How to Winterize an Outboard Motor: Flush, Fog, Stabilize and Store

Quick answer: To winterize an outboard, stabilize the fuel and run it through the engine, flush the cooling system with fresh water, change the engine oil (four-strokes) and the lower unit gear oil, and grease every fitting. Fog the engine only the way your owner's manual describes, since makers differ on EFI and four-stroke engines. Then replace the fuel/water separator filter, inspect the prop and anodes, and store the motor tilted down so water drains out.

An outboard is simple to winterize, but skipping steps is expensive. Water trapped in the powerhead or gearcase can freeze and crack castings. Stale fuel gums up injectors and carburetors. Old oil carries acids and moisture that sit on bearings all winter. Most owners can do this job in an afternoon on the trailer. Have your owner's manual open, because the exact procedure and fluids vary by maker and model.

Step 1: Stabilize the fuel and run it through

1. Add marine fuel stabilizer to the tank at the storage dose on the label.
2. Top off the tank to about 90 to 95 percent so there is less air space for condensation.
3. Connect water to the motor (Step 2), then run it 10 to 15 minutes so treated fuel reaches the lines, vapor separator tank, injectors or carburetor.

Small portable tanks can also be run dry or emptied, depending on your manual. See our marine fuel additive buying guide for which treatment to use.

Step 2: Flush with fresh water

Salt, silt and minerals left in the cooling passages cause corrosion and blockages over the winter. There are two common ways to flush.

Method	How it works	Engine running?
Flush muffs (ear muffs)	Cups seal over the lower unit water intakes and feed hose water in	Yes. The water pump draws water, so never run the engine without full water flow
Flush port quick connect	A hose fitting screws onto the engine's built-in flush port	Many makers say to flush with the engine off through the port. Check your manual

1. Use good water pressure and keep people and loose items clear of the propeller. Many technicians remove the prop before running on muffs.
2. Run or flush for the time your manual lists, typically several minutes, and confirm a steady telltale stream if the engine is running.
3. Disconnect the water and let the cooling system drain completely.

Step 3: Fog the engine, if your manual calls for it

Fogging oil coats cylinder walls, rings and valves to protect against corrosion while the engine sits. It is standard practice on many carbureted two-strokes and older engines. Modern EFI and four-stroke outboards are different. Some makers specify their own EFI storage fogging oil and procedure, some fog through the fuel supply, and some say fogging is not needed for their engines.

• Carbureted engines: Fogging spray is usually applied into the carburetor intakes while the engine runs on water, until it stalls or smokes heavily.
• EFI engines: Do not spray carburetor fogging oil into the intake unless your manual says to. It can foul injectors and sensors. Use the procedure and product your engine maker specifies.

When in doubt, follow the owner's manual or ask your dealer. It is the only reliable answer for your specific engine.

Step 4: Change the engine oil (four-strokes)

1. Warm the engine first so the oil drains fully.
2. Drain the oil, replace the oil filter and refill with the oil grade your manual lists.
3. Check the level on the dipstick with the motor upright.

Two-strokes have no crankcase oil to change, but top off an oil-injection reservoir so condensation has less room to form.

Step 5: Change the lower unit gear oil

1. With the motor upright, remove the lower drain plug, then the upper vent plug.
2. Let the oil drain and look at it. Dark, clean oil is normal. Milky, creamy or foamy oil means water has entered the gearcase, often through a worn seal.
3. Check the drain plug magnet for metal flakes. A fine paste is normal wear. Chunks are not.
4. Refill from the bottom drain hole with the gear lube your manual specifies until it flows out of the vent hole. Install the vent plug first, then the drain plug, using new washers.

If you see water in the oil, have the gearcase pressure tested and resealed before winter. Water left in a gearcase can freeze and crack it.

Step 6: Grease the fittings

Pump fresh water-resistant marine grease into every grease fitting until clean grease pushes out old grease. Typical points include the swivel bracket, tilt tube, steering pivot and trim pivots. Your manual shows the locations for your model. Also lube the prop shaft splines when the prop is off.

Step 7: Change the fuel/water separator filter

Replace the element on the boat's fuel/water separator, and the engine-mounted fuel filter if your service schedule calls for it. Drain any water from the bowl and dispose of the old fuel properly. Starting spring with clean filters is cheap insurance.

Step 8: Inspect the prop and anodes

1. Pull the prop and check for fishing line wrapped behind it, which can cut the prop shaft seal.
2. Look for dings, bent blades and a spun hub. Have damage repaired over the winter.
3. Check the anodes on the gearcase, trim tab and bracket. Replace any that are about half worn. Never paint them.

Our marine anode and zinc buying guide explains zinc, aluminum and magnesium anodes.

Step 9: Store it tilted down

Store the outboard in the vertical, tilted down position so any remaining water drains out of the powerhead and lower unit. Water trapped in a tilted-up motor can freeze in the cooling passages. Spray exposed metal with a corrosion inhibitor, disconnect the fuel line if your manual suggests it and cover the cowling to keep out pests and moisture.

Our picks for outboard winterizing

All in stock at Boat Supplies Direct.

Flushing

Flushmaster Flush Port Quick Connect for Yamaha, Mercury or Suzuki outboards, or the Whitecap Pro Dual Feed Motor Flusher for muff-style flushing.

Fuel

Star-Tron Stabilizer + Fuel Storage for storage, and Star-Tron Enzyme Fuel Treatment for every fill.

Fogging

STA-BIL Fogging Oil for engines where your manual calls for a standard fogging spray.

Grease

Star brite Pistol Grease Gun with Marine Grade Cartridge.

Fuel/water separator

Attwood Fuel/Water Separator, or the Attwood Universal Kit with Bracket if you are adding one.

Shop all fall layup and winterizing →

Frequently Asked Questions

Do I need to fog a four-stroke outboard?

It depends on the maker and model. Some specify an EFI fogging product and procedure, others say it is not needed. Follow your owner's manual.

Can I run my outboard on muffs without the prop removed?

Many owners do, but a spinning prop is dangerous. Removing it is safest. At minimum keep the engine in neutral and everyone well clear.

Is it better to flush with the engine running or off?

With muffs, the engine runs so the water pump moves water. Many flush port systems are designed for flushing with the engine off. Check your manual.

What does milky gear oil mean?

Water has gotten into the gearcase, usually past a worn seal. Have it pressure tested and resealed before storage, since trapped water can freeze.

Should I store my outboard tilted up or down?

Tilted down, in the vertical position, so water drains from the cooling system and lower unit.

How long does it take to winterize an outboard?

Usually a few hours for a single outboard if you have the parts and fluids ready. Twin engines or a first-time job take longer.

This guide is general information. Procedures, fluids and fogging requirements vary by engine maker and model, so always follow your owner's manual and consult a qualified marine technician if you are unsure.