Diesel bug is the boat owner’s enemy. This microscopic collection of yeasts, moulds and bacteria absorbed from the air lives for 24 hours and doubles in numbers every 20 minutes.

It’s the dead organisms that build up to make that sludge.

Dirty fuel, or sludge, can get into fuel lines, block filters and injectors, cause internal corrosion, reduce horsepower and even cause engine failure.

And the fuel tank is a wonderful place for them to live… access to food (carbon), water (so they can grow), oxygen to breathe and the perfect temperature. Sold!

So when Stu Davies warned me we might find diesel bug in the tanks of our Maxi 84 Project Boat, I waited with bated breath.

The 28ft cruiser had been sitting in a boatyard for 18 months, its fuel tanks half empty.

“Diesel bug grows on the interface between water and the diesel,” Stu explained, shining a torch in the fuel tank, and illuminating a layer of ominous black crud.

“You can see where condensation has trickled down the inlet pipe, gone into the tank, and where it’s run to the lowest part of the tank. The black marks are where the bugs have been.”

“Been?” This sounded promising. “So the bugs aren’t there anymore?”

“I don’t think that bug is live,” he clarified. “OK, if you want to be really pedantic, take the tank out and steam-clean it next season, but looking at the pickup it’s relatively clean and I don’t think that bug is going to affect it.”

Though we didn’t have a drain cock in the fuel tank, Stu pointed out the 5-micron primary fuel filter in the Volvo MD2020 engine.

This was our first defence against dirty fuel, and had a glass sediment bowl at the bottom to catch any water or dirt filtered out, which could be drained away.

Stu advised adding some Marine 16 diesel bug treatment when I next topped up and I’m pleased to report we had no fuel problems on our two-day delivery trip from Chichester Harbour to Poole.

Should you leave your diesel tank full over winter?

I was lucky. Despite the fuel tank having harboured the dreaded ‘bug’ in the past, the current fuel was in good shape.

But isn’t accepted knowledge to fill the fuel to the top of the tank when laying up? Maximus’s tank was half-full.

“In fact, our recommendation is to leave diesel tanks as low as possible over winter,” says Peter Weide of Marine 16.

“Decades ago, diesel would last forever. But gone are the days of filling up your tank and forgetting about it. Now that diesel has been refined to within an inch of its life it’s very unstable. It degrades so quickly, particularly because of the rapeseed, soya bean and other natural organic oils. Imagine a banana left in a bowl. It rots, and at the end of the day, the oil they put in your diesel rots and ends up as a sludge at the bottom of your food tank.”

But what about condensation? Presumably, there will be more of that if the fuel level is low, but is this the lesser of the two evils?

“Yes, there can be condensation problems and yes, you will get water in your fuel tank,” says Peter. “But when you come back in spring you can get that water out and top it up with nice, clean fresh diesel and an additive such as Diesel Fuel Complete. It’s best to do that at the start of the season.”

How long can I store diesel?

Manufacturers agree that modern diesel shouldn’t be stored longer than six months, explains Peter – and that’s including the two months it takes to get from the refinery to your marina pump.

During this time it can also change hands anything from seven to nine times.

“Crude oil goes through so many processes to produce the ultra low-sulphur finished product we have today… it’s unstable, degrades rapidly and leaves deposits everywhere, including the injectors.”

So what if your boat will be laid up for longer than six months? Should you empty the fuel tank altogether and clean it out?

A professional job with a mobile fuel cleaning company could cost more than £800.

Of course, you can do it yourself – that is if you can access your tank, and have the right equipment, such as a syphon pump, pressure washer, spare containers and a waterproof endoscopic camera.

Either way, it is a costly or time-consuming job.

“While it would be great if you could empty your tank, in reality that’s not going to happen if you’ve got 300lt of diesel,” points out Peter.

“Plus remember, even if you pay a company to do it, they can’t get behind the baffles [the long flat plates inside the tank to reduce sloshing]. As soon as you get past the breakwater and the fuel starts sloshing around, any sludge will come loose.”

This is where the Diesel Dipper comes in, a product Peter spent several years developing, that can be used while underway and continually sucks sludge from the bottom of the tank.

“On the deep seas, where I worked as a marine engineer, we’d have drain cocks on the diesel tank, but of course leisure boats don’t usually have them, so I needed a way to get to the bottom of the tank. The solution was a dip-tube flange. It had to be easily accessible as tanks are often housed in the hull.”

Often the engine doesn’t fail until the boat hits a few waves, starts heeling over, and all the sludge that was sitting at the bottom of the tank while the boat was on its mooring gets churned up and blocks the filters.

So although tank manufacturers make the fuel outlet a little higher than the bottom of the tank, this doesn’t prevent you from picking up dirty fuel.

The only way to avoid blocking the filters is to remove water and remove sludge from the tank.

However, leisure boat fuel tanks are often hard to access and rarely have drain cocks or inspection hatches.

I’ve found black sludge. Is it diesel bug?

So is all sludge ‘diesel bug’? Not necessarily, says Peter.

That sludgy black mess on the bottom of the tank and in your filters can be a range of things from oxidative, acidic or thermal degradation to incompatibility of fuels (if you’ve used different pumps), water, contaminates such as dust and rust, fatty acids from biodiesel (FAME) and asphaltenes (components of crude oil).

“FAME degrades then reacts with the element metals at the interface of the water and diesel producing salts of carboxylic acid, which is what the Victorians called soap.

“Some samples of filters I’ve seen are caked with the stuff, which can be scraped off and acts like soap in warm water!”

But whatever it is, you don’t want it finding its way into your filters and blocking them.

“That’s particularly true of touring boats,” says Peter. “You have to be very careful. When I was at sea we never mixed diesel and would always run the tank as low as possible before topping up.

“Of course, people want to keep spare fuel, so my advice is to rotate it.

“Don’t leave it in the jerry cans. Pour it into the engine from time-to-time and refill the cans.

“Try not to take on more fuel than you can use in six months.”

Can I filter my own fuel?

If you get caught out but don’t trust the fuel in your jerry cans, is there a crude way of filtering it to be on the safe side?

“The short answer is yes,” says Peter. “It’s called a fuel filter funnel. Racor makes one – it’s capable of removing contaminants down to 50 microns – but remember that’s still a very large micron. It will certainly help but it won’t achieve that much.”

Remove the water, remove the threat of diesel bug

Water is responsible for 90% of fuel issues. Peter warns that ‘free water’ gives rise to microbes.

Emulsified diesel can widen the platform for diesel bug and take it through the injection process which can lead to engine failure.

So if you have a drain plug, the advice is to open it up regularly.

Sludge sits at the bottom of the tank, so another solution is to fit a tank purifier, such as Marine 16’s Diesel Dipper (£1,074) which has its own pump and will suck crud from the bottom of the tank.

PBO’s Tony Davies reviewed this recently in his 1978 sports cruiser (see PBO July 2023).

He described it as “the biggest step forward in fuel cleanliness and, therefore, engine reliability since the introduction of the fuel filter,” and concluded that installation was well within the capabilities of an average DIY practical boat owner.

It’s too late. I’ve got diesel bug!

Over the years PBO has tested many brands of diesel bug treatment.

I remember going to a laboratory in Cardiff to report on this 20 years ago.

All products were effective to varying degrees when compared to an untreated sample of fuel, but fuel has changed dramatically since then.

Refiners are forced to use a much higher percentage of each barrel of crude oil, and have to produce fuel with even lower sulphur content, making it less stable.

The introduction into this of biofuel – with ingredients derived from rape seed, soya and palm oil – further accelerates the degradation.

Then, of course, engines have changed too, points out Peter.

“Today’s common rail injection systems work to super fine tolerances between the moving parts – of just a few microns (a human hair is 100 microns) – while relying on lower-quality fuel.”

So if you have diesel bug, it’s important to deal with it quickly. Only a biocide will kill diesel bug, but it struggles to shift by-products and get rid of water.

You still need to remove the deposits and water mechanically via a drain plug (if you have one) or the aforementioned Diesel Dipper.

In the past PBO has recommended dispersants, which guard against asphaltenes.

They don’t kill the bug, but lift water and debris (which are heavier than diesel and usually sit on the bottom of the tank) into suspension.

So, after a couple of filter replacements, you stand a better chance of cleaning your tank.

However, Peter says these are now actually a bad idea with modern diesel.

“Dispersants used to work well back when diesel was simple, and had glycol which allowed water to be absorbed – it went off into the engine and disappeared. It’s old technology. Now we have FAME, a very hygroscopic [moisture retaining] component of diesel, so you’re already battling absorption of moisture from the atmosphere. You’ve then got the free water at the bottom. The diesel is getting saturated. The worst thing you can do is put anything but super-clean, super-dry fuel into your tank.”

Treatment and diesel bug prevention

Marine 16’s biocide, Diesel Bug Treatment, can be used both for treatment and prevention, but in the latter case at a lower dose.

A 100ml bottle is sufficient to prevent build-up in 2,000lt of fuel, whereas if contamination is already present, you’re looking at 100ml to 100lt. But – and this is a big BUT – you must still get rid of the water.

“Whether you use a drain cock, syphon it out or the Diesel Dipper, you must get rid of the water in your tank – and do it carefully, because if you mix water with diesel it readily combines. And remember, no additive will remove the sludge either – you have to get rid of it mechanically. Our diesel bug treatment disperses into both the water and fuel phases in your tank and will remain sufficiently active for over a year at both high and low temperatures.”

Do we really need additives?

“There is no fuel additive in the world that will drastically improve your fuel consumption,” says Peter.

“We did a show in Amsterdam recently and heard people claiming their treatment would reduce fuel consumption by 10%. If that was true, BP and Shell would have bought it a long time ago!”

So what does a fuel additive do? “Well-chosen additives can play a very real part in the efficiency of modern diesel,” says Peter.

“They can improve lubrication, slow degradation and reduce deposits. For example, Diesel Fuel Complete increases the cetane number in diesel. It gives you more bang for your buck – makes it ignite quicker, so it might give you a marginal difference in fuel consumption.”

However, be careful not to select an additive that claims to remove water.

These work by allowing the fuel to absorb the water and modern engines just won’t tolerate this.

When it comes to storing fuel – perhaps over winter – fuel is susceptible to microbial infection, loss of cetane rating, lacquering, gumming and a change of colour as the heavier components of the fuel blend detach from the solution. In this instance, a fuel additive can help slow this down.

Continues below…

Fuel will degrade, especially at sea because 50% of the fuel in marine engines goes back into the tank.

“Imagine all that heat from the engine and pressure from the fuel pump as it goes round?” says Peter. “One little molecule could do that 20 times before it eventually gets burnt. The thermal degradation causes the asphaltenes to conglomerate and fall to the bottom of the tank. You cannot stop fuel degrading, you cannot make it last forever but you can slow it. With modern diesel, additives should be considered an essential rather than a desirable part of engine maintenance. There’s never been a greater need for fuel additives than now.”

---

Enjoyed reading this article?

A subscription to Practical Boat Owner magazine costs around 40% less than the cover price.

Print and digital editions are available through Magazines Direct – where you can also find the latest deals.

PBO is packed with information to help you get the most from boat ownership – whether sail or power.

• • • • Take your DIY skills to the next level with trusted advice on boat maintenance and repairs
      • Impartial in-depth gear reviews
      • Practical cruising tips for making the most of your time afloat

Follow us on Facebook, Instagram and Twitter