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Diesel Bug In The Fuel Tank
“Diesel bug” is microbial contamination — bacteria, yeasts and fungi living at the boundary between the diesel and any water beneath it. They feed on the fuel and need the water to survive.
It is far more a marine problem than a road problem, for one reason: boats have large tanks that sit part-full for months in humid conditions, which is exactly what the organisms need.
Is it safe to keep going?
You can usually get home. But the organisms produce acids that attack the tank, and the sludge blocks filters — typically at the worst moment, because motion is what stirs it up.
How you know
| Sign | What it means |
|---|---|
| Dark slime, jelly or stringy mats in the filter bowl | Established growth |
| Filters blocking repeatedly, worse in rough weather | Sludge on the tank floor |
| Sour or sulphurous smell from the tank | By-products |
| Black specks in a tank bottom sample | Biomass and sediment |
| Water in the separator | The habitat |
| Loss of power under load, clearing at idle | Restriction from partial blockage |
A tank-bottom sample and a test kit confirm it. Several other faults block filters, and it is worth knowing which one you have before treating.
Treating it properly — four steps, not one
1. Remove the water. Without water the organisms cannot live. This step decides whether the problem returns.
2. Shock dose with a marine biocide. Correct dose, full contact time. Underdosing selects for resistant organisms rather than killing them.
3. Remove the dead biomass. Killing a large colony converts it into a large quantity of sludge — which then blocks your filters. This is why boats often have worse filter problems immediately after treatment. Polishing or tank cleaning after treatment is part of the job, not an optional extra.
4. Fix how water is getting in. A leaking deck fill seal, a failed tank vent, condensation from long part-full storage, or contaminated fuel at the supplier. Skip this and the cycle restarts.
Diesel bug treatment · Fuel polishing and tank cleaning
What you can check yourself
Take a bottom sample. Drain the separator and look at the bowl. Check the deck fill seal and the vent. A test kit is inexpensive and removes the guesswork.
When to call a mechanic
When growth is confirmed and established. Treatment is straightforward; removing the biomass from a tank with baffles is not.
What marine engine work costs in Australia
Common on these engines
Tank-side, so brand-independent. Northern Australian humidity and heat favour it strongly.
Marine diesel fuel quality and storage
Frequently asked questions
Will biocide alone fix it? No. It kills the organisms and leaves the corpses and the water behind — both of which still cause problems.
Why did my filters get worse right after treatment? Expected. The treatment released a dead colony into the fuel as sludge. Polish and change filters afterwards.
Can I prevent it completely? Keep water out and it cannot establish. Full tanks over layup, sound deck fill and vent, routine separator draining.
Is it worse in northern Australia? Yes. Warm, humid conditions favour growth considerably over southern waters.
Practical summary
Use this fault guide safely
Stop now
Shut down for oil-pressure alarms, overheating, fuel spray, smoke in the engine space or loss of cooling-water flow.
Check safely
Start with visible leaks, strainers, fluid levels, alarms and loose connections—only when the machinery is safe and isolated.
Record evidence
Note rpm, temperature, smoke colour, alarm text, sea state and exactly when the symptom began.
Escalate
Give the evidence and engine model to a qualified marine mechanic; avoid repeated restart attempts.
Sources and verification
This page provides general Australian guidance. The current manual for the exact engine, gearbox or drive remains the authority for procedures, specifications and service intervals.
- 1Tank pickup
- 2Primary filter
- 3Lift pump
- 4Secondary filter
- 5Injection system