Diesel bug: water, biocomponent and clogged filters

The 2022 safety digest from the UK's Marine Accident Investigation Branch includes a case titled "The price of FAME?" The vessel was stopped, and the service engineer's investigation found the cause: clogged fuel filters, serious fuel system damage, and contamination inside the tank. The result was an expensive engine repair and a full clean of the fuel system.

Shortly before this, the supplier had started dispensing fuel with FAME added — fatty acid methyl esters, known as biodiesel. This requirement didn't apply to seagoing vessels, but some suppliers thought otherwise and were filling pleasure craft and fishing boats with gas oil containing up to 7% FAME.

Let's break down how this leads to engine failure.

There's always some water in the tank

The Royal Yachting Association puts it plainly: any diesel fuel contains water to some degree. There are three routes:

• it's dissolved in the fuel itself;
• it gets in through leaking caps, seals and vent tubes;
• it condenses on the walls with temperature swings.

The last is especially noticeable in common rail systems: fuel returns to the tank heated, the tank breathes, and moisture settles on the walls.

What the biocomponent adds

FAME is hygroscopic, meaning it attracts moisture, which the marine environment already has plenty of. Hence three consequences listed in the same MAIB breakdown:

• blends with biodiesel hold more water than ordinary diesel;
• FAME oxidises quickly, so such fuel can't be stored for long;
• fuel with FAME is more aggressive toward seals, and leaks appear.

Where the "bug" lives

Microorganisms — fungi, bacteria, yeasts — feed on hydrocarbons and settle at the interface between fuel and water at the bottom of the tank. Some species also grow on the walls without access to oxygen.

From there the mechanics take over: colonies produce slime and sediment, the sediment travels into the line, the filter clogs. A clogged filter starves the engine under load — exactly when you need it: leaving the marina, in a narrow channel, into the wind.

Continuing from the same breakdown: blocked filters, damaged pumps and injectors, a contaminated tank, and finally engine failure. Common rail engines are especially sensitive because of the high pressure, temperature, and large volume of fuel returned to the tank.

What to do before it stops

The routine is simple and dull, which is the point.

1. Remove the water, don't fight the microbes. Without water, the colony can't live. Drain the sediment bowl regularly and check the water separator in the fuel line — do this before departure, along with checking belts, hoses and levels.
2. Don't let the tank breathe with empty space. The less air above the fuel, the less condensation. A full tank over winter isn't superstition.
3. Check seals and vents. Seawater and rain get into the tank through these, not through the metal.
4. Don't store fuel with a biocomponent for long. It oxidises. Buy only as much as you'll burn in a season.
5. Carry spare filters and know how to change them at sea in a swell. This is where a cheap consumable is the difference between a delay and a tow.

If contamination has already happened, there are two options: drain and clean out the tank or treat with a biocide followed by filtration. The first is more expensive and more reliable, the second is faster and requires monitoring.

A quick check before the season

• Drain a sample from the very bottom of the tank into a clear container: water is visible at once, and so is slime.
• Hold the filter up to the light: a dark film and flakes are not "dust."
• Log the date of filter changes and sediment draining somewhere you'll find it a year from now.

Diesel forgives a lot, except water in the tank and inattention to filters.

Sources: Marine Accident Investigation Branch, Safety Digest 1/2022, case "The price of FAME?"; Royal Yachting Association, section on engine checks and fuel contamination prevention.