How to winterise a marine diesel engine

Change the oil and filter before layup rather than after, fill the fuel tank, check and protect the coolant, deal with the raw water circuit, and disconnect or maintain the batteries. In most of Australia this is about idle time rather than frost — the damage comes from months of sitting, not from freezing.

Layup deterioration is the dominant failure mode for lightly used boats in Melbourne, Geelong, the Gippsland Lakes, Hobart and the NSW South Coast. See why low hours is not good news.

Why the oil goes first

Used oil is acidic and holds moisture. Leaving it in the sump for six months means those acids sit against bearing surfaces the entire time.

Change the oil before layup, not in spring. This is the single most valuable item on the list and the one most often done the wrong way round.

The procedure

1. Run the engine to full operating temperature. Warm oil carries suspended contaminants out with it; cold oil leaves them behind.
2. Change the engine oil and the oil filter while it is still warm.
3. Change the fuel filters, or note their age so spring does not start with a blockage.
4. Fill the fuel tank. Less air space means less humid air drawn in through the vent, and less water condensing on the tank walls over months of temperature cycling.
5. Drain the water from the primary filter bowl and check the sample — see how to check for water in marine fuel.
6. Check the coolant condition and concentration. The corrosion inhibitors deplete on the calendar whether the engine runs or not; if it is old, change it.
7. Deal with the raw water circuit. Depending on your engine and location that may mean flushing with fresh water, draining, or running an antifreeze or inhibitor through it. Follow the engine manufacturer’s direction — this is the step where advice varies most by installation.
8. Consider removing the impeller, or at least note its age. An impeller left compressed for months takes a set and can split on the first turn.
9. Change or check the gearbox oil, and note it as a separate item — it is on a separate schedule with a different oil.
10. Clean the raw water strainer so it does not sit full of decaying weed.
11. Check anodes, including the pencil anode in the heat exchanger. They keep dissolving with the engine off.
12. Disconnect the batteries, or put them on a maintenance charger. A flat battery left over winter may not recover.
13. Close the seacocks, and leave a note where you will see it so you remember to open them in spring.
14. Record what you did and the date. Next spring you will not remember.

Running it monthly: useful or not?

Opinions differ and the manufacturer’s view should win. The case against a short monthly run is real: an engine started, run for fifteen minutes and shut down never reaches full operating temperature, which leaves moisture in the oil and unburnt fuel on the bores — arguably worse than leaving it alone.

If you do run it, run it long enough and hard enough to reach and hold proper operating temperature, which in practice means taking the boat out rather than idling at the berth.

Frequently asked questions

Should I change the oil before or after winter layup? Before. Used oil is acidic and holds moisture, and leaving it in the sump for months is what causes the damage.

Should the fuel tank be full or empty over winter? Full. Empty tank space breathes humid air and condenses water on the walls.

Do I need to winterise in Australia if it does not freeze? Yes, but for different reasons. The damage here comes from idle time — oil, fuel, impellers, coolant and anodes — rather than from frost.

Should I run the engine monthly? Only if you can run it long enough to reach full operating temperature. A short idle at the berth leaves moisture in the oil and is arguably worse than nothing.

What about the impeller? It takes a set if left compressed for months. Note its age, and consider removing it — many owners fit a new one at recommissioning.