In short: Winterise the outboard when the boat comes out, and before the first night frost at the latest. It takes a couple of hours and prevents frost damage, corrosion and starting problems in spring.
- Add stabiliser to the fuel and run the engine warm on flushing muffs, so the cooling system is flushed with fresh water.
- Change the engine oil, oil filter and gear oil while the engine is warm. Milky gear oil means water in the gearcase.
- Inspect the spark plugs and anodes, and grease the propeller shaft, cables and grease points.
- Store the engine upright, dry and preferably frost-free. Always follow the owner's manual.
Work in the order below and the engine only needs to run once: fuel first, then the fresh-water flush, then the oils while the engine is warm. The steps apply to most petrol outboards. Always follow your owner's manual, which has the final word on oil type, quantities, torque settings and order.
When should you winterise an outboard?
As soon as the boat comes out, and before the first night frost at the latest. Water left in the gearcase or cooling system can freeze and crack parts. In Denmark, haul-out typically falls in September or October, and frost can follow soon after.
If you leave the boat in a Danish yard over winter and travel home, do the whole job before you leave: an engine left until spring with salt water in its cooling passages and old oil in the sump corrodes in the meantime. Ask the yard where you may run an engine on flushing muffs.
What do you need to winterise an outboard?
- flushing muffs and a garden hose, or a tank of water for the gearcase
- engine oil and a new oil filter (four-strokes)
- gear oil, a gear oil pump and new screw washers
- fuel stabiliser and, if fitted, a new fuel filter
- new spark plugs and a plug socket
- water-resistant marine grease and a grease gun
- a drip tray, rags and gloves
Many parts come in a service kit for the engine. Stabiliser, oil and care products are under winterising, and tools under tools and winter.
Four-stroke or two-stroke: which steps apply?
Most steps apply to every outboard; the difference is the engine oil.
| Step | Four-stroke | Two-stroke |
|---|---|---|
| Protect the fuel | Yes | Yes, premix too |
| Flush the cooling system | Yes | Yes |
| Change engine oil and oil filter | Yes | No, but check the oil tank on engines with separate oiling |
| Change gear oil | Yes | Yes |
| Spark plugs, greasing and anodes | Yes | Yes |
| Fogging the cylinders | Only if the manual says so | Only if the manual says so |
Step 1: How do you protect the fuel over winter?
Add stabiliser, or run a carburettor dry if your manual allows it. Petrol with ethanol absorbs moisture and ages, and within months it can cause starting problems and blocked jets. Standard 95-octane petrol in Denmark has been E10, with up to 10 per cent ethanol, since 1 January 2020. If enough water gets in, water and ethanol can separate out at the bottom of the tank, which stabiliser cannot reverse.
- Stabiliser: Add it at the dose on the bottle. Running the engine in step 2 draws the treated fuel through the whole system. Fill a fixed tank almost to the top to leave less room for condensation.
- Empty carburettor: Many older carburettor engines can be laid up dry: at the end of step 2, close the fuel tap or disconnect the line and let the engine run until it stops. Manufacturers disagree on this, so follow your manual.
Fuel-injected engines should normally not be run dry; use stabiliser and follow the manual. Replace the fuel filter at the same time, and check the hoses and primer bulb for cracks.
Step 2: How do you flush the cooling system with fresh water?
Run the engine on flushing muffs with fresh water until it is warm. Salt and dirt left in the cooling passages cause corrosion as they dry, and brackish Baltic water leaves deposits too.
- Make the engine safe: remove the key and kill cord and pull off the plug caps. Take off the propeller, clear fishing line from the shaft and grease it with water-resistant marine grease. Refit the plug caps and kill cord, and keep children and others clear while the engine runs ashore.
- Fit the flushing muffs over the cooling water intakes on the gearcase, and turn the water fully on before you start the engine.
- Let the engine idle in neutral until it is warm and the thermostat has opened, typically 5–10 minutes. Watch the tell-tale, the small stream of cooling water, and do not rev the engine.
- Stop the engine before you turn off the water.
If the engine has a garden hose flushing connection, use it as the manual describes; on many engines it is used with the engine off. Never start the engine without cooling water. The impeller, the pump's rubber vane wheel, is damaged within seconds if it runs dry.
Step 3: Change the engine oil and oil filter
Used oil contains acids and moisture that attack the engine while it stands, so change it before storage.
- Begin while the engine is still warm from step 2.
- Drain the oil from the drain plug, or pump it out through the dipstick tube, as the manual describes.
- Replace the oil filter. Smear the new filter's seal with a little fresh oil and tighten it as stated on the filter or in the manual.
- Fill with the oil type and quantity the manufacturer specifies, and check the level after a few minutes.
Step 4: Change the gear oil and check for water
Gear oil normally needs changing every year. Look at it as it drains:
- Milky or creamy oil means water in the gearcase, often from a leaking propeller shaft seal. Have it investigated before the engine is used again.
- Metal chips or fragments are a sign of wear. A little fine metal paste on a magnetic drain screw can be normal; larger pieces are not.
- Stand the engine upright. Remove the lower drain screw first, then the upper vent screw, and let the oil run into the tray.
- Pump the gear oil the manual specifies into the lower hole until it comes out of the upper one.
- Fit the upper screw first, then remove the pump and fit the lower screw, both with new washers.
Step 5: Spark plugs, greasing and anodes
Spark plugs: Remove them when the engine is cold. Light brown electrodes are normal; replace plugs that are worn, sooty or damaged. Leave them out until the rest of this step is done.
Cylinders: Fog the cylinders against rust while the plugs are out only if the manual describes it, using the product and method it gives.
Anodes: Check the anodes on the gearcase, trim tab and bracket, and replace any that are about half gone now rather than in spring. For the right metal in Danish and Baltic waters, see Zinc, aluminium or magnesium anodes?
Propeller and greasing: Refit the propeller with a new split pin while the plugs are out, so the engine cannot start. Grease the nipples, steering, and throttle and gear cables with water-resistant marine grease. Finally, refit the plugs and tighten them to the torque in the manual.
Step 6: How should you store an outboard over winter?
- Store the engine upright so water can drain from the gearcase and cooling system. If it stays on the boat, leave it down, not tilted up.
- Lay the engine down only if the manual allows it, and a small four-stroke only on the side it specifies, or oil can run where it should not.
- Choose a dry, ventilated and preferably frost-free space, and use a breathable cover. Airtight plastic traps condensation and causes corrosion.
- Take the battery out and keep it charged, ideally on a maintenance charger.
What should you check on the outboard in spring?
- Check the oils and fuel, and use fresh petrol.
- Fit a charged battery with clean, tight terminals.
- Replace the impeller if it is due, and inspect hoses, primer bulb, cables and anodes.
- Test-run on flushing muffs before launching, and check that the tell-tale is steady.
For the annual service, see Outboard engine annual service: checklist and intervals.
Frequently asked questions
Can the outboard stay on the boat over winter?
Yes. Leave it down so water can drain out, under a breathable cover, with the battery out or on charge. Storing a small engine at home, dry and frost-free, is better still.
Should the fuel tank be full or empty over winter?
A fixed tank is usually filled almost to the top with stabilised fuel, leaving less room for condensation; a portable tank can be treated the same way. If the fuel stood untreated all winter, use fresh petrol in spring.
How long should the engine run on flushing muffs?
As a rule of thumb, 5–10 minutes at idle, until the engine is warm and the thermostat has opened. Follow the manual, especially if the engine has a flushing connection.
What does milky gear oil mean?
Water has got into the gearcase, typically past a leaking seal at the propeller shaft or gear shift shaft. Have the gearcase pressure-tested and the seals replaced before the engine is used again, because water causes rust and can freeze.
Any questions?
Not sure which filters, oils or service kit fit your engine? Write to us via the contact page with the engine's make, model, year and serial number, and we will help you.
