explicitly recommended; special transport regulations also apply inside vehicles.

In general: Canisters should be kept cool, dry, and protected from direct sunlight. They do not belong next to heaters, battery chargers, welding equipment, or other potential ignition sources. Gasoline vapors in particular are highly flammable.

The permissible storage quantity is also limited. Regulations in Germany are partly governed by state law. In Bavaria and North Rhine-Westphalia, for example, up to 20 liters of gasoline and 200 liters of diesel fuel may be stored in tightly closed, shatterproof containers in small garages; considerably stricter requirements apply to medium and large garages. Anyone wishing to store larger quantities should therefore always check local regulations.

From a practical standpoint, it is also advisable to label canisters with the fuel type and filling date (marker) and to consume the supply regularly and replace it with fresh fuel.

SeaHelp conclusion: Water is the biggest enemy

For normal seasonal breaks, no one needs to panic. Fresh fuel normally survives a winter without issues when stored sensibly. It only becomes problematic when long-standing fuel, water, contamination, and a lack of maintenance combine.