What happens when a generator starts during a power outage, but fuel quality no longer meets operating requirements? For many facilities, fuel degradation is the hidden cause behind unexpected generator problems.
Businesses store fuel on-site in tanks for emergencies. However, generator fuel degradation starts as soon as fuel enters storage. Diesel can become unstable within 6 to 12 months, especially in poorly maintained storage tanks.
Fuel quality declines due to oxidation, water contamination, microbial growth, and temperature changes. This blog explains a generator fuel checklist for facility teams and operations managers. It covers fuel quality checks, tank inspections, and emergency fuel planning.
Businesses can also contact Rhino Fuel for emergency fuel delivery services.
key takeaways
- A fuel checklist helps identify poor fuel quality, low fuel levels, and signs of deterioration.
- Fuel has a short lifespan and is able to sustain its quality for six months to twelve months, depending on storage.
- Maintenance and testing help to keep the fuel at a good quality.
- A comprehensive checklist includes tank inspection, emergency planning, and proper record keeping.
- The NFPA 110, Joint Commission, and CMS emphasize fuel system maintenance and documentation.
- Rhino Fuel provides generator fueling services on-site, which also reduces the chances of fuel theft.
What Is Generator Fuel Degradation?
Generator fuel degradation is the slow change in fuel quality after fuel sits in a storage tank for longer. Fuel quality deteriorates when oxygen, temperature, moisture, and microorganisms cause changes in it. Fresh fuel may become dark, sticky, and generate sludge over a period of time.
Diesel fuel degradation is quite frequent in standby generators. It is because fuel remains unused for many months or even years. Heat and oxygen degrade fuel quality, and water in diesel fuel allows microbial growth at the tank bottom.
Using the best generator gas in the required amount helps reduce this problem. This way, old fuel gets used, and fresh fuel enters the system regularly.
How Long Does Diesel Fuel Last Before It Degrades?
Unstabilized diesel can only be used for about six to twelve months. Water, elevated temperatures, poor storage tanks, and microbial growth lead to the faster degradation of diesel.
Moreover, water and sediments found in the fuel tank also deteriorate the stored diesel fuel quality. Blends of biodiesel degrade faster than ULSD fuel because of their increased biodiesel content from B5, B10, B20 up to B50. Standby generators use fuel infrequently, so stored fuel can spoil during storage.
For this reason, fuel quality standards play an important role in fuel management. ASTM D975 standard defines diesel fuel quality limits in the United States. It establishes restrictions for sulfur content, cetane number, density, flash point, and distillation range.
The 4 Main Causes of Generator Fuel Degradation
We know diesel fuel degrades during storage inside a generator system. Multiple factors lead to fuel contamination, which altogether increase the rate of fuel decomposition.
Oxidation alters the composition of fuel when it reacts with air inside the tank. Microbial growth forms at the fuel–liquid boundary and feeds on hydrocarbons. Similarly, temperature changes increase chemical reaction rate and raise vapor movement inside the storage space.
Cause 1: Diesel Fuel Oxidation
Oxidation starts due to the interaction of oxygen and the hydrocarbon composition in diesel fuel. This results in the formation of unstable compounds that form gummy substances in the diesel fuel tanks.
In the event of high oxidation levels, the color of the fuel will become yellowish-brown and then black. Heat, sunlight, and the metallic surface of fuel tanks increase diesel fuel oxidation.
Antioxidant additives minimize the reaction of oxygen with the fuel, preventing fuel oxidation. Therefore, businesses must follow the diesel fuel quality standards that set limits for stability and storage performance.
Cause 2: Water Contamination in Diesel Fuel
Generator fuel contamination mostly happens due to water that causes microbial growth and corrosion in fuel tanks. Water enters diesel storage tanks through condensation in empty tank space and small seal leaks. It also enters during fuel delivery transfer and vent breathing that draws moist air into the tank.
Various other fuel handling mistakes can also lead to contamination and reduce fuel quality. Water in diesel fuel appears as free water at the tank bottom and settles into a separate layer. It also appears as emulsified water that mixes inside the fuel and makes it cloudy.
Above all, dissolved water remains suspended at a molecular level and cannot be seen during inspection. Even a small water presence reduces combustion quality and affects engine performance.
Cause 3: Microbial Growth in Stored Diesel
Microbial growth occurs mainly at the fuel–liquid boundary where moisture and hydrocarbons exist together. These organisms feed on fuel components and slowly form black sludge, slime deposits, and foul odors within generator fuel tanks
The microbial growth in diesel also produces acidic byproducts. These byproducts corrode tank surfaces and damage generator components. Generator fuel contamination increases as these deposits spread through the system.
ASTM D6469 explains test methods and field practices for detecting microbial contamination in fuel systems. It also describes sampling methods and signs used to identify microbial activity in stored fuel systems.
Cause 4: Temperature and Storage Conditions
The changes in temperature also affect the generator fuel storage quality. Oxidation reactions increase in hot temperatures, which speeds up fuel degradation. Similarly, wax crystals form in winter, making the fuel thicker and reducing flow in equipment.
Businesses must follow proper generator fuel maintenance practices for all types of backup power generators. These practices include placing tanks in shaded areas, using insulation around storage units, and reducing direct sunlight exposure.
Temperature changes cause fuel tanks to expand and contract. This process, known as tank breathing, pulls outside air into the tank and pushes it back out. When warm air enters and later cools, moisture forms inside the tank. As this cycle repeats, moisture collects at the tank bottom and increases the chance of fuel contamination.
A Timeline of Diesel Fuel Degradation
This section explains how diesel fuel changes in quality over time inside storage systems. The table below shows a time-based view of fuel condition changes and related risks:
| Time Period | Fuel Condition | Effect |
| 0 to 3 months | No change in fuel quality | Fuel shows strong quality and stable behavior. Early oxidation starts inside storage tanks with a limited impact on performance. |
| 3 to 6 months | Noticeable changes | Oxidation becomes more visible. Moisture begins collecting in some tanks and changes the fuel condition. |
| 6 to 12 months | Fuel degradation increases | Fuel breakdown increases at this stage. Microbial activity risk grows inside stored fuel and affects fuel consistency. |
| 12 to 24 months | High degradation risk | Sludge formation, gum deposits, and contamination start appearing when no treatment takes place. Fuel condition drops further. |
| More than 24 months | Requires treatment | Fuel likely becomes unusable. Polishing or full replacement is required before use. |
How Generator Fuel Degradation Causes Equipment Failures
Degraded fuel clogs fuel filters and injectors, and it also damages fuel pumps and lines. Generators may show hard starts, smoke, and power loss when contamination builds inside the system.
Furthermore, acidic corrosion damages tank walls and fuel system components. Fuel pumps in diesel generators can also fail due to contaminated or degraded fuel. Common signs include hard starting, power loss, unusual noises, and fuel delivery issues. Therefore, proper generator fuel management is essential to prevent generator downtime.
If you want to learn more, read our guide on what are the symptoms of a bad fuel pump.
Warning Signs of Degraded Generator Fuel
The following signs indicate that fuel used by the generator has degraded or become stale:
- Fuel gradually starts losing its natural color and turns to dull yellow, brown, or even black.
- The presence of cloudiness and sticky matter is an indication that there is moisture and impurities in the fuel.
- There is an unpleasant, rancid smell similar to varnish in the fuel, which signals deterioration of the chemicals inside the fuel.
- Sediment settles at the bottom of tanks, showing dirt, rust, or degraded fuel components.
- Fuel filters clog more frequently because sludge and debris restrict flow.
- The generator struggles to start and may release smoke because of diesel fuel sludge formation.
How to Test for Generator Fuel Degradation
Here is how generator fuel is tested for degradation:
Visual Inspection
Diesel fuel testing starts by checking color, clarity, and sediment in a clear sample container. Inspectors hold the container against light to identify cloudiness or settled particles.
Field Testing
Field kits check for water, microbes, and stability using test strips or portable devices. Technicians dip strips into the fuel or load samples into handheld testers for instant readings.
Laboratory Testing
In laboratories, EPA diesel standards are used for accurate diesel fuel testing, which includes ASTM D2274, water content, and particulate analysis. Analysts use controlled equipment to heat, filter, and measure the fuel for oxidation and impurities.
Testing Frequency
Fuel quality testing is recommended every 6 to 12 months for stored diesel. Operators draw samples from storage tanks and submit them for analysis.
How to Prevent Generator Fuel Degradation
1. Fuel quality testing
Have the fuel quality tested once every six to twelve months to ensure proper generator fuel management.
2. Annual fuel polishing
Ensure diesel fuel polishing is done annually to clean the fuel of any water, sludge, and contamination. Coordinate with trusted suppliers for generator fuel polishing.
3. Tank fill levels
Fuel tanks should be filled to at least 75% capacity to minimize condensation and stabilize the fuel.
4. Microbial growth
Biocides can be used to ensure no microorganism develops in the fuel storage system.
5. Fuel treatment
Add stabilizers or antioxidants and use the best diesel fuel for long-term savings.
6. Tank component inspection
Inspect vents, seals, and gauges regularly to prevent fuel contamination.
7. Fuel monitoring systems
Install automated monitoring for continuous generator fuel management and early detection.
8. Supplier coordination
Partner with a fuel supplier for scheduled delivery and quality assurance.
When to Polish vs. Replace Generator Fuel
Fuel degradation requires either diesel fuel polishing or complete replacement, depending on its condition. When contamination is light, such as minor water or sediment, polishing is usually enough.
However, when degradation is severe with sludge or microbial growth, full replacement is necessary. Fuel polishing extends the usable life of stored diesel. It also supports generator downtime prevention and is more cost-effective than disposal and refilling.
Cost also plays an important role in this decision. Industry sources note that fuel polishing generally costs far less than full disposal and replacement. It is because businesses retain the fuel already stored in the tank and avoid disposal expenses.
Fuel polishing prices vary based on the tank volume. The costs usually decrease as tank size increases. For example, a tank with up to 500 gallons may cost around $1.80 per gallon to polish. Larger tanks between 2,500 and 6,000 gallons may cost around $0.60 per gallon.
Read our complete guide on generator refueling for businesses to understand best practices for maintaining fuel quality and improving efficiency.
How Fuel Suppliers Help Prevent Fuel Degradation
Fuel suppliers help maintain fuel quality through scheduled bulk fuel deliveries. This helps keep fuel fresh and reduces long storage periods that cause degradation.
Trusted suppliers provide fuel quality testing and tank monitoring services. They also offer reporting support to track fuel condition and identify issues early. Rhino Fuel supplies certified diesel for generators, helping ensure your business meets fuel quality compliance.
Frequently Asked Questions
generator fuel degradation
How long can gas sit in a generator without being used?
Gas can usually remain in a generator for about 1 to 3 months before it starts to degrade. After this period, it can lose effectiveness and may cause starting issues or poor performance if not treated or stabilized.
What is the 20/20/20 rule for generators?
The 20/20/20 rule for generators means running the generator for 20 minutes every 20 days at around 20% load. This helps keep internal parts active and reduces fuel buildup and system issues.
Can diesel still be used after sitting in the tank for 2 years?
Diesel can be used after sitting in a tank for 2 years, but it is degraded. It may contain contamination, water buildup, and microbial growth, and require testing or filtration before use.
Is 20-year-old gasoline still good?
Gasoline that is 20 years old is not usable. It breaks down completely. It forms gums and varnish that can damage engines and block the fuel system.
Complete Fuel Support for Generator Systems
Is your business facing generator fuel supply issues, or are you looking for on-site diesel delivery services? Rhino Fuel provides high-quality generator fuel at business locations with reliable fuel tank rental options for better fuel handling.
We offer mobile fuel delivery for jobsite refueling when demand rises and fuel levels drop. Contact Rhino Fuel to order fuel or view our service locations near your facility.
