Diesel and heating oil store better than gasoline, and what ruins them is microbial sludge growing where water meets fuel, not evaporation. Keep tanks full so there is little room for condensation, draw fuel from above the bottom, filter it and drain any water, and in freezing weather prevent waxing by blending in kerosene rather than gasoline, which would turn the tank into a gasoline-style vapour hazard.
Part of Fuel Siphoning & Storage
Diesel and heating oil are the same fuel in practice and the main fuel in farm, heating and truck tanks. They are easier to handle than gasoline but fail through water, microbes and cold.
Water Is The Root Cause Of Diesel Spoilage
Diesel in a tank collects water through condensation, and the water settles to the bottom. Bacteria and fungi, called the “diesel bug” and sometimes loosely “algae”, feed at the boundary between the water and the fuel. They form a dark slime or sludge that clogs filters and injectors (parent article). The parent article gives 6-12 months untreated and 12-24 months with biocide; treat these as indicative, because temperature and water content govern the real figure.
Cold behaviour. Diesel contains paraffin wax. In low temperatures the wax crystallises, first clouding the fuel and then gelling it, and a gelled fuel blocks the filter. Winter diesel is blended to resist this. The parent article says that if you have only summer diesel and temperatures will drop below -10 °C, a standard emergency practice is to add 10-20% kerosene or gasoline.
Use kerosene, not gasoline, for this
The parent article, Fuel Siphoning & Storage, names either kerosene or gasoline for this job. Use kerosene. Adding gasoline makes the contents of a diesel tank give off vapour and ignite more like gasoline, which is exactly the hazard diesel avoids. The 10-20% range is the parent article’s figure.
What You Need
- A clear jar, to check for water, sludge and clouding.
- A fuel filter, the parent article suggests a 10-micron element, and a water separator.
- Biocide made for diesel (the parent article names Biobor JF); follow the label dose.
- Kerosene, for cold-weather blending.
Method: Sample, Filter, Drain, Keep Full
Step 1: Sample outdoors with a hand pump or a clean bailer from just above the bottom of the tank into the clear jar. Let it settle.
Step 2: Read the jar. Clear to straw-coloured fuel with nothing at the bottom is good. A cloudy haze in cold weather means wax. A dark layer, flecks or slime and a sour smell mean microbial growth. A separate layer of clear liquid at the bottom is water.
Step 3: Remove the water with a pump taking from the bottom, or a water separator, before using the fuel.
Step 4: Filter through a filter of the fineness the engine’s maker specifies; the parent article suggests 10 microns. Change filters when flow slows.
Step 5: Treat. Add biocide at the label rate if the fuel is to be stored.
Step 6: Keep tanks as full as practical. Less air space means less condensation.
Step 7: Cold weather. If winter diesel is not available, blend kerosene into the fuel before the temperature falls, not after it has gelled.
Worked Example
A 200-litre drum of summer diesel is to be protected against cold with the parent article’s 10-20% kerosene, taking the percentage as a share of the finished blend. At 10%: 200 × 0.10 = 20 L kerosene, 180 L diesel. At 20%: 200 × 0.20 = 40 L kerosene, 160 L diesel. If the parent article meant the percentage of the diesel alone, the kerosene volume would be a little higher; since it does not say, the lower end of the range is the safer starting point and the blend can be adjusted by the engine’s behaviour.
Common Mistakes
- Ignoring a slow fuel filter. A filter clogging with dark slime is the first sign of microbial growth.
- Filling the tank from the bottom of a long-stored batch. That is where the sludge lies.
- Adding gasoline to prevent gelling. The fire and explosion risk goes up and the tank is no longer a diesel-class hazard.
- Waiting until the fuel has gelled. A gelled filter must be warmed and cleaned; prevention is easier.
Safety
Diesel burns, and mists and hot surfaces ignite it
Diesel and heating oil give off far less vapour than gasoline, but they still burn, and a spray or a rag soaked with them can catch fire from a hot surface. Do not use gasoline to dilute diesel, and do not warm a gelled fuel line or tank with a naked flame. Never test any fuel by lighting a sample. Spilled fuel in a basement is also a slip hazard; contain and absorb it with sand or soil.
What’s Next
- Learn next: Fuel Stabilization & Treatment — biocide and other treatments in detail
- Combine with: Fixed Tanks & Depots — where large diesel stores sit