What is diesel fuel quality testing

Regular diesel fuel quality testing is essential, no matter why or how you use diesel fuel. If you store diesel fuel for regular or emergency use, it’s vital to understand how fuel quality impacts performance and equipment function. Poor quality can lead to inadequate performance and can damage equipment. In extreme cases, poor quality can lead to equipment failure — that’s why regular testing is crucial. 

Fuel quality testing reveals combustion speed and efficiency as well as stability and contaminant buildup. Each of these factors affects fuel performance. Scheduling fuel quality tests for your stored fuel will ensure adequate performance and equipment protection. Be sure to plan for annual standardized fuel quality tests. In addition, follow all best practices for fuel storage and maintenance to help lengthen its usable life span. Learn more about diesel fuel quality testing — what it entails and why it’s necessary. 

Defining Diesel Fuel Quality

Diesel fuel quality is important because it directly relates to performance. Higher quality fuel lights quicker and burns more thoroughly. In addition, uncontaminated fuel protects parts and equipment from damage. The main way to measure fuel quality is by determining its cetane number, which highlights the speed and efficiency of combustion. Other diesel tests measure stability and contamination levels, which also affect how well it functions. In some cases, sending samples to a fuel testing laboratory will be necessary for accurate measurement. 

Common Diesel Fuel Contaminants

Impurities, which accumulate in stored diesel fuel, degrade its quality and performance. Proper storage techniques and fuel tank maintenance can avoid the buildup of these impurities. Some common contaminants include: 

  • Water molecules: Through the process of hydrolysis, water molecules break down diesel fuel. They also provide a breeding ground for microbial life. 
  • Microbial life: Organisms like bacteria and fungi interfere with diesel fuel in multiple ways. They produce corroding acids, clog filters, restrict fluid flow and damage equipment. 
  • Dirt: Sediment buildup is another contaminant issue. Sand, dust and other dirt particles in diesel fuel impact engine performance and can lead to equipment failure. 

Fuel Quality Testing With the ASTM D975 Specifications

The American Society for Testing and Materials (ASTM) outlines D975 specifications for diesel fuel testing methods. It includes 13 standardized tests with acceptable limits. In most cases, it’s best to test for ASTM D975 at least once per year. Regular testing will ensure contaminants or other variables have not compromised your fuel quality. Remember, the water and debris that can accumulate during storage will impact how well fuel functions. Adequate fuel quality testing will ensure you have safe and usable fuel when you need it. 

The Value of Diesel Fuel Testing

Fuel quality testing can uncover many different essential factors, some of which determine whether or not fuel is usable. Here are some of the things you can measure with diesel fuel testing: 

Oxidative Stability

Oxidative stability testing reveals a fuel sample’s stability, which is a vital characteristic of any diesel fuel. Testers put fuel through an accelerated stability scenario and measure how much sludge and varnish form as a result. Greater sludge deposits correspond with decreased stability. 

Fuel that fails the oxidative stability test will leave sludge deposits inside the storage tank and inside of other equipment upon use. In addition, sludge deposits result in incomplete fuel burning, which leads to black smoke. In extreme cases, the fuel can cause engine failure. Performing oxidative stability tests is necessary for ensuring safe and functional fuel. 

Cetane Rating

Diesel fuel’s cetane rating determines its performance — how quickly and how completely the fuel burns. The equivalent for regular gasoline is the octane rating. 

A decreased cetane rating can lead to equipment breakdown, so it’s a fundamental measurement. The cetane rating of stored fuel can degrade over time due to environmental factors and contaminants. For that reason, regular cetane rating tests for stored fuel are important. 

Water Content

Since water molecules can cause fuel to degrade and microbial life to accumulate, testing fuel for water content is also a priority. Excess water content can cause equipment malfunctions and poor performance. Water in fuel also leaves it vulnerable to contaminant buildup. Another issue is that water can cause freezing in fuel tanks, lines, pipes or filters. For those reasons, ensuring a safe maximum of water content in fuel is vital. 

Sediment Content

Checking for sediment is also necessary, as sediment accumulation can lead to failed or damaged equipment along with poor performance. Sediment can include dirt, dust, sand and similar particles. While a visual inspection can alert you of extreme buildup, a more detailed test will reveal performance quality. 

Sulfur Content

Measuring sulfur content ensures compliance with the Environmental Protection Agency’s (EPA’s) regulations. The EPA has set a maximum 15 parts per million (ppm) sulfur content limit for diesel fuel. The strict limit helps protect ecosystems as well as public safety — in the past, excess fuel sulfur content was a major environmental concern. Today, sulfur content tests will help you avoid liability by maintaining compliance. 

Microbial Content

Fuel testing will also address microbial content. Keep in mind, microbial life requires water to thrive, so reducing water buildup can also reduce microbial formation. Testing for microbes is necessary because their accumulation degrades fuel and clogs equipment. In extreme cases, microbial life can cause engine failure. In addition, microbes like bacteria and mold can present a human safety issue upon exposure. Fuel testing should always include microbial content measurement. 


Time and environmental factors cause diesel fuel to degrade, which impacts performance. Upon use, degraded fuel can lead to black smoke, trouble starting and engine stalling. Proper storage techniques, regular fuel testing and storage tank servicing can extend diesel fuel’s usable lifespan. For instance, keeping storage tanks full with scheduled tank refills will limit the amount of space available for contaminants to build up. In addition, fuel quality testing will alert you of degradation early so you can take steps to replace unusable fuel. 

Contact Foster Fuels to Learn More

Many different industries require diesel fuel storage for regular or emergency use, and routine fuel quality testing is an essential part of fuel storage. Quality testing ensures safe and effective performance upon use. ASTM tests measure oxidative stability and cetane rating, as well as water, sediment, sulfur and microbial content. Each of these ratings and measurements impacts performance. Poor diesel fuel quality test results indicate unusable fuel or fuel that might damage equipment. In addition, content tests determine compliance with legal standards, protecting your company from liability. 

If you store diesel fuel, be sure to schedule regular quality tests. Testing for fuel quality will ensure high-performance standards, protect your equipment from damage and limit liability concerns. If you’re interested in learning more about diesel fuel quality testing, reach out to the experts at Foster Fuels Mission Critical. We can be your strategic fuel partner providing award-winning risk management services.