Diesel Bug — Removal & Prevention
Identify, treat, and prevent microbial contamination in your vessel's fuel system.
WATER + WARMTH + FUEL = DIESEL BUG
Diesel bug is a consortium of bacteria, mould, and yeast that lives at the fuel-water interface. It produces sludge that clogs filters, corrodes tanks, and can leave you stranded at sea. The key to control is eliminating water in the tank.
Warning Signs
Cloudy / hazy fuel
Fuel that appears cloudy or hazy instead of bright and clear is a strong early sign of microbial growth.
Clogged fuel filters
Frequent or rapid filter blockage with slimy, dark sludge indicates active contamination.
Engine rough running / power loss
Stuttering, smoking, or loss of revs under load — especially after fuel has sat unused — points to restricted fuel flow.
Slime in the tank
A brown, black, or grey sludge layer between fuel and water at the tank bottom is a hallmark of diesel bug.
Fuel contamination odour
A sour, rotten-egg, or unusual smell from the fuel indicates bacterial activity.
Corrosion in tank / injectors
Pitting or corrosion on metal surfaces inside the fuel system is caused by acids produced by the microbes.
Removal Procedure
Confirm the Contamination
Draw a fuel sample from the tank bottom (where water and bugs settle) using a hand pump or sampler jar. Inspect for cloudiness, slime, or a water layer. A simple bug-test dip slide can confirm microbial presence.
Isolate the Tank
Identify which tank(s) are affected. If your vessel has multiple tanks, avoid transferring contaminated fuel into a clean tank. Isolate the affected tank from the engine supply.
Drain Water & Sludge
Pump out all free water and bottom sludge from the tank. This is where the microbes live and breed. Dispose of contaminated waste responsibly at a marina fuel facility.
Add a Biocide Treatment
Add a marine-grade fuel biocide (e.g. Marine 16, Biobor JF, or Stanadyne) at the manufacturer's recommended dose. A biocide kills the microbes — a simple fuel polish alone will not.
Agitate the Fuel
Circulate or slosh the fuel to mix the biocide throughout the tank. Run the fuel return line back to the tank for at least 30–60 minutes to ensure the treatment reaches all contaminated areas.
Let It Work
Allow the biocide to sit for the manufacturer-recommended time (typically 24–48 hours). The dead microbes will drop to the tank bottom as sludge.
Polish the Fuel
After the kill period, circulate the fuel through a fine filtration polisher (2–5 micron) to remove dead biomass and debris. Continue until the fuel runs bright and clear.
Replace Fuel Filters
Replace all primary and secondary fuel filters. They will have captured dead microbes and must not be reused. Carry spares — you may need to replace them again soon as residual debris clears.
Bleed the System
Bleed air from the fuel lines, primary filter, and injector pump following your engine manual. Ensure clean, bubble-free fuel reaches the injectors before starting.
Test Run the Engine
Start the engine and run under load. Watch for clean exhaust, smooth idle, and full RPM. If filters clog again quickly, repeat the polish and filter replacement cycle.