Skipping winterization costs more than doing it. Water left in the powerhead freezes, expands, and cracks the block. Old fuel gums up carburetors. Moisture in the lower unit destroys bearings. A $50 winterization prevents $2,000+ in spring repairs.
Here's the complete process with the details that matter.
Table of Contents
Before You Start
You'll Need:
- Fuel stabilizer (Sta-Bil Marine or similar)
- Fogging oil spray
- Fresh gear oil (80W-90 marine grade)
- Engine oil and filter (4-stroke only)
- Spark plug wrench
- Flush muffs or flush adapter
- Waterproof grease
- Corrosion inhibitor spray
Time Required: 1-2 hours depending on engine size
Step-by-Step Winterization
Step 1: Stabilize the Fuel
Add fuel stabilizer to a full tank—not a partial tank. A full tank prevents condensation from forming inside.
Dosage: - Sta-Bil Marine: 1 oz per 5 gallons - Mercury Quickstor: 1 oz per 6 gallons - Yamaha Fuel Stabilizer: 1 oz per 10 gallons
Run the engine for 10-15 minutes to circulate stabilized fuel through carburetors or injectors.
Step 2: Flush the Engine
Connect flush muffs to a garden hose. Run the engine at idle for 5-10 minutes with fresh water flowing.
Water temperature matters: Use warm water if possible—it does a better job dissolving salt deposits.
On Yamaha engines with built-in flush ports, you don't need to run the engine. Connect the hose, turn on water, flush for 3-5 minutes.
Step 3: Fog the Engine
With the engine running on flush muffs:
- Remove the air intake cover/flame arrestor
- Spray fogging oil directly into the carb throats or throttle body
- Spray until the engine stumbles and smokes heavily
- Shut off fuel supply and keep spraying until the engine dies
Alternative method (spark plug removal): 1. Remove spark plugs 2. Spray 2-3 seconds of fogging oil into each cylinder 3. Crank engine briefly (without plugs) to distribute oil 4. Reinstall plugs
Step 4: Change the Gear Oil
Do this now, not in spring. Contaminated oil sitting all winter accelerates corrosion.
- Remove bottom drain plug, then top vent plug
- Let oil drain completely (5-10 minutes)
- Check for water (milky appearance) or metal flakes
Capacity by engine: - 25-40 HP: 10-12 oz - 50-90 HP: 16-20 oz - 115-150 HP: 22-28 oz - 200+ HP: 32-40 oz
Fill from the bottom until oil comes out the top vent. Install vent plug first, then drain plug quickly.
Step 5: Change Engine Oil (4-Stroke Only)
Old oil contains acids and combustion byproducts. Leaving it in over winter etches bearing surfaces.
Typical capacities: - Yamaha F115: 3.7 quarts with filter - Mercury 150 FourStroke: 5.0 quarts with filter - Honda BF90: 4.0 quarts with filter
Use manufacturer-recommended oil weight (typically 10W-30 or 10W-40 marine 4-stroke).
Step 6: Protect the Cylinders
After fogging, the cylinders have a protective oil coating. To distribute it evenly:
- With spark plugs removed, crank the engine 3-4 revolutions
- This spreads fogging oil over cylinder walls and piston rings
- Reinstall plugs hand-tight
Step 7: Battery Service
- Disconnect negative terminal first
- Clean terminals with baking soda paste
- Fully charge the battery
- Store in a cool, dry place (not freezing)
- Trickle charge monthly or use a battery maintainer
Temperature matters: A fully charged battery won't freeze until -70°F. A discharged battery can freeze at 20°F.
Step 8: Grease and Lubricate
Apply waterproof grease to: - Prop shaft and thrust washer - Steering tube and pivot points - Throttle and shift linkages - Tilt tube pivot pins
Spray corrosion inhibitor on: - Electrical connections - Powerhead exterior under the cowling - Starter and alternator
Step 9: Final Steps
- Remove the prop and inspect for line/fishing line wrapped on the shaft
- Apply grease to prop shaft splines
- Reinstall prop or leave off for storage
- Leave cowling vents open for air circulation
- Cover the engine with a breathable cover (not plastic)
Brand-Specific Notes
Yamaha F-Series: - Built-in flush port on most models—no need to run engine while flushing - VMAX SHO models: extra attention to supercharger oil level - Use Yamalube 4M for 4-strokes, Yamalube 2M for 2-strokes (fogging)
Mercury FourStroke: - Drain plug on oil pan may require 15mm socket - Verado models: check supercharger oil level - Use Mercury Premium Plus 4-Cycle oil
Johnson/Evinrude E-TEC: - Self-winterizing feature: engine fogs itself when you turn the key off after the last use - Still need to stabilize fuel and change gear oil manually - No engine oil to change (direct injection 2-stroke)
Storage Tips
Position: - Store upright or tilted down slightly (not tilted up) - This allows any water to drain from lower unit - Prop should be clear of the floor
Location: - Dry, ventilated area - Avoid direct sunlight on rubber and hoses - Rodents love to chew wiring—use repellent or traps
Cover: - Use a breathable cover, not plastic - Plastic traps moisture and promotes mold - Cowling vents should remain open
FAQ
Can I skip fogging if I use fuel stabilizer?
No. Fuel stabilizer protects the fuel system. Fogging oil protects the cylinders, rings, and valves. They do different jobs.
What if I winterized without changing the gear oil?
Change it in spring before the first run. But if there's water in the oil, it's been corroding the gears all winter. Check carefully for damage.
How long can stabilized fuel sit?
Most stabilizers protect fuel for 12 months. After that, fuel quality degrades. If the boat sat for 18+ months, drain and replace the fuel.
Should I disconnect the fuel line?
Yes, if storing with fuel in the tank. This prevents fuel from seeping past carburetor needle valves and flooding the engine.
What about ethanol fuel?
E10 absorbs moisture from the air. Stabilize it, and don't let it sit longer than 90 days if possible. E15 or higher is not recommended for marine engines.
Bottom Line
Winterization takes 1-2 hours and costs under $100 in supplies. Skipping it can mean a cracked block, scored cylinders, or corroded gears—repairs that cost 10-50x more.
Do it right, and your engine starts on the first pull come spring.
Find service kits and maintenance supplies at SeaSierra.
