Diesel shelf life and the infamous bug

Stu Davies

Julian Orchard wants to know what the shelf life is for diesel and what he can do to prevent diesel bug. Stu Davies answers his question

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I have a 44ft sailing boat with a 55hp Volvo Penta diesel supplied by a 280lt stainless steel diesel fuel tank.

The boat is kept on a mooring in Pembrokeshire’s Milford Haven estuary and in past years I’ve always made sure the tank is filled to the brim over the winter to reduce condensation, but now with the higher level of bio content in the fuel and the shorter shelf life I’m not sure if this is the best thing to do any more.

I’ve done a bit or research but am still not sure what’s the best option I probably only use around 100lt per year so the tank will be half full at the end of the season and some of the fuel in my tank is getting old.

Should I drain the tank and dispose of the fuel, top up with fresh diesel or try and get a supply of hydrotreated vegetable oil fuel instead?

Julian Orchard

Stu Davies replies:

There are mixed messages about the shelf life of diesel fuel but real life shows that diesel still has a vastly superior shelf life to petrol.

It doesn’t have ‘light ends’ like petrol that can evaporate and make the fuel less useable.

Instead it’s the increase in the bio content of diesel that’s the issue concerning us, and whether we should fill the tank to the top over winter.

My boat in Portugal always uses road diesel: I filled the tank to about two thirds over two years ago but with Covid it was very little used for a year.

Black substance coming out of a hose caused by diesel passed its shelf life

Tarry ‘diesel bug’ substance sucked from tank. Credit: Stu Davies

There’s been a two gallon jerry can of diesel in the locker all that time as well. In the middle of last year I was having issues with the tank level gauge reading wrongly.

There’s an easy solution – just remove five screws holding the sender unit in the tank and carefully lift the float and electronic sensor out to clean the contacts.

This also allows access to the tank – just – and the condition of it from the point of view of the bug can be easily checked.

Some diesel bug does collect on the bottom – despite being regularly treated with Marine 16 biocide – and so I do a periodic clean of the black, tarry substance that collects at the lowest point of the tank, which is where the stack pipe for the diesel injection system is fitted.

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The stack pipe end is located about 12mm above the tank and if the tarry goo builds up to that level then the system will suck up the sticky black stuff and cause problems.

I’ve owned the boat for 12 years now and have cleaned the bottom of the tank three times using a pipe on a stick, as shown in the photos.

It’s connected to my Pela oil changing pump. My wife, Laura, pumps the Pela and I squint into the tank and hoover up the sticky stuff with the pipe on the stick.

Black traces are left but any loose stuff is soon removed.

A hole giving access to a fuel tank

Tank gauge sender unit hole is the only access to inside the fuel tank. Credit: Stu Davies

Basically I have resigned myself to the fact that a check every few years, doing what I do, is now a regular maintenance item.

I also carry on board some 5-micron filter material which I can fit in my funnel and use to polish diesel fuel by pouring it through the filter material.

The tank was down to about a third full by this time and so I drained most of it into jerry cans through the filter.

I also checked the spare jerry can of fuel and it was clean but I polished that as well.

A view down a sender hole in a diesel tank, which is useful for checking if diesel shelf life has been reached

View down the sender unit hole after cleaning out. Credit: Stu Davies

The polished fuel was put back in to the cleaned tank with another dose of Marine 16 and then the boat was taken out and run hard for about an hour on engine.

All was well. My regime now is that I don’t fill up to the top.

My diesel road fuel has been in the tank for two years with no apparent harm.

Diesel bug does grow in the tank (to a lesser extent) despite preventative treatment. Polishing it and reusing it is what I do.

There is a bung at the bottom of the tank and I keep saying to myself that one of these days when the tank is empty-ish, I’ll remove it and fit a valve to periodically drain the bottom of the tank!

So my personal opinion and experience is that two year old diesel fuel is good to use, that bug treatment helps, and that fuel tanks and fuel should be a ‘maintenance’ item.

The interior of the tank should be accessed every couple of years, physically cleaned as I do, and if there are any concerns, polish the fuel!

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