How to Winterize an Outboard Engine

When boating season ends and temperatures start to drop, it’s time to prepare your boat for winter storage. One of the most critical maintenance tasks is winterizing your outboard engine. Proper winterization protects your motor from corrosion, freezing damage, and costly repairs — ensuring that it’s ready to start smoothly when spring arrives. Whether you have a two-stroke or four-stroke engine, following the right steps will save you money and extend the life of your outboard.

1. Why Winterizing Matters

Outboard engines are exposed to moisture, fuel residue, and salt (in marine environments). When left untreated, these elements can lead to rust, corrosion, and internal damage. Freezing water trapped inside cooling passages can expand and crack engine components. Fuel left in the system can deteriorate and form varnish or gum that clogs injectors and carburetors.

Winterization prevents these issues by removing water, stabilizing fuel, lubricating internal parts, and protecting the engine from the harsh winter environment. Even if you store your boat indoors, skipping this process risks damage and performance loss.

2. Flush the Cooling System

The first step is to flush the engine with fresh water. Attach a set of “ear muffs” (a flushing device) to the water intake ports on the lower unit and connect them to a garden hose. Start the engine and let it run for about 10 minutes. This removes salt, dirt, and other debris from the cooling system.

If your boat was used in saltwater, use a salt-neutralizing flush solution. Always run the engine in neutral and make sure there’s a steady flow of water from the telltale (the small stream from the cooling system). This indicates proper circulation.

After flushing, shut off the engine and disconnect the fuel line from the engine to let it burn the remaining fuel in the carburetor or fuel rail. When it stalls, you’ll know the system is clear.

3. Drain and Stabilize the Fuel System

Fuel begins to break down after just a few weeks. Over time, it forms sticky deposits that can clog injectors or carburetor jets. To prevent this, add a fuel stabilizer to your tank before winter storage.

Here’s how:

Add the recommended amount of stabilizer to the tank.
Run the engine for 10–15 minutes to allow the treated fuel to circulate through the entire system.
Turn off the engine and top off the fuel tank to reduce air space and minimize condensation.

If you have a portable fuel tank, it’s best to store it separately in a cool, dry place. For built-in tanks, make sure vents are closed and lines are secure.

4. Change the Engine Oil and Filter (Four-Stroke Engines)

Old oil can contain acids and contaminants that damage internal engine parts during storage. For four-stroke outboards, change the engine oil and replace the oil filter before winter.

Warm up the engine slightly to make the oil drain easier.
Remove the drain plug and collect the old oil in a pan.
Replace the oil filter and refill with fresh marine-grade oil according to the manufacturer’s specifications.
Check the oil level with the dipstick once filled.

If you have a two-stroke engine, you can skip this step — but make sure to check and replenish your two-stroke oil reservoir.

5. Fog the Engine Cylinders

Fogging oil protects the engine’s internal metal surfaces from corrosion. Without this protection, moisture can cause pitting or rust in the cylinders and bearings.

To fog your engine:

Remove the engine cover and air intake silencer.
Start the motor and spray fogging oil into the air intake or carburetor while it’s running. The engine will begin to smoke — that’s normal.
Just before it stalls, give it one last heavy spray and then shut it off.
Remove the spark plugs and spray a small amount of fogging oil into each cylinder.
Turn the flywheel manually a few times to distribute the oil evenly, then reinstall the spark plugs.

This step creates a thin protective coating that prevents internal corrosion throughout the winter.

6. Drain and Inspect the Lower Unit

Next, remove the drain and vent plugs on the lower unit to drain the gear oil. Check the oil’s color and consistency.

If the oil looks milky or contains water, it indicates a leaking seal that must be repaired before refilling.
Once drained, replace the gaskets and refill the lower unit with fresh gear oil using a pump.

This step prevents freezing and protects the gears from rust and corrosion.

7. Grease, Protect, and Inspect

Lubricate all grease fittings, steering mechanisms, and moving parts using marine grease. Apply corrosion inhibitor spray on external metal surfaces, bolts, and linkages to prevent rust.

Inspect the propeller for nicks or damage. Remove it to check for fishing line or debris wrapped around the shaft, then apply a light coat of waterproof grease before reinstalling.

Wipe down the exterior with a clean rag and apply a protective wax or silicone spray to shield it from moisture during storage.

8. Store the Engine Properly

Finally, store your outboard in an upright position to allow any remaining water to drain from the cooling system.

If possible, store it indoors in a dry, temperature-controlled environment.
If storing outside, keep it covered with a ventilated motor cover to prevent condensation and moisture buildup.
Disconnect the battery and store it indoors on a trickle charger.