How do I winterise the Yanmar engine on a Sun Odyssey 36i?

The best way to winterise the Yanmar 3YM30 fitted to a Sun Odyssey 36i is to flush the raw water circuit with marine antifreeze, change the oil while warm, and remove the impeller. Leaving clean oil in the sump and a protected cooling system prevents internal corrosion and frost damage over the cold months.

Oil and Fuel Preparation

Run the engine until it reaches operating temperature to suspend contaminants in the oil. Extract this old oil via the dipstick tube using a vacuum pump. The Yanmar 3YM30 takes 2.8 litres of 15W-40 mineral oil. Fit a new Yanmar oil filter (part 119305-35170). Smear a little fresh oil on the rubber gasket and tighten it by hand plus a quarter turn.

Fill the 130-litre diesel tank to the brim to prevent condensation forming on the internal walls. Add a shock dose of a diesel biocide like Marine 16. Change your primary fuel filter (usually a Delphi 296 or a Racor element depending on the factory specification) and the secondary engine-mounted Yanmar filter (104500-55710). Bleed the fuel system now so the engine is primed and ready to start in the spring.

Flushing and Antifreezing

The 3YM30 is indirectly cooled. You have two circuits to protect. First, check the internal coolant circuit using an antifreeze hydrometer. It should be a 50/50 mix of ethylene glycol and water, protecting down to -35°C. Drain and replace this coolant if it is older than two years.

Next, winterise the raw water circuit. Close the saildrive or shaft raw water seacock. Open the lid of the raw water strainer. Start the engine and immediately pour a bucket of fresh water through the strainer to flush out the salt. Just before the fresh water runs out, pour in 5 litres of non-toxic marine antifreeze (propylene glycol). Shut the engine down once you see coloured antifreeze spitting out of the exhaust at the transom. Never run the engine without water flow. The hot exhaust gases will melt the plastic waterlock muffler in minutes.

Impeller and Saildrive Checks

Leaving the raw water impeller compressed in its housing all winter causes the rubber vanes to take a permanent set. This leads to poor pumping performance or premature failure in spring.

• Remove the water pump cover plate using an 8mm spanner or socket.
• Pull out the impeller (Yanmar part 128990-42200) using an impeller puller or two flat screwdrivers.
• Inspect it for cracks or missing rubber chunks. Store it in a plastic bag if it looks perfect. Throw it away and buy a new one for £35 if you have any doubts.
• Slacken the alternator belt to relieve tension on the bearings over winter.