Winterizing a boat engine – step by step

Properly winterizing your engine can save you thousands of kronor and hours of repairs when the season starts again.
This guide shows you how to do it yourself — whether you have an inboard or outboard motor.

1. Preparations before haul-out

• Ensure the boat is on land and stable before you start working.
• Have all tools and materials close at hand — hoses, bucket, glycol, oil pumps, rags, gloves, spray oil, etc.
• Retrieve the engine's instruction manual. Follow the manufacturer's recommendations for your engine (e.g., what type of antifreeze is approved).

2. Warm-up and flushing

Start the engine and let it run until it reaches operating temperature — this opens the thermostat.
Now it's time to flush fresh water through the system: use a flushing muffs or a hose connected to the raw water intake.
Ensure the water runs clear, without salt or dirt, and never pressurize the cooling system.
This removes deposits and salt that can otherwise cause corrosion.

3. Drain and winterize the cooling system

There are two main types of cooling systems — direct raw water cooled and internal heat exchanger systems.
The methods differ slightly:

a) Raw water cooled engine

1. Disconnect the suction line from the raw water pump on the inside.
2. Connect a hose to the suction line and place the end in a bucket with 50% fresh water / 50% glycol (use the type supported by the engine).
3. Start the engine at idle. Let the glycol mixture circulate until it reaches the exhaust and appears clear.
4. Turn off the engine and let the fluid remain. It acts as a seal against frost and corrosion.
5. Check and clean the raw water filter before closing the system.

b) Internal cooling / heat exchanger system

1. Flush the system with fresh water to remove salt and dirt.
2. Check the coolant level in the expansion tank and replace with proper antifreeze if needed (according to manufacturer's recommendation).
3. If the engine has zinc anodes in the heat exchanger, unscrew and inspect them — replace if more than 50% worn.

4. Oil and filter change

1. Warm up the engine for a few minutes before draining — warm oil is easier to remove.
2. Use an oil pump or drain via plugs. Make sure to remove any sludge that has settled at the bottom of the oil pan.
   Check the oil volume against the instruction manual and ensure all old oil is removed.
3. Fill with new oil of the correct volume and quality for your engine.
4. Start the engine briefly so the new oil lubricates all components.
5. Check for leaks at the oil filter and ensure the dipstick shows the correct level.
6. Also service the sterndrive/reverse gear if your installation has one.

5. Fuel system and injection

1. Add fuel stabilizer to the tank before the last run. Run the engine until it stops to empty lines/injection.
2. Drain water separator and clean/replace filter.
3. For gasoline with carburetor: remove spark plugs and spray preserving oil into the cylinders. Turn the engine manually a couple of revolutions.

6. Impeller, hoses and other parts

1. Remove the impeller, inspect and clean. Store in a dark plastic bag so the rubber doesn't dry out.
2. Check cooling water and exhaust hoses: bend/pull them to find dry cracks — replace worn parts.
3. Check belt tension (guideline ~10 mm play in the middle of the belt) and adjust if necessary.
4. Lubricate moving parts (controls, steering links, shaft, nuts) and contact surfaces with moisture-protective spray.

7. Electrical system and batteries

1. Remove the battery and store it in a cool but frost-free place. Charge it fully and check the acid level if relevant.
2. If the battery remains: disconnect the negative terminal to reduce self-discharge.
3. Spray moisture protection on electrical contacts, starter relay, and instruments to counteract corrosion.

8. Final checks and conclusion

• Check all plugs and drain holes — nothing should be left open where water can collect.
• Ensure hatches and ventilation openings are protected but that some ventilation exists to avoid condensation.
• Cover the boat, but consider ventilation. Document the work in a checklist/service book.