Why isn’t fuel pre-treated to prevent diesel bug? Ask the experts

Stu Davies

PBO reader Glynn Jones wants to know why fuel suppliers don't add diesel bug additives to their products. Engine whisperer Stu Davies has this response…

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Glynn Jones writes: “If I have understood the issue correctly, the addition by the oil industry of biodiesel to our boat fuel is the main reason that ‘diesel bug’ is now such an annoyingly common problem for boat owners.

“Fortunately, numerous additives are now on sale that enable us to render the fuel suitable for use. All we have to do is measure out the recommended dosage and add it to the fuel whenever we top up the tank.

“In order to ensure it mixes properly, it’s probably best to do this before adding the fuel to our tank, so we need to know exactly how much our tank will take in order to calculate how much chemical to add; if we get it right, we should have the problem under control.

“Has it occurred to anybody else that our suppliers are actually selling us a product that is not ‘fit for purpose’ and that if they added the appropriate chemicals at source we’d be certain that it was done, the proportions would be correct and the fuel would be safe to store in our boat fuel tanks?

“Why is it left up to us to treat the fuel we purchase in order to make it fit to use? I am informed that many marinas in the USA dose their fuel with the appropriate chemical to protect their own storage tanks as well as their customers.

“So far I have avoided the diesel bug problem myself, but I find the process of self protection rather clunky and believe that our marinas have a duty of care to sell us a product that is fit for use in the hostile environment that they share with us.”

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PBO engine expert Stu Davies replies: “Basically we use our fuel in a hostile environment and our market segment is such that we are insignificant. I doubt that fuel companies would make a fuel specially for boat owners’ use, and if they did would we be willing to pay the price for it?

“We moaned enough when the price of red diesel went through the roof – speciality fuel would probably be an eye watering price. Our use of fuel in small quantities encourages the growth of diesel bug. It has always been an issue, biodiesel just exacerbates it (bear in mind that the bug occurred before FAME appeared on the scene).

“Big government and the EU are driving the use of biofuel not big companies. I personally have strong views on the use of food to produce fuel, but that’s another argument!

“One thing you could consider is using premium diesel such as Shell V-Power or search out a supplier of FAME-free red diesel. In many respects, we are similar to classic car owners and the use of non-leaded petrol in old engines – their owners manage to deal with it.

“I consider diesel fuel to now be a maintenance item, and dealing with diesel bug is much like changing filters on a regular basis:

  1. Dose the fuel and/or source FAME-free fuel
  2. Open the tank on a regular basis and clean any black residue out

“I have documented how I do it with a Pela oil change pump previously. I dose my fuel every time I fill the tank, and change my fuel pre-filters annually and the secondaries every two years. I don’t have any problems.”

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This feature appeared in the August 2023 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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