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Contaminated fuel is a nightmare scenario for any industry. The damage that can be done to an aircraft or vehicle when contaminated fuel is used can be catastrophic, causing you to spend thousands. 

In this post, we will discuss how contaminated fuel can occur and what to do when it does. We will also provide a list of the warning signs of contaminated fuel so you can make the right choice in your next maintenance task.

What is Fuel Contamination

Fuel contamination is when different contaminants like water, microbes, etc., enter the fuel storage tank from the atmosphere, and in optimal condition, they thrive and degrade your fuel quality. 

Though using FAME (Fatty Acid Methyl Ester) fuel is an excellent alternative to mineral diesel fuel or gasoline to reduce the earth’s carbon footprint, it may not be favourable to your fuel storage business. It is more prone to contamination and requires a lot of maintenance than traditional fuels. 

Causes of Fuel Contamination

Let’s go through the causes of fuel contamination:


Water is the leading contaminant that can cause equipment wearing issues besides fuel contamination. Water is always present in modern ULSD (Ultra-low sulfur diesel) fuel. But if the quantity exceeds the approved limit (500 ppm), contamination starts. 

But how can water enter your fuel storage tank? 

  • As biodiesel is hygroscopic, it tends to absorb water from the atmosphere. 
  • Water can also find its way into your storage tank if there is any leak there. Even small leakage or open vents allow water to get penetrated and mix with the clean fuel inside. 
  • If the tank is half-filled, air and water vapour get deposited above the fuel. The water vapour expands in the day when the temperature is high and condenses and creates free droplets inside as the temperature falls at night.

While dissolved water can affect your fuel stability, emulsified and free water is more challenging to deal with and cause severe water contamination. 


Another most common type of fuel contamination is microbial contamination. The microbes that cause microbial growth in fuel are often called diesel bugs (fungi, bacteria, mold, yeast), and their by-products look like slime or coffee-like buildup on the tank walls or the interface between water and oil inside the tank. 

Microbes are naturally present in biofuels at a limited percentage and live on the alkanes in the fuel. 

Moreover, modern ULSD fuels are less resistant to microbes, and when they get an optimal environment (presence of water and 10-40°C), their growth accelerates. 


Different particulates also cause fuel contamination, and if they remain unchecked, they can be hazardous.

Some particulates are:

  • Abrasives
  • Asphaltenes
  • Dust and grime
  • Ferrous substances, and more

Warning Signs of Contaminated Fuel

Do you suspect your fuel is getting tainted? Check out the following warning signs of contaminated fuel and take necessary steps:

Fuel Filter Clogging

Fuel filters are specially designed to filter out any particles from the fuel before it reaches the engine. If your fuel filters are getting clogged frequently and need replacement, the possibility is, you are running on contaminated fuel. 

Fuel Pump Failing

The result of unchecked fuel filter clogging is the fuel pump failing, which means your engine cannot get a steady fuel flow.

Some symptoms of fuel pump failure are:

  • Sudden power loss in acceleration
  • Engine fails to fire.
  • Whining sound from the fuel tank
  • Engine sputtering at high speed
  • Low milage
  • Engine surging, etc.

Injector Failure

Fuel injectors are essential to supply an accurate blend of air and fuel to the combustion chamber to power the engine. They also ensure your vehicle can run smoothly and efficiently. 

As a result of contaminated fuel, your fuel injectors can partially or fully fail, and show the following signs:

  • Poor engine performance and decreased RPM
  • You require refill fuel more frequently.
  • Fuel injectors may deliver the engine more fuel than required resulting in poor fuel economy.
  • Frequent engine surging and power loss

Fuel Tank Corrosion

Another prominent sign of fuel contamination is a corroded fuel tank.   Microbial growth is the leading cause of fuel tank erosion. So, if you notice that your storage tank is getting corroded, the reason might be the microbes inside the tank. 

Fuel Appearance

Clean fuel looks bright, and if it looks cloudy or gets dark or brown, the possibility is, it is getting contaminated. 

While cloudy fuel indicates there is water accumulation inside the tank, dark/black appearance means it is tainted with particulates. 

And if you notice brown sludge underneath the water, the contamination is due to microbial growth.

Other signs of fuel contamination:

  • Fuel smells like ‘rotten egg.’
  • Heavy and black exhaust upon engine start
  • Corrosion in piston rings and cylinder lining

How to Fix Contaminated Fuel

If you identify fuel contamination and do not take the necessary steps to fix it before it gets too late, it can cause havoc inside your engine, leading to complete engine failure!

Let’s look at some viable options to fix contaminated fuel:

Fuel Testing

If you want to avoid the hassles of fixing contaminated fuel, and spending thousands or identify contamination early, fuel testing within every 6-months is essential.

For fuel testing, you have a handful of options, including:

  • Flash Point (D93): To determine if the fuel meets the minimum flash point to ignite appropriately.
  • Water and Sediment (D2709): To measure the sand and water content in the fuel. 
  • Microbe Count: To identify the number of microorganisms in fuel/unit. 

Tank Cleaning

Based on the level of contamination, you may need to clean your fuel storage tank. You have to shift the fuel into a sealed container and deep clean the tank, fuel filters, linings, and exclude the sludge buildup if you find any. Once the tank is clean, you can transfer the treated fuel back into it and ensure proper sealing. 

Fuel Polishing

Fuel polishing is the advanced mechanical fuel cleaning process to exclude contaminants like microbes, sediments, etc., from the stored fuel through circulation and filtration and maintain its quality. In this process, you have to empty the tank, filter out the contaminated fuel and refill the tank with the remaining clean fuel. 

Fuel Treatment

If the microbial growth is not severe, it can be treated with biocides, chemically-formed antimicrobial pesticides that can eliminate microorganisms from biofuel and prevent their growth. While using a biocide, make sure it is authorised, compatible with the additives you use, fuel, tank parts. 

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