Can I winterize my Mercury FourStroke outboard myself?

Yes. For a standard Mercury four-stroke outboard, DIY winterization is a realistic option for a mechanically comfortable boater. Supply cost runs $120-$220 (fogging oil ~$20-$35; AMSOIL fuel stabilizer ~$19.19; lower unit gear lube ~$25-$50; marine grease ~$15-$20). The process takes 2-4 hours and doesn't require specialty tools or a lift. The key steps: flush the cooling system, stabilize the fuel and run it through the system, fog the engine, drain and refill the lower unit, grease all fittings, and service the battery. The main reason to use a dealer instead is if you find water in the lower unit, if the engine is under active warranty and you're not documenting service carefully, or if the engine has known diagnostic issues. Dealer shop rate at HBW is $140/hr.

What happens if I don't winterize my Mercury outboard?

Skipping winterization puts your engine at risk of freeze cracking, fuel system degradation, lower unit damage, and battery failure. Water remaining in the powerhead cooling passages freezes and expands, this can crack the block or head, often requiring a complete engine replacement. Untreated fuel forms varnish over the winter that clogs injectors and carburetors, causing starting and running problems in spring. Based on our own repair order history, a "won't start" diagnostic averages $540; a full engine repair averages nearly $2,000. Unchanged gear lube won't reveal a leaking lower unit seal, which continues to cause internal damage while the boat sits. Total repair cost from skipped winterization can easily exceed $1,000-$3,000+ depending on what fails.

What fogging oil should I use on a Mercury outboard?

Mercury recommends Mercury Precision Fogging Oil for their four-stroke outboards. Using Mercury-branded fogging oil is advisable if your motor is still under warranty, using compatible OEM-recommended products protects your warranty documentation position. If your engine is out of warranty, a quality marine fogging oil from another reputable brand (such as Yamaha Stor-Rite or Star Brite) will perform the same protective function. The key is using a product specifically designed for marine engine fogging, not a substitute like WD-40 or light oil, which don't provide adequate cylinder wall protection over a long storage period.

Do I need to run the engine during winterization?

Yes, running the engine is essential for two steps. First, you need to run the engine on flush muffs with fresh water flowing to clear the cooling system of lake water and sediment. Second, you need to run the engine after adding fuel stabilizer to ensure the stabilized fuel circulates through the fuel injectors or carburetor, just adding stabilizer to the tank without running it doesn't protect the fuel delivery components. You also need the engine running briefly (and still warm) to fog properly. If you can't run the engine for any reason before storage, take it to a shop, partial winterization can leave you worse off than a complete job.

How long does DIY outboard winterization take?

Plan for 2-4 hours for a first-timer working through a single outboard. An experienced boater who has done the process before can complete it in 90 minutes to 2 hours. Twin-engine setups roughly double the time. The longest step is usually the cooling system flush and fuel stabilization run, which takes 10-20 minutes of engine-running time. The lower unit drain and refill takes 15-20 minutes if you're careful and don't strip anything. Allow time to let the engine cool slightly before removing any covers or doing your final grease and inspection work. First-time DIYers should budget a full afternoon.

What lower unit oil should I use on my Mercury outboard?

Mercury recommends Mercury Precision Lubricants gear lube in the weight specified for your model. Check your owner's manual for the correct specification, most Mercury four-stroke outboards use a specific SAE grade (typically 80W-90 or Mercury High Performance Gear Lube, depending on the model). AMSOIL synthetic gear lube is another quality option at $24.99-$26.49/L (available at marinecatalogue.ca). Using the wrong weight or an incompatible aftermarket product isn't recommended, particularly on engines under warranty.

Should I store my outboard up or down for winter?

Store the outboard in the fully tilted-down (vertical) or slightly tilted-up position, never in the fully trimmed-up position for extended storage. Storing fully trimmed up traps water in the cooling passages and puts mechanical stress on the tilt/trim system components over a long storage period. Trimming slightly down allows residual water to drain from the lower unit and water pump area. After the final cooling system flush, let the motor rest in a trimmed-slightly-down position overnight before doing your final grease work.

What is the most important step in outboard winterization?

The cooling system flush and the fuel stabilization run are the two most consequential steps. The cooling flush removes water that would otherwise freeze and crack the powerhead or exhaust system. The fuel stabilization run ensures treated fuel actually reaches the fuel delivery components, if you only add stabilizer to the tank without running the engine, the injectors and carb are still exposed to untreated fuel over winter. The lower unit drain is close behind in importance, it's your main diagnostic for a failing seal. If forced to choose, prioritize the cooling flush and fuel stabilization. Skipping either of those creates the most expensive failure modes.

Does DIY winterization affect my Mercury warranty?

Mercury's warranty does not require dealer-performed maintenance to remain valid, but it does require that maintenance be performed according to the intervals and procedures specified in your owner's manual, using appropriate products. DIY winterization is compatible with warranty coverage if you use OEM-recommended lubricants and fogging products, follow the correct procedures, and keep records. The risk with undocumented DIY service is that if a warranty claim arises, you may be asked to demonstrate that maintenance was performed correctly. If you're in an active warranty period and not confident about documentation, having a dealer service the engine at least once, then doing it yourself afterward, is a reasonable approach.

What's the difference between winterizing a two-stroke and a four-stroke Mercury?

The core process is similar, cooling flush, fuel stabilization, fogging, lower unit drain, battery service. The main differences: two-stroke carbureted engines need the carb bowl drained or run dry as part of fuel system prep, since fuel sitting in a float bowl over winter is prone to varnishing. Four-stroke EFI engines need fuel stabilizer circulated through the injection system by running the engine after treatment. Four-strokes also require an oil check (and oil change if due) as part of the service. At HBW, our 40-200 HP 2-stroke winterization rate is $206.89 vs. $337.84 for a 40-60 HP 4-stroke, the difference reflects the additional steps involved.

Can I Winterize My Mercury Outboard Myself? (Complete DIY Guide + When to Call a Dealer)

Language: English --- DIY Mercury winterization risk checklist, 8 things that go wrong when boat owners winterize themselves ## Quick Answer You can winterize your own Mercury outboard if you're comfortable with fogging oil, lower-unit drain-and-fill, and fuel stabilizer, and...

Mercury Platinum Dealer·Family-owned since 1947·Mercury dealer since 1965·Gores Landing, ON·Quote builder available

Language: English

DIY Mercury winterization risk checklist, 8 things that go wrong when boat owners winterize themselves

Quick Answer

You can winterize your own Mercury outboard if you're comfortable with fogging oil, lower-unit drain-and-fill, and fuel stabilizer, and if you have the supplies and an hour of focused work. The step that kills motors is leaving water in the powerhead through a freeze. If you're not certain the cooling system is clear, pay a dealer to do it. A professional winterization costs a fraction of a cracked block.

For professional winterization and winter boat storage in the Kawarthas: hbw.wiki/service.

Fall checklist

Winterizing your Mercury? Five steps in this order

Spring no-starts usually trace back to one of these five steps being skipped in fall.

Fuel

Is the tank near full with treated fuel (marine stabilizer added)?

Top the tank to 95 percent with stabilizer mixed in. Empty space invites condensation, which is the most common cause of spring no-starts.

Fogging oil

Have you fogged the cylinders through the spark plug holes or air intake?

Mercury recommends running fogging oil through the intake while the motor runs, then a shot directly into each cylinder. Skipping this is the #1 cause of corroded cylinder walls.

Lower unit oil

Has the lower unit oil been changed and inspected for water?

Milky oil means a seal failure. Better to find it in fall when you have time to fix it, not in spring at the launch ramp.

Water flush and drain

Have you flushed with fresh water and let the motor drain fully tilted down?

Run the motor on muffs with fresh water for 5 to 10 minutes, then tilt straight down to drain. Leftover water in the block freezes and cracks aluminum.

Battery

Is the battery off the boat or on a maintainer for the winter?

A frozen, discharged battery is a $200 spring purchase. A trickle charger or full pull-out is $50 of prevention.

🔧

Want us to handle it?

Full winterization service runs around $250 to $400 depending on motor size. Includes fuel stabilizer, fogging, lower unit oil, flush, drain, and battery prep. Book at /service or call (905) 342-2153.

I skipped winterization once. Just one year. The block cracked and the repair quote came back at $7,800. The $400 winterization service every fall is the cheapest insurance I buy on this boat.

Ron H.HBW Customer, Peterborough 2026

DIY Mercury outboard winterization, six steps in order

DIY Mercury Outboard Winterization: What to Do, What to Skip, and When to Call a Dealer

Ontario's boating season has a hard stop. Unlike some parts of the country where an outboard can sit in the water year-round, here it comes out in October and faces five or six months of temperatures that will crack an unprotected powerhead if you leave water in it.

Winterization isn't optional maintenance. It's the difference between a motor that starts in May and one that needs a powerhead, or gets scrapped.

This guide covers what DIY winterization actually involves, which steps are realistic for most owners, which ones are easy to get wrong, and when it makes more sense to bring the motor to a dealer.

In the Aug, Nov 2025 season, we completed 584 winterizations at HBW. The May and June diagnostic work that follows every spring includes a consistent number of motors that weren't winterized correctly, DIY or otherwise.

Who Should DIY and Who Shouldn't

DIY is realistic if:

You're comfortable working around a motor and have done basic maintenance before
You have a smaller motor (under 60 HP is more manageable for a first DIY attempt)
You have storage access with space to work around the motor
You can run the motor briefly at home (with flush muffs or a water supply)
You have the right supplies and aren't guessing at quantities or procedures

Consider professional service if:

You've never done this before and aren't confident with mechanical work
You have a larger, more complex motor (especially V6, V8, or Verado)
You're not sure you can clear the cooling system properly
You're rushed and likely to skip or hurry steps
The motor is stored somewhere you can't access it easily for prep

For engine repairs, we only service Mercury and Mercruiser.

What You Need Before Starting

Pick these up at a Canadian Tire, marine store, or from your dealer's parts counter before the last day on the water:

Mercury Quickstor or equivalent fuel stabilizer
Mercury Storage Seal fogging oil (or Mercury Premium Fogging Oil)
Mercury High-Performance Gear Lube, volume depends on motor size; a quart typically covers motors under 60 HP, larger motors need more
New gear lube drain plug gaskets, cheap, replace every year
Spark plug socket (size varies; 13/16" is common on many Mercurys, check your manual)
Anti-seize compound for spark plug threads
Marine grease for grease points
A bucket and rags
Flush muffs (earmuffs that go over the lower-unit cooling intake) or another method to supply cooling water

You need a way to run the motor briefly during this process. Flush muffs connected to a garden hose are the most common DIY method and work well.

The Winterization Sequence

Do these in order. Don't skip steps. If you're not sure what you're looking at at any point, stop and bring the motor in.

Step 1: Stabilize the Fuel

Do this before your last run of the season, not after.

Add fuel stabilizer to the tank at the rate on the bottle, typically 1 oz per 2.5 gallons (10 L) or as directed.
Run the motor for at least 10 minutes to circulate the stabilized fuel through the entire system, lines, injectors (FourStrokes), or carburettors (older 2-strokes).
This prevents the fuel from breaking down and gumming up your fuel system over winter.

Ethanol note: If you're running pump gas with ethanol (E10), this step matters more. Ethanol absorbs water and can cause phase separation in stored fuel. Ethanol-free fuel is available at HBW on-site, filling up with it at the end of season simplifies this step considerably.

Don't: drain the tank completely dry. Modern FourStroke fuel systems need some fuel in the system. What you want is stabilized fuel, not an empty tank.

Step 2: Flush the Cooling System

After the last run (or during winterization), flush the motor with clean fresh water. This removes salt (for salt water boats), sediment, and any organic debris from the cooling passages.

Connect the flush muffs to a hose, run the motor at idle for at least 5 minutes, then shut down.

Critical: The motor must reach operating temperature during this step for the thermostat to open and allow water through the full cooling circuit. Five minutes at idle on warm motor is better than two minutes on a cold one.

After shutting down, let the water drain completely from the motor while tilted down.

Step 3: Fog the Engine

Fogging oil is sprayed into the intake (for most FourStrokes) or carburettors (for older 2-strokes) while the motor is running. It deposits a protective oil film on the internal cylinder walls, protecting against rust and corrosion during storage.

FourStroke process (typical): With the motor running at idle and connected to flush water:

Remove the air box cover to access the intake.
Spray fogging oil in short bursts into the intake. The motor will smoke, this is expected.
Continue until oil smoke is coming steadily from the exhaust.
Shut down while still spraying, this leaves a coat of oil on the cylinder walls.

Older carbureted 2-strokes: Process is similar but through the carburettor throat(s). Follow your model-specific procedure.

The procedure varies by motor model, consult your Mercury operator's manual for the exact method for your motor. Getting this step wrong (under-fogging, or fogging incorrectly) is one of the most common DIY mistakes.

Step 4: Drain and Refill the Gearcase

The lower unit gearcase contains gear lube that absorbs water through the seals over a season of use. If you leave that water-contaminated lube in over winter, it can freeze, expand, and damage the gear case or seals.

Remove both drain/fill plugs from the gearcase, the lower one first, then the upper one.
Let the old gear lube drain completely into a container.
Inspect what drains. Milky or foamy gear lube indicates water intrusion, this is a sign you need seal inspection before assuming the gearcase is healthy.
Using a gear lube pump, fill from the bottom hole until lube comes out the top hole with no air bubbles.
Install the top plug first, then the bottom plug. Replace the gaskets, they're cheap and the small cost of a new gasket beats a gear lube leak.

Use Mercury-branded gear lube appropriate for your motor. The spec is in your owner's manual.

You can build a live CAD quote for your repower online at Mercury Repower Centre.

Step 5: Lubricate All Grease Points

Every Mercury has grease fittings on the steering and tilt/trim pivot points. These need marine grease before storage. Dry or under-greased pivot points seize over winter and result in stiff, damaged components in spring.

Follow the lubrication diagram in your owner's manual. Common points:

Tilt/trim tube and pivot
Steering cable end (if applicable)
Swivel bracket
Propeller shaft (anti-seize compound on prop shaft, not regular grease)
Any other fittings shown in your manual

Don't skip the prop shaft. A prop that corrodes onto the shaft over winter is a spring headache.

Step 6: Battery, Remove and Store Properly

The battery should not overwinter in the boat. Cold temperatures accelerate discharge in lead-acid batteries, and a discharged battery can freeze and be permanently damaged.

Remove the battery and store it in a cool, dry, frost-free location. Put it on a battery tender (trickle charger designed for long-term storage), this maintains charge without overcharging and dramatically extends battery life.

A battery that stored properly over winter starts spring at full capacity. A battery that sat in a cold boat all winter may start the season already compromised.

Step 7: Final Check, Water Drain

Before putting the cover on:

Tilt the motor to allow any remaining water to drain from the cooling system
Confirm you can see water drain from the motor
The motor should be stored in a position that allows gravity to drain any remaining water from the cooling passages

For most outboards, storing tilted up (or close to vertical if the motor can be removed and stored on an engine stand) works. Check your model's storage orientation recommendation.

Step 8: Cover and Store

Cover the motor with a vented motor cover or cowling cover. A non-vented plastic wrap traps condensation. The motor needs to breathe slightly while in storage.

If the boat goes into covered indoor storage, this is less critical. Outdoor or unheated storage needs proper covering.

What Gets Missed Most Often

Based on what we see in spring diagnostics at HBW:

Incomplete cooling flush. The motor didn't get to temperature, so the thermostat never opened and the full cooling circuit wasn't cleared.
Fogging oil skipped entirely. "I forgot" is the most common reason. The consequence shows up as surface corrosion on cylinder walls in spring.
Milky gear lube ignored. If gear lube came out milky and the owner just refilled without addressing the seal leak, the new lube goes milky again by spring.
Battery left in the boat. Dead or frozen battery in May.
Stabilizer added but motor not run afterward. The stabilizer never reached the injectors or carbs, so they saw unstabilized fuel all winter.

When to Let a Dealer Do It

The procedure above is accurate and complete, but it's also a sequence of steps where one mistake can result in expensive consequences in May. If you're not fully confident in any of the steps above, particularly fogging and cooling system clearance, professional winterization is the better economic decision.

The cost of professional winterization is a known quantity. The cost of a cracked powerhead is not, and it's substantially higher.

If you want professional winterization: hbw.wiki/service. We booked 584 winterizations in the 2025 fall season, the shop knows these motors.

Related at HBW

The full topic hub: How Much Does Boat Winterization Cost in Ontario? (2026 Price Guide) -- start here if you want the complete picture.

Two related guides in the same cluster:

Frequently Asked Questions

Can I winterize a Mercury FourStroke the same way as an older 2-stroke? The core steps are the same, but the procedure for fogging and fuel stabilization differs. FourStrokes have fuel injection systems rather than carburettors, and the fogging process is different. Always follow the procedure in your model's Mercury operator's manual, not a generic guide.

Do I need to change the spark plugs as part of winterization? It's a good time to inspect them, but it's not required at every winterization. If the plugs are at or near their service interval, change them in fall so the motor starts fresh in spring. Mercury's service schedule has plug replacement intervals, check your manual.

What if I pull the boat out of the water in November but can't get to it until December? The deadline is before it freezes hard enough to crack the block. In Ontario, that usually means before sustained temperatures below -5°C to -10°C. Don't cut it close, once it's frozen, the damage is done.

Is it okay to run the motor without water to fog it? No. Dry-cranking an outboard even briefly can damage the water pump impeller. Always have the cooling system supplied with water when running the motor, including during fogging.

Should I drain the fuel tank completely for winter? Not recommended for modern FourStrokes. Leaving stabilized fuel in the system is better than an empty system, which allows seals and gaskets to dry out. The stabilizer should be run through the full fuel system. Ask your dealer if you're unsure about your specific motor.

My gear lube came out milky. Is that a big problem? Yes. Milky gear lube means water in the gearcase, which means a seal is leaking. Fresh gear lube will go milky again by spring if the seal isn't addressed. This is worth having looked at before or during winterization.

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Reviewed by the Harris Boat Works service team. HBW handles Mercury and Mercruiser service from Gores Landing on Rice Lake. For engine repairs, we only service Mercury and Mercruiser. About Harris Boat Works.

CTA

Want it done right without the Saturday morning? Request professional winterization at hbw.wiki/service. Harris Boat Works, Gores Landing, Mercury Platinum dealer. Mercury dealer since 1965, family marina on Rice Lake since 1947.

Phone: 905-342-2153

Ready to price it out? Build a live CAD quote for your repower online at the Mercury Repower Centre.

Frequently Asked Questions

Can I winterize my Mercury FourStroke outboard myself?: Yes. For a standard Mercury four-stroke outboard, DIY winterization is a realistic option for a mechanically comfortable boater. Supply cost runs $120-$220 (fogging oil ~$20-$35; AMSOIL fuel stabilizer ~$19.19; lower unit gear lube ~$25-$50; marine grease ~$15-$20). The process takes 2-4 hours and doesn't require specialty tools or a lift. The key steps: flush the cooling system, stabilize the fuel and run it through the system, fog the engine, drain and refill the lower unit, grease all fittings, and service the battery. The main reason to use a dealer instead is if you find water in the lower unit, if the engine is under active warranty and you're not documenting service carefully, or if the engine has known diagnostic issues. Dealer shop rate at HBW is $140/hr.
What happens if I don't winterize my Mercury outboard?: Skipping winterization puts your engine at risk of freeze cracking, fuel system degradation, lower unit damage, and battery failure. Water remaining in the powerhead cooling passages freezes and expands, this can crack the block or head, often requiring a complete engine replacement. Untreated fuel forms varnish over the winter that clogs injectors and carburetors, causing starting and running problems in spring. Based on our own repair order history, a "won't start" diagnostic averages $540; a full engine repair averages nearly $2,000. Unchanged gear lube won't reveal a leaking lower unit seal, which continues to cause internal damage while the boat sits. Total repair cost from skipped winterization can easily exceed $1,000-$3,000+ depending on what fails.
What fogging oil should I use on a Mercury outboard?: Mercury recommends Mercury Precision Fogging Oil for their four-stroke outboards. Using Mercury-branded fogging oil is advisable if your motor is still under warranty, using compatible OEM-recommended products protects your warranty documentation position. If your engine is out of warranty, a quality marine fogging oil from another reputable brand (such as Yamaha Stor-Rite or Star Brite) will perform the same protective function. The key is using a product specifically designed for marine engine fogging, not a substitute like WD-40 or light oil, which don't provide adequate cylinder wall protection over a long storage period.
Do I need to run the engine during winterization?: Yes, running the engine is essential for two steps. First, you need to run the engine on flush muffs with fresh water flowing to clear the cooling system of lake water and sediment. Second, you need to run the engine after adding fuel stabilizer to ensure the stabilized fuel circulates through the fuel injectors or carburetor, just adding stabilizer to the tank without running it doesn't protect the fuel delivery components. You also need the engine running briefly (and still warm) to fog properly. If you can't run the engine for any reason before storage, take it to a shop, partial winterization can leave you worse off than a complete job.
How long does DIY outboard winterization take?: Plan for 2-4 hours for a first-timer working through a single outboard. An experienced boater who has done the process before can complete it in 90 minutes to 2 hours. Twin-engine setups roughly double the time. The longest step is usually the cooling system flush and fuel stabilization run, which takes 10-20 minutes of engine-running time. The lower unit drain and refill takes 15-20 minutes if you're careful and don't strip anything. Allow time to let the engine cool slightly before removing any covers or doing your final grease and inspection work. First-time DIYers should budget a full afternoon.
What lower unit oil should I use on my Mercury outboard?: Mercury recommends Mercury Precision Lubricants gear lube in the weight specified for your model. Check your owner's manual for the correct specification, most Mercury four-stroke outboards use a specific SAE grade (typically 80W-90 or Mercury High Performance Gear Lube, depending on the model). AMSOIL synthetic gear lube is another quality option at $24.99-$26.49/L (available at marinecatalogue.ca). Using the wrong weight or an incompatible aftermarket product isn't recommended, particularly on engines under warranty.
Should I store my outboard up or down for winter?: Store the outboard in the fully tilted-down (vertical) or slightly tilted-up position, never in the fully trimmed-up position for extended storage. Storing fully trimmed up traps water in the cooling passages and puts mechanical stress on the tilt/trim system components over a long storage period. Trimming slightly down allows residual water to drain from the lower unit and water pump area. After the final cooling system flush, let the motor rest in a trimmed-slightly-down position overnight before doing your final grease work.
What is the most important step in outboard winterization?: The cooling system flush and the fuel stabilization run are the two most consequential steps. The cooling flush removes water that would otherwise freeze and crack the powerhead or exhaust system. The fuel stabilization run ensures treated fuel actually reaches the fuel delivery components, if you only add stabilizer to the tank without running the engine, the injectors and carb are still exposed to untreated fuel over winter. The lower unit drain is close behind in importance, it's your main diagnostic for a failing seal. If forced to choose, prioritize the cooling flush and fuel stabilization. Skipping either of those creates the most expensive failure modes.
Does DIY winterization affect my Mercury warranty?: Mercury's warranty does not require dealer-performed maintenance to remain valid, but it does require that maintenance be performed according to the intervals and procedures specified in your owner's manual, using appropriate products. DIY winterization is compatible with warranty coverage if you use OEM-recommended lubricants and fogging products, follow the correct procedures, and keep records. The risk with undocumented DIY service is that if a warranty claim arises, you may be asked to demonstrate that maintenance was performed correctly. If you're in an active warranty period and not confident about documentation, having a dealer service the engine at least once, then doing it yourself afterward, is a reasonable approach.
What's the difference between winterizing a two-stroke and a four-stroke Mercury?: The core process is similar, cooling flush, fuel stabilization, fogging, lower unit drain, battery service. The main differences: two-stroke carbureted engines need the carb bowl drained or run dry as part of fuel system prep, since fuel sitting in a float bowl over winter is prone to varnishing. Four-stroke EFI engines need fuel stabilizer circulated through the injection system by running the engine after treatment. Four-strokes also require an oil check (and oil change if due) as part of the service. At HBW, our 40-200 HP 2-stroke winterization rate is $206.89 vs. $337.84 for a 40-60 HP 4-stroke, the difference reflects the additional steps involved.

Harris Boat Works · 5369 Harris Boat Works Rd, Gores Landing, ON · (905) 342-2153

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