Dealing with Diesel Bug

Dealing with Diesel Bug
There is a myriad of things that can go wrong with your engine, but sometimes the culprit isn't a mechanical failure, it’s a biological one - diesel bug.
Diesel bug, or diesel plague, refers to the growth of microorganisms like bacteria, yeast, and mold within your fuel tank. It’s a contamination that can go unnoticed until it is too late, so knowing how to identify and treat it is key.

The foundation of microbial contamination
For microorganisms to be able to contaminate your fuel tanks, there needs to be more than diesel present. What sets the entire process in motion is the presence of water.
Water can get into your boat’s fuel in several ways. 

• Condensation due to the daily changes in air temperature
• Contaminated diesel during refuelling
• A leaking tank or filler cap
• Human error

Water is heavier than diesel, so settles at the bottom of the tank. The interface, or separating layer, between the water and the diesel offers the microbes a viable habitat. This is especially true in warmer climates, where the higher temperatures result in your fuel tank offering perfect conditions for the rapid proliferation of these tiny stowaways.

The problem with diesel bug
The problem is not the bacteria itself, but the sticky, dark-coloured metabolic products produced. As these organisms consume the hydrocarbons and nutrients in the diesel fuel, they excrete waste that forms a black, brown, or green bio-sludge. This sludge is problematic in two major ways: mechanical and corrosion.

Mechanical
The slime can clog vital components such as fuel filters, diesel lines, or the very small openings in the injection nozzles. When the boat encounters rough seas, the previously dormant sludge, which sat on the tank bottom, gets stirred up and moves to block the fuel intake, causing the engine to stutter or die completely. 

Corrosive
The sludge can be acidic, actively promoting bio-corrosion in the tank and on key engine components. This bio-corrosion can subsequently lead to long-term engine damage.

Checking for contamination
A quick, practical way to diagnose potential contamination is to take a small sample from the very bottom of the tank using a sludge dipper or clear tube; if you see water or particulate matter, you know there may be an issue. 

Preventing and controlling the growth
There are a few different actions you can take to prevent and control the growth of diesel bug.

Drain the water
Since water is the essential catalyst for microbial growth, the most effective preventative measure is regularly checking and draining any accumulated water from the bottom of the fuel tank. It’s best for this maintenance to be scheduled, not reactive. 

Keep tanks high
A handy practice is to keep the tank full, especially when laying up the boat or during periods of temperature fluctuation. A full tank minimizes the surface area of the tank walls that is exposed to air, which in turn significantly reduces the amount of condensation that can form inside the tank.

Choose premium fuels
Consider using premium fuels that inherently have a lower biodiesel content, as the fatty acid methyl esters in biodiesel are particularly nutritious for the microbes.

Biocides
There are biocide additives for conventional diesel that help inhibit the growth of these organisms, or even completely remove them. Depending on the concentration of the treatment, the service life of the product, and local climatic conditions, even a small dose of these chemicals may be sufficient to control the microorganisms and prevent bio-corrosion. 

Install a fuel polishing system
Polishing the fuel involves using a dedicated external filtration system to circulate and filter the fuel, removing water and particulate matter. These systems usually use a 12V pump to take the fuel from the bottom of the tank and filter it through a fuel filter and water separator, before returning it to the tank.

Addressing a serious infestation
Once diesel plague has struck with full force, there is usually no way of avoiding an extensive fuel tank clean. While biocides are highly effective at killing the living microbes, they are not designed to dissolve the existing bio-sludge without residue. If a significant amount of sludge has formed and the engine is stuttering, adding a biocide will kill the organisms, but the physical slime remains in the tank, ready to clog the system again.

The process of cleaning a tank requires the contaminated fuel to be safely pumped out, the sludge physically removed, and the tank walls cleaned. It’s a good opportunity to check that the tank is structurally sound, and it may be worth consulting a specialist for advice tailored to your vessel’s fuel system design.

Clean tanks, happy engine
Being vigilant with fuel quality and water separation and knowing how to treat diesel plague effectively is a smart move for all diesel engine owners. The last thing you want is to end up dead in the water, with clogged lines and an engine that just won’t go.

Have any specific maintenance tips or experiences managing diesel bug? Share them in the comments to build a wealth of knowledge for the NFL community!