PBO reader Martyn Crossland asks: “I have a 1989 Beneteau Oceanis 320 that I bought in Greece in 2009. I spent two months restoring her, and then had two months wonderful sailing in the Ionian, after which I had her hauled out, originally just for the winter.

“For various reasons I’ve not been able to sail since. Consequently, the tank is now full of 11-year-old diesel. I’ve topped it up two or three times meanwhile, to avoid leaving air in the filling pipe, in the hope of at least limiting diesel bug, but my question is, what procedure would you recommend when I return to the boat to sail?

“Would it be best just to pump the whole lot out (hoping that the boatyard has somewhere to put the stuff!), rinse the tank and start again, or might it be possible to check the state of the diesel and possibly filter it? Oh yes, there is no access trap at all in the tank, as far as I can see.”

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PBO engine expert Stu Davies replies: “Diesel doesn’t usually degrade like petrol does. It doesn’t evaporate and if kept in a dark, sealed container – which basically your fuel tank is – it should be OK.

“And there is a way of getting in to your tank – the fuel level sender. Ry removing five or six screws the sender can be wriggled out leaving a hole which you can utilise to inspect the interior to see if there is any dirt or bug in your tank.

“If it looks dirty or clouded then you can ‘polish’ the fuel by passing it repeatedly through a filter. If your Beneteau is the same as mine, then there’ll be a CAV 296 primary filter on or near the fuel tank.

“These are rated at about 5 micron size and will be more than adequate to clean or ‘polish’ your fuel. These filters are cheap and available just about anywhere.

Plumbing in a pump to run diesel through a filter system will help clean the fuel so it can be used

“So you could buy an electric fuel pump, similar to this one pictured from ASAP Supplies, and use it and the filter to ‘polish’ your fuel. To set it up, disconnect the suction pipe from your tank to your existing filter and connect the pump in between the tank and filter.

“Then disconnect the outlet pipe from your filter and fit a length that can go back to your tank through the fuel level sender hole. Connect up the pump then start polishing your fuel through the existing CAV 296 filter.

“Basically by putting the electric fuel pump in between your tank and the filter you are using it to suck fuel from the tank, push it through the filter and return it to the tank.

“If you let the pump run for a few hours, all the fuel in the tank will be pushed through the filter several times. I’d change the filter a couple of times.

“If you have a serious colony of bug the best cure will be to empty and remove the tank and clean it with a pressure washer – but the diesel can still be polished and reused.”

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This feature appeared in the December 21 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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