How to maintain your diesel fuel for your generator

The makeup of diesel fuel has changed drastically over the last thirteen years. In 2007 the EPA mandated that all diesel fuel must have ultra-low sulfur content. Sulfur content is now limited to 15 ppm as opposed to the limit of 500 ppm pre 2007. The higher levels of sulfur that used to be in diesel fuel helped to lubricate the seals in fuel injectors and pumps and acted as a biocide to help prevent microbial growth such as fungus, mold, and bacteria. Nowadays, diesel fuel must be carefully monitored, tested, and maintained in order for it to stay stable for long periods of time in storage.


Asphaltenes from diesel fuel can build up on the bottom of your fuel storage tank over time, causing sludge. This sludge represents energy value that cannot be contributed when the fuel is burned by your equipment. Unaddressed, this leads to fuel instability, filter plugging, and a reduction of energy availability. Regular fuel polishing and cleaning of your fuel tank can help mitigate the issues caused by sludge buildup.


The normal pH for diesel fuel should be between 5.5 and 8.0. The lower the pH, the higher the potential for corrosive damage to metal components. The overgrowth of microbes is the main contributor to diesel fuel acidity. To remedy the acidity, lower the microbial content in your fuel using EPA approved biocides. The pH could also be affected by trace amounts of ethanol mixing with your diesel fuel. When ethanol vapors cross over the vent pipes into the diesel tanks, they combine with any of the water found at the bottom of the fuel tank. Certain bacteria feed off of the ethanol particles and convert them to acidic acid. The acidity produced by these bacteria is extremely destructive to storage tanks and fuel distribution systems.


The number one rule to follow when it comes to storing diesel fuel is: Keep all water out of your fuel tank. If any water is found in your fuel tank, it needs be removed immediately. Finding a company that will perform this service may be difficult, because it requires special licensing to handle, transport, and dispose of petroleum-contaminated water (PCW). So your best bet is to keep water out of your fuel from the beginning.

Chemicals are available that can be used to bind trace amounts of water to suspend it in the fuel. It can then be burned harmlessly by your engine. This can only be done with very small amounts of water.

You can combat water accumulation by keeping your fuel tank full to prevent room for condensation accumulation, testing fuel for water every 30 days using a water finding paste, investing in an underground storage tank, and checking the tank for any pooled water after rain.


Most diesel fuel contains 2-5% biodiesel no matter what company it is produced by. The biodiesel content provides the lubrication that is now lacking from regular low sulfur diesel, but microbes thrive in it. Biodiesel is a lot less stable than regular, low-sulfur diesel, and can make cold water gelling problems an issue when there is biodiesel content of 20% or greater in the fuel.

Microbial Contamination

To help prevent microbe growth, you should be checking your fuel tanks for water at regular intervals. In addition to this, you should be paying attention to your filter lifespans (for shortening) and any other operational signs that could indicate that a microbial issue may be present.


Diesel fuel should be stored between 20-70 degrees Fahrenheit. If the fuel cannot be stored at temperatures below 70 degrees Fahrenheit, 12 months is the longest amount of time it should be stored, even with proper maintenance and the correct fuel additives.

Mechanical Processing

Mechanical fuel processing and polishing uses filters and water separators to mechanically remove particulates and water from stored fuel. Fuel polishing can remove water and sludge, but will not remove microbes.


There are many different tests you can perform on your stored diesel fuel to determine what additives your fuel may need to remain stable, or to determine if your fuel is past the point of no return and needs to be replaced. See our article about the different types of diesel fuel testing available, here. Diesel fuel is crucial to many business applications and their backup generators, and the fluctuations in pricing make it advantageous to stock up on fuel when the price is low. But maintaining that fuel can be an expensive process in and of itself. Weighing the pros and cons of storing fuel vs buying fuel at the fluctuating price will help you make the best decision for your home or business.

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