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Fuel Contamination

Diesel fuel can contain many types of contaminant; all of them can be harmful to your engine.


Can be present in fuel in three states; dissolved, emulsified and free.

Dissolved Water

Water that is chemically dissolved, or absorbed, into water and distributed molecule by molecule. Just like sugar in tea.

Emulsified Water

Where very small droplets of water are suspended in the fuel. Just like oil and vinegar in a salad dressing.

Free Water

Water that falls out of suspension in the fuel and gathers at the bottom of the fuel tank. Whilst dissolved water can affect fuel stability, it is free and emulsified water that are more problematic. Not only can it cause fuel system and engine damage but it also promotes Microbial growth, or diesel bug, in fuel systems.

Diesel Bug

Microbial growth within a fuel tank or system is commonly known by the generic term ‘diesel bug’. Diesel bug isn’t a single type of organism, there are up to 100 different types of bacteria, moulds and yeasts that have been found inhabiting fuel systems.


Single cells, typically 1-10 micron in size, 20-30 minute generation time (the time for the population to double), one cell can multiply into 2 million in 7 hours. Bacteria will degrade fuel over time.


Type of fungi, long multi-cellular filaments. There is little indication that they degrade fuel but due to the long strands are effective at blocking filters etc.


Type of fungi, relatively slow growing. Typically 3-4 micron in size.


A biofilm is a complex structure of microbes that adheres to the walls of the fuel tank. It begins to form when free floating microbes land on a surface and attach themselves to it. This attachment is initially reversible, although if they are not removed they start to change their structure and become irreversibly attached to the surface. These microbes then start to divide and attract other microbes to join the colony. Bio-films are complex structures that, given time, can grow to millimetres thick and contain billions of microbes. Sometimes, perhaps following fuel tank turbulence, chunks of the bio-film will slough off and will block fuel filters. The bugs in bio-films also excrete acid, this acid will erode a metal fuel tank quite easily and many holed tanks are as a result of biofilm formation.


It is often claimed that ‘sludge’ is found within diesel fuel systems, and it is often interpreted as being one of the above contaminants. In reality it is likely that it is formed from a combination of many of the above contaminants.


These are present in all petro-diesel fuel to a greater or lesser extent, although they tend to increase with changes to fuel temperature and fuel oxidation. They are hard, brittle particles that are not soluble in fuel, and are generally less than 2 micron in size making them harmless to the fuel injection system. However they can agglomerate into larger particles which can easily block engine filters. These particles tend to collect at the bottom of a fuel tank and can form an oily sludge that is often confused with microbial contamination.

Gums and Other Organic Contaminants

The oxidation stability of bio-diesel is inferior to that of petro-diesel. Meaning that as the fuel comes into contact with oxygen chemical reactions break down the fuel into peroxide, organic acids and gummy sediment. These soft, sticky substances can stick to fuel filters and engine components and cause acid erosion.

Other Particles

Other particles can be present in fuel with diverse sources such as road dust and grit, soot, fuel tank rust, engine wear particles.
All particles, regardless of source have the potential to cause wear or damage to the fuel injection system or engine.

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