Winterizing Your Outboard Motor

Winterizing an outboard motor is one of those tasks that’s genuinely unpleasant to regret skipping. A motor that sat all winter with untreated fuel, unlubricated internals, and water in the lower unit is a spring repair bill waiting to happen. I’ve done this the right way and the lazy way over the years, and the difference in what you find in April is stark. Here’s the process I follow now.

What You’ll Need Before You Start

Get everything staged before you begin so you’re not making runs to the hardware store mid-process. You’ll need fuel stabilizer, fogging oil, water muffs, engine oil and a filter if you’re running a four-stroke, marine-grade grease, gear lube for the lower unit, a set of wrenches and screwdrivers, a bucket, and a drain pan. Having the manual nearby for torque specs and fill amounts is worth the thirty seconds it takes to find it.

Stabilize the Fuel First

Old, degraded fuel is one of the most common causes of spring no-start problems. Modern ethanol-blended fuel starts to break down and absorb moisture in as little as 30 days. Left all winter in a tank and carb, it turns into a varnish that clogs passages and makes the engine hard or impossible to start.

Add fuel stabilizer — Star Tron or STA-BIL Marine formula are both solid choices — following the dosage on the bottle for your tank size. Then run the engine for 10 minutes on the water muffs so the stabilizer circulates through the carb or fuel injection system completely. If you’re on a carbureted motor and want maximum protection, some people drain the carb bowls entirely after the run. Either approach works; the stabilized fuel route is simpler.

Flush the Engine

Flushing removes salt, sediment, and mineral deposits from the cooling passages. This is non-negotiable if you’ve been in saltwater, and still worth doing in freshwater. Hook up the water muffs over the lower unit intakes and connect a garden hose. Start the engine and let it idle in neutral for 10-15 minutes with good water flow. You’re not revving it — just a steady idle to let the water circulate and push out whatever’s been accumulating in there. Shut it down only after the engine is fully flushed.

Apply Fogging Oil

Fogging oil coats the cylinder walls, pistons, and internal metal surfaces to prevent rust and corrosion over the storage period. With the engine running at idle, spray the fogging oil directly into the carburetor throat or air intake in short bursts. The engine will sputter and smoke — that’s normal, that’s the oil burning through. Keep spraying until the engine shuts down under the load of the oil. That means the internals are coated. Don’t skip this step; it’s what keeps your cylinder walls from rusting against the piston rings.

Change the Engine Oil (Four-Strokes)

If you’re running a four-stroke, change the oil and filter now, not in the spring. Used engine oil contains acids and combustion byproducts that will continue to attack metal surfaces all winter if you leave them in there. Drain while the oil is still warm from the flush run — it flows out completely. Fill with fresh oil of the correct viscosity per your manual, double-check the level, and call it done.

Service the Lower Unit

The lower unit houses the gears, seals, and bearings that transfer power to the prop. Check the gear lube annually at minimum. Remove the lower drain plug — the bottom one — then the upper vent plug. Let the old oil drain completely into a pan. Watch it carefully as it drains: clear or light amber is fine. Milky or whitish oil means water has infiltrated through a failing seal, and that needs a mechanic’s attention before storage, not after. If it looks good, refill from the bottom plug with fresh gear lube until it runs out the top vent, reinstall both plugs, and you’re set.

Lubricate Moving Parts

Work through all the grease fittings on the motor with marine-grade grease — pivot points, tilt tube, steering linkage, throttle shaft, and shift linkage. Apply anti-corrosion spray to exposed electrical connections and any bare metal. Spray or wipe CorrosionX or CRC 6-66 on the whole motor exterior. This stuff is genuinely underrated for preventing the kind of surface rust that makes motors look 10 years older than they are after just one winter.

Inspect and Service the Propeller

Remove the prop. This is a good time to check for damage — nicks, bent blades, or cracks in the rubber hub insert. Any significant damage to a blade affects how the prop runs and should be addressed. More importantly, check for monofilament wrapped around the prop shaft, which can cut through the shaft seal and introduce water into the lower unit. Clean the shaft, apply fresh grease, and reinstall the prop with the cotter pin or locking nut properly secured.

Battery Storage

A battery left discharged all winter loses significant capacity and may not recover. Remove the battery from the boat, clean the terminals with a baking soda and water solution, and put it on a smart trickle charger — a Battery Tender or similar — in a dry garage. These chargers maintain the battery at full charge through winter without overcharging. A properly maintained battery lasts 4-6 years; one that sits dead all winter may not make it to season two.

Cover and Store

A good cover keeps dust, moisture, and animals out. Store the motor in a vertical position if at all possible — horizontal storage over winter allows oil to settle and can create starting issues in spring. Avoid storing directly on concrete, which wicks moisture. A dry, ventilated garage space is ideal. If you’re storing in a space that gets damp, a desiccant pack inside the cover is cheap insurance.

One Check Mid-Winter

Peek at the setup once during the off-season. Check the cover for tears, look for signs of moisture intrusion, and verify the battery charger is still running. Takes five minutes and catches problems before they become bigger ones. The motor will be ready to drop in the water the first warm day of spring instead of spending that day chasing down what went wrong.