FAQ – Diesel Fuel Analysis

What is diesel fuel analysis?

Diesel fuel analysis identifies causes for fuel filter plugging, smoking, loss of power, poor injector performance, malfunctioning throttle position sensors and sticking valves.

Why should I do diesel fuel analysis?

Periodic fuel analysis can identify fuel conditions detrimental to engine performance which are typically easily fixed with treatment.

When should I have my fuel tested?

  • If there is an engine performance problem, take a sample of the engine fuel and have it tested for performance properties.
  • If you have bulk delivery shipments to your terminals, each shipment should be tested for basic properties.
  • Possible contamination is cause for testing.
  • Large bulk reservoirs should be tested at least twice a year for basic properties – Water & Sediment, and Bacteria, Fungi & Mold.
  • If you are blending #1 & #2 Diesel Fuel for winter applications, Cloud Point, Pour Point will tell you if you have reached the desired level of protection.
  • If you are blending anti-gel additives with bulk fuel, testing may be necessary.

What are the most common diesel fuel analysis tests? 

API Gravity

A measure of the fuel’s density. This test is required to calculate the cetane index.

Bacteria, Fungi Mold

A dip slide is incubated for a set period of time, providing ideal conditions for any biological growth. Treatment with a biocide is recommended for any positive growth.

Cetane Index

A measure of the combustion quality of diesel fuel.

Cloud Point

The temperature at which wax crystals are first observed as fuel is cooled. This is an indication of the potential for cold temperature fuel filter plugging.

Flash Point

The lowest temperatures at which fuel emits a flash when exposed to flame. The minimum requirement for #1 diesel is 38C, where the minimum for #2 is 52C.


Measures the ability of the fuel to prevent friction and wear. Lubricity became a critical property to monitor with the introduction of ultra-low sulfur fuels because sulfur helped boost this property.

Pour Point

The lowest temperature at which fuel will still flow.


Measures the amount of sulfur in fuel in ppm.

Thermal Stability

Measures the tendency for fuel to produce asphaltenes at high temperatures, which are a common cause of fuel filter plugging.


Viscosity is a measurement of a fluid’s resistance to flow at temperature. In other words, viscosity is the film strength or thickness of the fuel. A change in viscosity can be an indication of mixing fuel types or contamination.

Water & Sediment

The percent of water and/or sediment observed when a measured volume of fuel is placed under centrifuge.

For more test methods performed at POLARIS Laboratories®, download our complete test list.