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Outboard Winterization Checklist

Nov. 11 2025 Maintenance By Patrick Pershing

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🧊 Outboard Winterization Checklist

Here are the essential steps every boater should perform before storing their outboard:

  1. Prepare the fuel system

  2. Change engine oil and filter (4-stroke engines)

  3. Fog the engine

  4. Change gear lube

  5. Inspect the prop shaft

  6. Check and top off power trim/tilt fluid

  7. Inspect sacrificial anodes (zincs)

  8. Lubricate all grease points

  9. Apply corrosion inhibitor

  10. Touch up paint

  11. Check and maintain the battery

  12. Store engine upright

Let’s break down what each step does — and how to do it.

🔧 DIY vs. Professional Winterization

Should you do it yourself, or bring it to Cope Marine?

DIY winterization appeals to mechanically minded boaters and those who enjoy working on their equipment. But professional winterization comes with important advantages:

  • Certified technicians spot issues early (water intrusion, worn seals, loose bolts, cable misalignment, cracked hoses).
  • They handle proper fluid disposal.

Professional winterization isn’t just about fluids — it’s a full engine health inspection.



🧴 SUPPLIES CHECKLIST 

Before starting, gather:

  • Marine fuel stabilizer

  • Engine oil and filter (for 4-stroke engines)

  • Gear lube + lower unit pump

  • Fogging oil (for storage protection)

  • Marine grease

  • Rust/corrosion inhibitor spray

  • Basic hand tools

  • Prop wrench

  • Spark plug socket and gap tool

  • Flushing muffs + garden hose

  • Oil drain pan

  • Funnels and rags

  • Battery charger/maintainer

❗ Tip: Marine-grade lubricants and oils are formulated for moisture resistance. Automotive products are not.

✅ STEP-BY-STEP OUTBOARD WINTERIZATION

 

1. Prepare the Fuel System

Fuel breaks down over time, causing varnish, deposits, and clogged injectors/carburetors. Ethanol-blended fuel absorbs moisture, accelerating corrosion and phase separation.

  • Fill tank to 95% full (older vented tanks benefit from being full to reduce condensation).

  • Add the correct amount of marine fuel stabilizer.

  • Run the motor for 10–15 minutes so stabilized fuel circulates through the entire system.

If you end the season with old fuel: consider pumping it out and refilling with fresh, ethanol-free fuel if possible.

2. Change the Engine Oil & Filter (4-stroke engines only)

Old oil contains moisture and acidic combustion byproducts that cause internal corrosion during storage.

  • Warm engine slightly so oil flows well.

  • Drain oil completely and replace the oil filter.

  • Refill with marine-grade oil per your manual’s recommendation.

After refilling, start the engine briefly to circulate clean oil and check for leaks.

3. Fog the Engine (Internal Corrosion Protection)

Fogging prevents rust on internal components during long storage.

  • Remove spark plugs.

  • Spray fogging oil into each cylinder.

  • Rotate prop or manually rotate flywheel to distribute oil.

  • Install new spark plugs (correctly gapped).

Some modern engines have electronic fogging modes via the control system — check your manual. For direct fuel injected (DFI) two-stroke engines such as Mercury OptiMax models, instead of using fogging oil, squirt one ounce of DFI outboard oil into each cylinder through the spark plug hole. A small oil can with a long flexible neck works well for this task.  

4. Change Gear Lube

Any water inside the lower unit can freeze, expand, and crack the casing.

  • Remove lower drain plug and vent plug.

  • Inspect drained lube:

    • Milky or white? → indicates water contamination.

    • Metal shavings? → could indicate gear wear or bearing issues.

  • Pump new gear lube from bottom to top until it flows out the vent. Replace plugs and seals after changing gear lube. 

If water is present, schedule a seal inspection.

5. Inspect the Prop Shaft

  • Remove prop and thrust washer.

  • Check for fishing line wrapped around shaft — a common cause of seal failure.

  • Grease shaft lightly and reinstall or store the prop separately (theft deterrence).

6. Check Power Trim/Tilt Fluid

Trim fluid should be clean and full. Milky appearance indicates water intrusion and should be serviced.

7. Inspect Sacrificial Anodes (Zincs)

These prevent corrosion on underwater metals.

  • Replace when more than 50% deteriorated.

  • Never paint over anodes — that makes them ineffective.

8. Lubricate All Grease Points

Use marine grease to protect:

  • Steering pivot points

  • Tilt tube

  • Prop shaft

  • Any grease fittings noted in your manual

9. Apply a Corrosion Inhibitor

Spray a corrosion blocker on:

  • Powerhead surfaces

  • Metal components

  • Lower unit (optional)

This displaces moisture and forms a protective barrier.

10. Touch Up Paint

Bare metal equals corrosion risk. Lightly sand, apply primer, paint, and clear coat.

11. Check & Maintain the Battery

  • Remove battery from boat.

  • Store in cool, dry place.

  • Keep on a maintenance/float charger — not a trickle charger.

Proper storage can double the life of a marine battery.

12. Store Engine Upright

Trim down into vertical position so water drains from:

  • Cooling passages

  • Lower unit

Never store an outboard tilted up — trapped water can freeze and crack components.

🎉 Spring Startup Will Be a Breeze

After proper winterization, spring readiness takes minutes:

  • Reinstall battery

  • Reinstall prop

  • Perform a quick visual inspection

  • Launch and enjoy

🛥 Prefer Cope Marine to handle it all?

We offer:

  • Full professional outboard winterization

  • Boat Storage 

  • Off-season upgrades (electronics, accessories, props,)

📞 O’Fallon, IL — (618) 632-6353
📞 Branson West, MO — (417) 272-1277

We stock everything you need to winterize your outboard yourself or let our team handle it with complete winterization services- ensuring your boat is fully protected for the off-season.

Stop in or give us a call — let’s protect your investment so you’re ready to hit the water next spring!

Click here to request additional service information.