diesel fuel quality

7 More Things to Consider When Maintaining a Diesel Fuel System

In our blog post last week, we discussed the first 7 of 14 characteristics of diesel that must be considered and maintained to ensure optimal performance from your diesel fuel engine.

Today we’ll discuss the second half of this list:

  1. Heat Value
  2. Ignition quality/cetane rating
  3. Viscosity
  4. Sulfur content
  5. Water and sediment content
  6. Carbon residue
  7. Flash point
  8. Pour point
  9. Cloud point
  10. Active sulfur copper strip corrosion content
  11. Ash
  12. Distillation
  13. Specific gravity
  14. Winter Fuel issues


We measure the corrosion tendency of diesel fuel by its reaction with the copper, brass or bronze parts that make up the fuel system components. Its corrosion tendency is measured by heating the fuel to100°C (boiling temperature) and submerging a  strip of polished copper into the fuel for three hours. The results are interpreted three ways: as slightly tarnished, moderately tarnished or as having a dark tarnish.


Diesel fuel contains ash-forming materials such as abrasive solids and soluble metallic soaps. These abrasive solids cause wear on injection components, pistons, rings and cylinder bores. Ash from soluble metallic soaps may contribute to engine deposits and wear.

To determine the amount of ash in diesel fuel, analysts burn a measured amount in an open “pot” until all of the carbon deposits have been consumed. The remaining ash is weighed as a percentage of the weight of the original fuel sample.


Volatility is liquid’s ability to transform into a vapour. As volatility goes down, carbon deposits and component wear could increase with some engines producing more smoke with this increase.

Specific Gravity

The specific gravity of diesel fuel is measured with a hydrometer and is defined as the ratio of fuel density to water density.

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Specific gravity changes the spray pattern of diesel fuel when it is injected into the cylinder. To determine if fuel quality is a factor in an engine performance issue, we use the hydrometer reading for fuel quality and temperature.

Winter Fuel issues – Cloud Point and Pour Point

Wax content is another important factor that may affect the performance of a diesel engine in cold weather. This was is referred to as “paraffin.” Because of the wax’s relationship between temperature and solubility, separation is often a problem in the handling and use of diesel fuel during cold weather operation.

Paraffin, a solid, waxy crystalline mixture is composed of straight-chain saturated hydrocarbons that naturally occur in all diesel fuels. This paraffin melts at a temperature of approximately 40 to 60° C (104 to 140° F).

The content of this wax varies greatly and depends on the crude oil from which is was originally produced and processed. As the fuel is cooled, it reaches a point where it becomes saturated with wax; any further cooling causes the wax to separate from the fuel.

At colder temperatures, the hydrocarbons in the fuel become less soluble and start to precipitate and form wax crystals. The temperature at which these wax crystals become large enough to make the fuel appear hazy, or cloudy is called the cloud point.

The cloud point indicates when the fuel filter begins to plug up. As fuel continues to cool, the wax crystals precipitate and grow. The lowest temperature at which the fuel can flow is referred to the pour point. Fuel additives can help by reducing the pour point temperature, however, using good quality No. 1 diesel fuel is your best option for optimal performance in cold weather.

Now that you are a diesel fuel expert, you understand why it is so important to use good quality fuels from a trusted filling station. Good fuel will protect your investment and ensure consistent, reliable operation of your engine over its long service life.

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