How to Winterize Your Outboard Motor

As the boating season winds down and temperatures start to drop, one of the most important jobs on your fall checklist is winterizing your outboard motor. Skipping this step—or doing it halfway—can lead to cracked engine blocks, corroded internal components, gummed-up fuel systems, and costly repairs come spring. The good news is that winterizing an outboard motor isn't complicated, and with the right tools, a little time, and this guide from Steveston Marine, you can protect your investment and get your boat ready for a trouble-free launch next season.

Whether you own a small four-stroke kicker motor or a powerful offshore outboard, the fundamentals of winterization are largely the same. Below, we'll walk through why winterizing matters, what you'll need, and a full step-by-step process you can follow at home or in your garage.

Why Winterizing Your Outboard Motor Matters

An outboard motor left unprepared for winter is vulnerable in several ways:

• Freeze damage: Water trapped inside the engine block, cylinder head, or cooling passages can freeze and expand, cracking cast components. This is one of the most expensive mistakes a boat owner can make.
• Fuel degradation: Ethanol-blended gasoline absorbs moisture over time and can separate or turn stale, leading to varnish and gum buildup in carburetors and fuel injectors.
• Corrosion: Saltwater and even freshwater residue left inside the engine can corrode internal metal parts, including pistons, cylinder walls, and the powerhead.
• Battery failure: Cold temperatures are hard on batteries, and a battery left connected and uncharged all winter may not hold a charge by spring.
• Pest infestations: Mice and other small animals love nesting in cowlings and air intakes over the winter months if motors are left uncovered and unsealed.

Taking a few hours in the fall can save you thousands of dollars in repairs—and a lot of frustration—when it's time to hit the water again.

What You'll Need

Before you get started, gather the following supplies. Most are available in-store or online at Steveston Marine:

• Fuel stabilizer (specifically formulated for ethanol-blended fuel)
• Fogging oil
• Fresh engine oil and oil filter (for four-stroke motors)
• Gear lube for the lower unit
• Marine grease
• A flushing muff or dedicated flush kit
• A battery charger or maintainer
• Corrosion inhibitor/protectant spray
• A cover for the motor and/or the boat

Step 1: Stabilize and Run Fresh Fuel Through the System

Start by treating your fuel tank with a marine-grade fuel stabilizer. This prevents the gasoline from breaking down, separating, or forming varnish deposits over the winter months. Add the stabilizer according to the bottle's instructions—dosage is typically based on the size of your tank—and top off the tank to roughly 90–95% full. Leaving a small amount of airspace reduces condensation while a mostly full tank limits the amount of air (and moisture) that can interact with the fuel.

Once the stabilizer is added, run the engine for 10–15 minutes so the treated fuel circulates through the fuel lines, filters, and carburetor or injectors. This ensures the stabilized fuel reaches every part of the fuel system, not just the tank.

Step 2: Flush the Cooling System

This is one of the most critical steps, especially if you boat in saltwater. Salt, sand, and mineral deposits can build up inside the cooling passages and block water flow, leading to overheating next season.

Attach a flushing muff to your outboard's water intake, connect a garden hose, and run the engine at idle (never above idle when out of the water) for about 10 minutes with fresh water flowing through. This clears salt, silt, and debris from the cooling passages. If you have a portable outboard, you can also use a flush tank or bucket designed for this purpose.

Step 3: Fog the Engine

Fogging protects the internal components of your engine—pistons, cylinder walls, and valves—from corrosion during storage. While the engine is running (typically during the final minute of your flushing process), spray fogging oil into the air intake or carburetor throat according to your motor's specifications. You'll want to do this until the engine begins to run rough or emits visible smoke, which indicates the oil has coated the internal surfaces. Turn the engine off immediately afterward.

For fuel-injected motors, follow your manufacturer's specific fogging procedure, as some require fogging oil to be introduced through a different port or method.

Step 4: Change the Engine Oil and Filter (Four-Stroke Motors)

If you run a four-stroke outboard, change the oil and oil filter before storage. Old oil can contain moisture and combustion byproducts that are corrosive if left sitting in the engine over winter. Run the engine briefly to warm the oil (this helps it drain more completely), then drain the old oil, replace the filter, and refill with the manufacturer-recommended oil grade.

Two-stroke motors don't require this step in the same way, but you should still check and top off any oil injection reservoirs.

Step 5: Service the Lower Unit

Water intrusion into the lower unit gear case is a common and often overlooked issue. Drain the old gear lube by removing the lower and upper drain plugs. If the lubricant appears milky or cloudy, that's a sign water has gotten in, and the unit may need to be inspected by a technician for a damaged seal.

Refill the gear case with fresh marine gear lube, working from the bottom hole upward until lube flows out of the top, then replace both plugs with new sealing washers if needed.

Step 6: Grease All Fittings

Apply marine-grade grease to all the fittings on your outboard, including the tilt tube, swivel bracket, steering system, and throttle linkages. This prevents corrosion and keeps moving parts operating smoothly. Refer to your owner's manual for the specific grease points on your model.

Step 7: Disconnect and Charge the Battery

Remove the battery or disconnect the terminals to prevent slow drain over the winter. Clean any corrosion from the terminals with a wire brush and a baking soda solution, then store the battery in a cool, dry place. Use a smart charger or battery maintainer (sometimes called a trickle charger) to keep it topped up throughout the winter—this significantly extends battery life and ensures it's ready to go in spring.

Step 8: Protect Exterior Surfaces

Spray a corrosion inhibitor on the powerhead and other exposed metal surfaces to guard against rust and oxidation. Wipe down the cowling and check for any cracks or damage that should be repaired before storage. If your motor has a propeller, remove it, inspect it for damage, grease the propeller shaft, and reinstall it—or store it separately if you're storing the motor detached from the boat.

Step 9: Cover and Store Properly

Once everything is treated and dry, install an outboard motor cover to keep out dust, moisture, and pests. If possible, store the outboard in an upright position to allow any residual water to drain properly and prevent water from pooling in the cylinders. Store the boat and motor in a dry, sheltered location—whether that's a garage, carport, or covered storage facility—and consider adding rodent deterrents if you're storing in an area where mice are common.

A Few Extra Tips from Steveston Marine

• Check your owner's manual first. While the steps above apply broadly, always check your specific outboard's manual for torque specs, fluid types, and any model-specific winterization instructions.
• Don't skip the test run. Before storing, make sure the engine starts and runs smoothly after fogging and servicing so you know everything is in working order.
• Book early for spring. If you'd rather leave winterization to the professionals, our service department fills up quickly in the fall. Booking early ensures your outboard is ready to go the moment the weather turns.
• Consider a full service package. Winterizing is also a great time to have a technician inspect your impeller, thermostat, spark plugs, and fuel lines so small issues don't turn into big ones.

Let Steveston Marine Help

Winterizing your outboard motor doesn't have to be a hassle. Whether you prefer to do it yourself with quality products from our store, or you'd rather have our certified marine technicians handle the job for you, the team at Steveston Marine is here to help you protect your investment all winter long. Contact us today to shop winterizing supplies, find your nearest store location, or book your outboard motor service appointment.

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Frequently Asked Questions

How often should I winterize my outboard motor? You should winterize your outboard once per year, typically in the fall before the first hard freeze, or anytime the motor will sit unused for an extended period in cold weather.

Do I need to winterize an outboard motor in a mild climate? Yes. Even in regions that rarely freeze, fuel stabilization, fogging, and corrosion protection are still recommended to prevent stale fuel, varnish buildup, and rust during any extended period of storage.

Can I winterize my outboard motor myself, or should I hire a professional? Most steps—fuel stabilization, flushing, fogging, greasing, and battery care—can be done at home with basic tools and the right products. However, if you notice milky gear lube, unusual noises, or you're unsure about a step, it's best to have a certified marine technician inspect the motor.

What happens if I don't winterize my outboard motor? Skipping winterization can lead to frozen and cracked engine blocks, corroded internal components, clogged fuel systems from stale gasoline, and a dead battery—all of which can mean expensive repairs or a motor that won't start in spring.

How long does winterizing an outboard motor take? For most single-outboard setups, the full process—including flushing, fogging, oil changes, and greasing—takes about 1 to 2 hours, depending on motor size and how comfortable you are with each step.

What products do I need to winterize an outboard motor? At minimum, you'll need fuel stabilizer, fogging oil, marine gear lube, engine oil and filter (four-stroke), marine grease, a corrosion inhibitor, and a battery maintainer. All of these are available at Steveston Marine, in-store or online.