What Is Diesel Bug?
A colony of micro-organisms breeding at the fuel tank’s water-diesel interface — the silent saboteur of an underused yacht.
Diesel bug is the common name for a mix of bacteria and fungi that grow at the interface where water in the tank meets diesel. They live in the water and feed on the diesel; the black-brown slimy sludge they produce blocks tanks and filters.
Yachts that are little used and lay up for long periods are the perfect environment: condensation water building up in a tank that is not kept full, plus fuel left stagnant for months. The table below sets out the signs.
Diesel bug symptoms
Symptom | What it means |
|---|---|
Black slime in the filter | The clearest sign — the colony is established |
Engine cuts out in rough seas | Tank sludge is stirred up and reaches the filter |
Filters clog repeatedly | The new element darkens within weeks |
The problem is sly: on the first rough patch of weather, tank sludge gets stirred up and reaches the filter, and the engine stops at exactly the worst moment — in a seaway or during manoeuvring. Prevention is cheaper than cure: fill the tank before winter lay-up (to reduce the condensation space) and, if needed, use a biocide additive.
If the colony is already established, additive alone will not solve it; the dead biomass will still end up in the filters, so tank cleaning and fuel polishing need to be considered.