How to Winterize an Outboard Motor: Step-by-Step Guide (2026)

Water left inside an outboard can freeze, expand and crack the powerhead or lower unit, and untreated fuel goes stale over the winter. An hour or two of work in the fall saves a costly repair, and a motor that starts first time in spring. This guide covers the steps for most 2-stroke and 4-stroke outboards.

Always check your owner's manual first. Engine makers differ, especially on fogging fuel-injected (EFI/DFI) motors and on oil and gear-lube specs. Use this guide as a checklist, and the manual for the exact details.

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What you need

• Marine fuel stabilizer, e.g. STA-BIL Fuel Stabilizer 10 oz (see all stabilizers)
• Fogging oil, e.g. STA-BIL Fogging Oil 12 oz or Star brite Star-Tron Fogging Oil
• Flush muffs ("ear muffs") and a garden hose, or a flushing system
• Lower-unit gear lube and new drain/vent plug gaskets (the type in your manual)
• 4-strokes: engine oil and a new oil filter
• Marine grease and a grease gun, corrosion-inhibitor spray
• A battery maintainer for winter storage

Step 1: Stabilize the fuel

Fill the tank nearly full (less air space means less condensation) and add marine fuel stabilizer at the dose on the bottle. Then run the motor for 10–15 minutes on flush muffs so treated fuel reaches the fuel lines, carburetors or injectors. If you use a portable tank, you can treat it the same way.

Step 2: Change the engine oil (4-strokes only)

With the motor warm, drain the oil, replace the filter and refill with the outboard oil grade in your manual. Old oil holds acids and moisture that can corrode the engine over the winter. 2-strokes skip this step, since their oil is mixed with the fuel, but top up the oil reservoir if your motor has one.

Step 3: Change the gear oil

Remove the lower drain plug, then the upper vent plug, and let the gear lube drain into a pan. Look at the old oil: milky or creamy oil means water got past a seal. Get it fixed before spring, because water left in the gearcase can freeze and crack it. Refill from the bottom hole until lube comes out of the top hole, put the top plug in first, then the bottom plug, with new gaskets.

Step 4: Flush with fresh water

Attach flush muffs (or your boat's flush port) and run the motor at idle until it reaches operating temperature, with a steady "tell-tale" stream of water. This rinses out salt, sand and silt. Never run an outboard without a water supply to the cooling system.

Step 5: Fog the engine

Fogging oil coats the cylinders and internal parts to prevent rust while the motor sits.

• Carbureted motors: with the engine idling, spray fogging oil into the air intake until it smokes heavily and stalls, or shut it off as directed on the can.
• Fuel-injected (EFI/DFI) motors: don't spray into the intake unless your manual says so. Most makers specify their own procedure (often running a fogging mix through a portable tank). Follow the manual.

Step 6: Drain the water and store it upright

After flushing, trim the motor fully down/vertical so all water drains out of the cooling passages and lower unit. Store it that way for the winter. A motor left tilted up can trap water that freezes.

Step 7: Grease, protect and inspect

• Grease every fitting: steering, swivel and tilt tube.
• Spray corrosion inhibitor on the powerhead under the cowl.
• Check hoses, fuel lines and the primer bulb for cracks or soft spots, and replace the fuel/water separator filter if you have one.
• Check and replace worn sacrificial anodes.

Step 8: Pull the propeller

Remove the prop, clear any fishing line wrapped behind it (it can cut the prop-shaft seal and let water into the gearcase), check the blades for damage, grease the shaft splines and reinstall it, or store it inside.

Step 9: Take care of the battery

Charge the battery fully, disconnect it, clean the terminals and keep it on a maintainer or recharge it monthly. A fully charged battery is much less likely to freeze than a flat one.

Before you cover the boat

• Pull the drain plug and tag the key "PLUG OUT" so you remember it in spring.
• Take home life jackets, flares, electronics and anything that can hold moisture.
• Check your full boat winterizing guide for the hull, plumbing and cover.

Quick FAQ

Do I need to winterize if I live where it doesn't freeze?

You can skip antifreeze, but if the boat will sit for more than a month or two, still stabilize the fuel, change the oil and gear lube, fog the engine and maintain the battery.

2-stroke vs 4-stroke: what's different?

Only 4-strokes need an engine-oil change. Both need fuel stabilizer, gear oil, flushing, grease and battery care. Fogging applies to both, but fuel-injected motors follow the maker's procedure.

Can I do it myself?

Yes, most owners can winterize an outboard with basic tools. If you see milky gear oil, damaged seals or anything you're unsure about, have a marine technician look at it before spring.

This guide is general information. Always follow your engine manufacturer's manual and warranty requirements.