Are you experiencing frequent issues with your diesel engine and suspecting it is running on fuel tainted with microbes and diesel bug? 

When microbial degradation of diesel fuel causes 80% of engine malfunction, taking proper preventive measures to check diesel bugs is crucial. Be sure to use the best diesel bug test kit to check your fuel regularly.

If left untreated, they may wreak havoc in your diesel-powered engine, causing complete system disruption and making you struggle with the mounting cost! 

In this post, we will cover common diesel contamination causes and how to prevent diesel bugs.

What is a Diesel Bug?

Diesel bugs are microorganisms and their succeeding reaction products that cause microbial contamination in biodiesel and feed on the nutrients, water, and hydrocarbons in the fuel. 

They look like blackish or dark brown thick concentrated slime or sludge-like buildup. They exist between the layers of diesel and water in the storage tank and smell like rotten eggs. 

Types of Diesel Bugs

Including some airborne microscopic species, there are around 100 microbes that can contaminate diesel fuel and cause engine breakdowns. 

Some leading types are:


  • It is a 0.3-0.6 mm single-cell microorganism that can deteriorate diesel quality drastically.
  • One single bacteria cell can multiply into 2-million cells in biodiesel in only 7-hours.
  • If in groups, they can be doubled in 20-30 minutes!


  • It is a microbe with a complicated organic structure that forms a group and sticks to the vessel walls. 
  • Producing sludge, they can promptly reproduce in the optimal environment.
  • It neutralises the acidic environment in the diesel, and the tank corrodes over time.
  • Creates the optimal environment for other microbes to thrive and block filters. 


  • It is a slow-growing yet harmful fungus.
  • Yeast sizes approximately 3-4 mm.


  • It is another type of fungus that develops in the form of multicellular filaments termed hyphae.
  • Can cause filter clogging in diesel engines.

What Causes Diesel Fuel Contamination

  • When your fuel contains water, it spurs microbial growth. While you would never want the diesel to get mixed with water, you cannot avoid it entirely. 
  • Modern Ultra-low Sulfur Diesel (ULSD), a mixture of 7% biodiesel and 93% petroleum diesel, can significantly lower the carbon footprint in the environment but is not advantageous for your vehicle engine. It contains a low quantity of sulfur to keep water separated from diesel, and as the FAME biodiesel is hygroscopic, it absorbs more water from the atmosphere. 
  • Water can also enter the storage vessel by rainwater ingress, humidification, and condensation due to different temperature exposure for ground-level storage tanks.
  • Modern FAME biodiesel also contains different minerals and hydrocarbons that can make the fuel unstable by attracting more water. They promote microbial development by serving as a good food source for them. 
  • These microbes spur if the temperature inside the fuel tank is between 10-40 degrees Celsius in the presence of water. 

Signs of Diesel Bug Contamination

So, how can you recognise the signs of microbial contamination? 

Check for the following symptoms:

  • Do you require changing the fuel filters as they get blocked so frequently? It can be an outcome of diesel bug contamination as their byproducts can clog filters. 
  • There may be coffee-like residues in the fuel filters.
  • Fuel pump failing. The pump cannot maintain a uniform flow while transferring fuel to the engine that intervenes in the mechanical strokes and engine functionalities. 
  • Sudden power loss in the engine.
  • As tainted diesel cannot burn completely, the engine may need more fuel to generate the same amount of power, resulting in an increased fuel consumption rate. 
  • Sudden deceleration and jerks at acceleration
  • Decreased engine RPM
  • Your fuel injectors require replacement or frequent cleaning.
  • You may notice a reduction in fuel pressure through injector nozzles. 
  • Fuel discoloration
  • The diesel smells like rotten eggs.
  • Dark exhaust upon engine start. 
  • In the worst case, complete engine failure!

How to Check Diesel Bug

Though microbes are present at a certain degree in all biodiesel, they may cause you to spend thousands to make the system operate optimally again if you ignore the signs. 

Let’s get insight into some preventive measures to avoid expensive fuel cleanups:

Always Buy from a Trustworthy Dealer

Reputed dealers take proper preventive measures to sustain the diesel quality. They also adopt industry-grade best practices to avoid microbial development and ensure you get the fuel in the best condition. 

Replace Fuel Filters Periodically

Fuel filters ensure the engine runs on clean fuel by filtering out the dirt and smudges. Thus, by replacing fuel filters systematically, you can make sure only clean diesel gets delivered to the engine. 

Always Keep the Tank Full

If you leave the diesel tank half-filled, it will stimulate microbial growth by condensing water over time. Keep the tank topped up so that it cannot get enough room for water accumulation. 

Ensure Proper Maintenance

Besides buying from a reputed source, you must ensure top-notch system monitoring and maintenance to prevent microbial contamination in diesel. You must clean the storage tank systematically, recycle the fuel by periodic water separation, discharge water from the base of the tank, etc., to keep the diesel in its top-notch condition. 

Treat the Fuel with Biocide

As we have already mentioned, microbes can thrive in the presence of water and optimal temperature when they get minerals to live on. If you suspect your fuel is getting tented by microorganisms, treat it with fuel polishing and biocides before it is too late. Biocides can effectively kill microbes and prevent their further development. 

Other best practices to prevent diesel bugs:

  • Turn over the biodiesel regularly as microbes can multiply more rapidly if the diesel is left idle.
  • Check if there are leaks and wear in the tank to avoid water spilling. 
  • Seal the tank properly so that air cannot get into it. 
  • If you diagnose microbial growth in your diesel, make sure you completely clear out the storage tank to decontaminate it properly. Check the fuel lines and filters for corrosion and clean them robustly. 
  • Install fuel contamination units for constant monitoring.