Diesel Bugimage

Diesel Bug

Engine Maintenance


As part of our MAN dealership capabilities, two of our engineers recently attended a research catamaran.

The work scope was a 2000-hour service on two MAN D2848 LE 423 V8-900 engines.

And during the work, they came across a common but avoidable issue…

When inspecting the primary fuel filters, they found that the bowl from the filter in use contained a large amount of sludge in the bottom, and the filter itself contained a large amount of sludge in the top.

This sludge is caused by bacteria in the diesel, also known as “diesel bug”*

Sludge in filters and water bowl, and particles in diesel caused by Diesel bug.

In this case, the bug was likely caused by condensation forming on the inside of the tank, whilst the vessel was out of use for several months.

When this happens, it can cause water contamination and provide the bacteria with the perfect environment to grow.

How do you know if your engine has diesel bug?

  • Brown or black sludge in filters
  • A layer of sludge on the bottom of the tanks
  • Black particles in the diesel

This is preventable, and we have a few pointers to help you avoid it:

  • Ensure diesel is stored correctly
  • Reduce the amount of water that can form in the tank (filling diesel tanks completely before a vessel is laid up to prevent condensation forming on the inside of the tank)
  • Fit a drain valve on the lowest point of the tank and frequently drain any water
  • There are also diesel additives that can help prevent it from forming and multiplying

If you discover you already have diesel bug, it can be very difficult to eliminate and can often return after treatment, here’s what we suggest:

  • The diesel will require treatment through a fuel polishing system
  • Clean the tanks and fuel system with a biocide and flush them
  • Consider using additives that help to kill and prevent diesel bug from reoccurring

* Diesel bug is a living contaminant that includes microbial bacteria, fungi and algae. It lives in the interface between diesel and water just off the bottom of the tank and breaks down the alkanes in the fuel using oxygen from the water and excretes a brown or black sludge. This black sludge will very quickly clog fuel filters and choke the engine.

Diesel bug can double in size every 20 minutes given the correct environment, so diesel must be stored correctly.

If you have any concerns with the performance or health of your engines, get in touch, we’d be happy to carry out a health check for you.

Next week we’ll share our top six tips on oil care.

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