How should I choose a diesel fuel additive for my marine engine?
My yacht's diesel sometimes sits in the tank for several months, and I'm unsure whether to buy a stabiliser, a diesel bug treatment or an all-in-one additive. What should I check before choosing one, and should I add it every time I refuel?
Answer
Choose a diesel additive that your engine manufacturer permits and that meets a specific need: storage stability, microbial prevention or a diagnosed fuel-system problem. Check the tank bottom and primary fuel filter first, because an additive cannot remove accumulated water, sludge or dead diesel bug.
Check engine requirements and fuel
Read the fuel section of the manual for your engine's exact model and year. A bottle labelled "suitable for marine diesel" does not establish approval for your engine, particularly if its injection equipment or warranty carries specific requirements.
For example, Yanmar's YM fuel guidance from 2019 and 2025 permits B7 diesel: up to 7% biodiesel blended with conventional diesel, subject to the specified fuel quality requirements. This fuel specification does not give blanket approval for aftermarket additives or higher biodiesel blends.
Before buying anything, establish:
- What fuel you have: check the supplier's grade and biodiesel content rather than relying on colour.
- How long it stays aboard: a boat that regularly burns through fresh fuel has different needs from one laid up for six months.
- Whether contamination is present: drain the water separator into a clear container and let the contents settle. Water collects beneath the diesel.
Before opening any part of the fuel system, stop the engine, eliminate ignition sources and have a suitable container ready to collect drained fuel.
Match the additive to the job
Read the product's technical sheet as well as its front label. "Conditions fuel" tells you little. Look for a stated purpose, compatible fuel types, treatment rate and any restrictions.
- Storage stabiliser: helps slow fuel deterioration during extended storage. Add it while the fuel is fresh; it cannot restore badly degraded diesel.
- Biocide: targets microbial contamination commonly called diesel bug. Choose a product permitted for that use where you keep the boat, with clear preventive and remedial dosing instructions.
- Water-handling additive: some formulations help small amounts of moisture remain dispersed in fuel for combustion. Use one only if your engine manufacturer permits this approach. Physically remove free water first.
- Injector detergent or cetane improver: consider these only for a defined need. Neither is a first response to blocked filters, air leaks or an engine that struggles to reach its normal operating speed.
An all-in-one product can suit routine prevention if its functions match your needs and your engine manufacturer permits it. Do not assume every conditioner contains a biocide, or that "diesel bug protection" means it can clear an infested tank.