How do I start a Yanmar marine diesel engine safely after it has been sitting?
My boat's Yanmar diesel has been sitting unused for several months. What should I check before starting it, and should I crank it without letting it fire first?
Answer
Check the oil, fuel supply, cooling system and battery before starting; after a long lay-up, Yanmar specifies a five-second crank with the fuel supply disabled to distribute oil. Then start in neutral with the cooling-water seacock open, release the starter as soon as the engine fires, and check the warning lamps and exhaust-water discharge.
Pre-start engine checks
Identify the model on the engine plate and find its operation manual. A 1GM10, an older 2GM20 and a 3YM30 can have different stop controls and starting arrangements. The boat's installation also determines the cooling-water and exhaust layout.
A few weeks between outings normally calls for routine pre-start checks. Several months in storage, an unknown service history or a winter lay-up calls for closer inspection before cranking.
- Engine oil: check the dipstick with the boat reasonably level. The oil should sit between the marks. Investigate milky oil, an unexplained rise in level or a strong diesel smell before starting.
- Coolant: on freshwater-cooled engines, check the expansion tank and, when cold, the pressure-cap filling point specified in the manual. Raw-water-cooled models have a different arrangement.
- Fuel: inspect the primary filter or water separator for water, sediment or dark sludge. Deal with contamination before drawing it into the engine.
- Cooling-water circuit: inspect the strainer, hoses, clips and pump. Refit any winterising drain plugs and confirm the impeller is fitted and serviceable.
- Battery and drive belt: charge the starting battery, clean and secure its terminals, and check the alternator belt for cracks and correct tension.
Ventilate the engine space. Keep hands, clothing and tools clear of belts and pulleys whenever the engine is cranking or running.
Circulate oil after storage
Yanmar's YM instructions call for cranking without starting after a long period of non-use because oil may no longer cover all operating parts. This brief crank distributes oil; do not keep turning the engine until the oil-pressure lamp goes out.
- Open the cooling-water seacock and fuel cock, put the gearbox control in neutral, and turn on the battery isolator.
- Disable fuel using the method specified for your engine. The 2018 YM instructions describe pulling out and holding the engine stop knob throughout cranking.
- On the YM arrangement described in the February 2025 manual, hold in the emergency stop button on the back of the fuel pump while operating the panel's start switch.
- Crank for about five seconds, listening for unusual noises, then release the starter.