Diesel Bug, Stale Petrol and Lay-Up: Keeping Boat Fuel and Engines Healthy in Thai Heat

Ask any engineer in a Thai marina what stops more boats than worn engines and the answer is fuel. Diesel tanks grow a black slime that blocks filters on the first rough day out; petrol left in an outboard through the monsoon turns to varnish in the carburettor; and an engine that sat for four months restarts with rust on the cylinder walls. None of it is dramatic and all of it is preventable. This guide covers the three problems — diesel bug, stale petrol and lay-up corrosion — and the Liqui Moly Marine products that address each, with the dosage and method always taken from the label rather than from folklore.

Diesel bug: what it is and why Thailand breeds it

“Diesel bug” is the boatyard name for a mixed colony of bacteria, yeasts and moulds that live at the interface between diesel and the water that collects at the bottom of a tank. The organisms need water to live and fuel to eat; a tank with a layer of condensation on the bottom gives them both. They produce a dark biomass that is drawn into the fuel pick-up and clogs the primary filter, and their by-products are acidic enough to corrode tank bottoms, injectors and pumps. The classic symptom is an engine that runs fine at the dock and loses power in a seaway, when the slime is stirred up off the tank floor.

Warm fuel, humid air and long idle periods are the ideal conditions, which describes a boat on a mooring in Phuket or Pattaya between charters exactly. Modern low-sulphur diesel and biodiesel blends also hold more dissolved water than the fuel of twenty years ago, which is why the problem has become more common, not less.

Prevention first, then treatment

Three habits do most of the work. Keep tanks as full as practical when the boat is idle so there is less air space to condense in. Drain water from the primary filter bowl on a routine, not when the engine stumbles. And keep a biocide in the fuel so the colony never establishes.

Liqui Moly Marine Diesel Protect is that biocide: Liqui Moly describes it as an efficient solution against bacteria, yeast and mould, suitable for long-term preservation, with cleaners and corrosion protection for the whole fuel system and a cetane booster for easier starting when the engine is recommissioned. The preventive and shock dosages are on the label — use those figures, because over-dosing a biocide does not help and under-dosing does not work. If a tank is already heavily contaminated, treatment kills the organisms but does not remove the dead biomass; expect to change filters once or twice as it clears, and for a badly fouled tank a professional fuel polish is the right first step before treatment.

For engines that are simply hard to start or smoky rather than infected, Marine Super Diesel Additive is the maintenance product: Liqui Moly’s combination of cleaning, protective and cetane-boosting agents that improves ignition, cleans injectors and protects the fuel system against corrosion. Many owners run it every fill during the season and switch to Diesel Protect for lay-up.

Petrol outboards: stale fuel and gum

Petrol does not grow bugs but it does age. Left in a tank or a carburettor bowl for months in tropical heat it oxidises, drops gum and varnish in the jets and, where ethanol is blended in, absorbs water and separates. The result is the outboard that starts on the first pull in April and will not idle in November. Two Liqui Moly products cover it. Marine Fuel Stabiliser preserves and protects fuel against ageing and oxidation, prevents corrosion through the fuel system and is formulated to stabilise petrol with organic (ethanol) content — add it at the label dose before the last run of the season so treated fuel reaches the carburettor or injectors. Marine Fuel System Cleaner is the recovery product for an engine that has already been sitting: it removes deposits on injection valves, intake valves, spark plugs and in the combustion chamber on both two- and four-stroke petrol engines.

Lay-up: protecting the engine itself

An engine that will not run for weeks needs its internals protected from the humid air that replaces the exhaust gases as it cools. Marine Storage Fogging Oil lays a long-term corrosion-protection film on cylinder walls, piston crowns and rings and ensures a smooth restart after shutdown; the method — sprayed into the intake with the engine running, then through the plug holes — is on the can and in your engine handbook. Follow the handbook’s lay-up procedure for the rest: cooling system flushed and, for an inboard with raw-water cooling, descaled with Marine Anti-Limescale if the heat exchanger has been running warm; gearcase oil checked for water; every exposed electrical connection and moving fitting treated with Marine Multi-Spray, which drives out moisture and protects the vessel’s electrical system.

For the boat-wide lay-up checklist — covers, ventilation, batteries and the rest — see our southwest monsoon lay-up guide for the Andaman coast.

A season in four bottles

All four are held in stock at Drake Marine in Trang — genuine Liqui Moly, supplied through the Thai importer — with courier delivery nationwide in one to three days and carton pricing for fleets. The full range is on our Liqui Moly Thailand page; the oil-choice side of the story is in which oil for your outboard.

Часто задаваемые вопросы

How do I know if I have diesel bug?

Black or brown slime in the primary filter bowl, filters that block faster than they used to, and an engine that loses power in rough water are the usual signs. A sample drawn from the bottom of the tank will show a dark layer at the fuel/water interface.

Does a diesel biocide clear a blocked filter?

No — it kills the organisms so they stop growing, but the dead biomass still has to be filtered out or polished from the tank. Fit new filters after treatment and carry spares for the first few runs.

How much Diesel Protect do I add?

Use the preventive and shock-treatment dosages printed on the Liqui Moly label; they differ, and the label is the manufacturer’s instruction. Do not exceed them.

Should I stabilise petrol in an outboard that is used every week?

Fuel that turns over weekly rarely needs it. Stabiliser is for fuel that will sit — through the monsoon, or in a portable tank kept as a spare — and it must be added before storage, because it prevents ageing rather than reversing it.