This is a topic that comes up frequently in the boating community, so here we will look at what is diesel bug, how do you know if you’ve got it, how to treat it and ongoing treatment

What is diesel Bug

Diesel bug as it’s commonly known is a microbial bacteria that is found in diesel fuel, it survives on any water condensation found in its storage tank be it on the vessel or even ashore. The fuel itself contains hydrocarbons and bio-diesel is particularly prone to the bug. Thankfully the microbes have a short lifespan but do multiply rapidly and create waste. On death they sink to the lower parts of the tank where a thick sludge is formed.

How do you know you’ve got it

The less obvious signs are loss of power when accelerating or the engine misses a beat and you notice a variation in engine revs caused by a partial blockage to the fuel filters. In extreme cases the filters or pick up lines may become blocked and starve the engine of fuel, the normal order of work here would be to try and re-bleed the engine but it will not bleed due to a blockage in the fuel system. In our case, I had some gaskets to replace on the upper side of the fuel tank and whilst it was open I would take advantage of this and dredge the tank just to check for any water build up. Needless to say I was quite shocked at what I extracted as we had suffered none of the above mentioned symptoms.

Temperature plays a vital role in the microbes reproduction, just one of the joys of sailing in Spain and with it having been a particularly warm summer we would have experienced more condensation in the tank and despite keeping it topped up I am led to believe that all boats can / will suffer it’s simply a case of trying to manage your fuel by keeping the tank full to minimise air space, introducing a fuel additive and turning the fuel over regularly. Chances are you already have it, you just don’t know it yet.

How to treat it

Fuel additives can kill the microbial bacteria but very few dissolve the sludge and residue they leave sitting in the lower reaches of your fuel tank. So whilst this minimises the initial bacterial growth it doesn’t solve the problem. If you have a reasonable size tank, fuel polishing is probably the most effective solution here, fuel polishing is done where a circulating pump passes the fuel through a centrifuge and series of filters of varying microns before returning the clean fuel to the tank, there is very little waste using this method, However we must look at why water is getting into the fuel in the initial stages, it’s primarily condensation but other factors such as buying fuel in low turnover fuelling points will not help and tank breather valves and worn o-rings on filler caps can allow water ingress. so check these areas as well or else the problem will return quickly.

Without access to specialist equipment I used our extraction pump (oil) and attached a stiffer plastic pipe to the hose, this allowed me to move the hose around various parts of the tank and extract the worst of it. Fuel is lost using this method but it’s a cheap solution. We had Diesel Bug back in 2010 and the mechanic polished the fuel and since then we regularly use Marine 16 fuel additive to the tank and have never noticed a problem since, my conclusion is that further to such a hot summer and potentially 7 year build up we need to increase the quantity of additive.

Ongoing treatment

Regular use of additive is going to help and has kept us going for the last 7 years trouble free so I have confidence in the product, I believe we need to increase dosage during the warmer months and ensure the tank is topped up even more regularly, the o-ring on the deck filler cap appears in good order but his will be replaced just in case. Regular fuel filter changes will reduce the chance of blockages and any evidence of sludge should be present in the primary filter.

Conclusion on Diesel Bug

Government guidelines and environmental pressures are pushing for increased bio content on fuels to increase from 7% to 12%, once this happens the problems for boaters will certainly only get worse.The best action here is prevention, so by keeping the fuel tanks full, buying diesel from busy high turnover fuel stations and using an additive there appears little else we can do. Keep checking primary filters, especially on boats with glass bowl filters to detect the first signs of any blooming and turn the fuel over regularly, I am often amazed when I talk to sailors that have not topped up tanks for the last 5 years and still have half a tank of fuel. Regular checks will keep you moving and if you suspect you have the bug get it sorted before casting off – remember the engine on a sailing yacht never breaks down at a convenient time, it breaks down when there is either no wind or your entering/leaving a harbour or confined piece of water. Happy sailing

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