Diesel Bug - Microbial Growth

If you own a diesel generator, then you may have heard of Diesel Bug. It’s something that should be a concern to you, as it’s a type of fuel contamination that occurs after a period of time. We strongly advise that you seek regular diesel generator servicing to prevent this issue from happening, which will end up lengthening the lifespan of your equipment. In today’s blog post, we’re going to explain everything there is to know about Diesel Bug, and what it will do to your diesel engine.

What is Diesel Bug?

Essentially, this is just a blanket term for any type of microbial growth that occurs in diesel fuel. It’s something that’s only just come to light in the last few years, largely thanks to a change in the type of fuel used. Right now, most diesel contains just under 10% biodiesel, which is more environmentally friendly than the petrochemical diesel that makes up the remaining 90-odd percent.

The problem is that biodiesel is also more prone to attracting water, which basically creates the perfect environment for a Diesel Bug to grow and develop resins that attach themselves to your fuel tanks, leading to problems for your generator.

How does Diesel Bug form?

Diesel Bug mainly forms when there’s water in the fuel. This can quickly enter a fuel tank through rainwater, condensation, or even contaminated diesel. It occurs when bacteria feed on the hydrocarbons found in the fuel, which is known as the lag phase. This is followed by the log phases where the bacteria grow in the water. The water will usually be found at the bottom of the tank, with the fuel lying on top of it. Diesel bug tends to grow where these two lines meet, in what’s called the rag layer.

When this starts to happen, it presents some serious problems to a diesel generator as the fuel becomes contaminated with tiny cells that can end up blocking filters and damaging essential parts of the generator. As a result, your diesel generator can break - and many manufacturers are voiding warranties when this happens.

How many types of Diesel Bug are there?

As we mentioned earlier, Diesel Bug is something of a general term for all kinds of microbial contamination in diesel fuel. In total, there are four main types of Diesel Bug that you’re likely to see in your fuel tanks.

Bacteria: This is the most common form of Diesel Bug, and they are tiny in size. The problem with bacteria is that it experiences exponential growth. The number of cells can double in as little as half an hour, with one cell creating millions of daughter cells in just a few hours. This is problematic as it severely decreases the quality of your diesel fuel.

Biofilm: As Diesel Bug grows on the rag layer between fuel and water, it will eventually become a planktonic form called biofilm. This gets sucked into your diesel engine, which can block filters and clog up pathways. But, the biofilm is a very sticky substance that can adhere to the walls of the tank and excrete acid. As such, it can erode a fuel tank as well, leading to leakages and further contamination.

Mould: When Diesel Bug occurs, it can create mould spores in the fuel tank. While research doesn’t suggest this degrades film like bacteria, mould is rather large in size, which results in blocking filters.

Yeast: This is similar to mould and presents the same issues, but it grows a lot slower.

Why is Diesel Bug so problematic?

We’ve touched upon this a few times already, and the main issue is that it will significantly decrease the lifespan of your diesel and lead to problems with the overall generator. Depending on what type of Diesel Bug you have, this can cause issues with your filters or fuel injectors. The microbial contaminants will clog up the fuel system and result in your generator breaking down - along with your fuel degrading in quality and dropping below the European standards.

This is especially problematic when you take into account that most manufacturers are taking action against Diesel Bug and all forms of fuel contamination. They see it as your responsibility to keep your fuel clean, which is why your warranty is void if your diesel generator breaks thanks to Diesel Bug.

Consequently, you will have to spend a lot of money on repairs or a brand new generator - and this will happen time and time again if you don’t know how to prevent it.

How do you prevent Diesel Bug?

If your diesel fuel tank is already suffering from this problem, then it is possible to have it professionally cleaned to remove the existing contamination. However, prevention is always the best approach. Thankfully, Diesel Bug can easily be prevented, and all you need to do is maintain a consistent service record.

We strongly advise that you consider servicing your diesel generator at regular intervals to check that everything is as it should be. This can help prevent Diesel Bug by testing your fuel, giving your tank a clean, and ensuring that the conditions remain healthy. The simple fact is that diesel fuel tanks are going to be susceptible to microbial growth if they’re not maintained and looked after. It might seem like an additional cost to service your generator but think about all the money you’ll save as well. It costs far less for regular servicing than it does to repair a broken generator or buy and install a new one.


To round of this piece, we just want to summarise some of the key points to take from it. Diesel Bug refers to any type of microbial contamination in diesel fuel. This happens thanks to the inclusion of biodiesel, which attracts water, which encourages microbial growth. There are four different types of Diesel Bug to be wary of, and they can block filters, degrade your fuel quality, and cause your generator to break down.

Any generator that malfunctions thanks to fuel contamination will have its warranty voided, so you need to service yours regularly. Servicing helps prevent diesel bug, meaning you have one less thing to worry about.