How do you winterize an outboard motor?

01What you’ll need

Gather it all before you start, because mid-job runs to the marine store are how jobs get abandoned. Both West Marine and Mercury publish supply lists, and they agree almost completely:

• Fuel stabilizer — marine formulation
• Fogging oil — check this one carefully: carbureted and EFI formulas are different products (more in section 04)
• Non-toxic propylene glycol marine antifreeze — never automotive ethylene glycol, which is highly toxic to pets and wildlife and makes disposal problematic (West Marine)
• Gear lube plus a gear lube pump, and new gaskets or O-rings for the drain and vent screws
• Engine oil and filter, plus a filter wrench — 4-strokes only
• Flushing muffs and a garden hose, or a winterizing kit that mounts to the muffs with an antifreeze tank and valve
• Drain pan, shop rags, basic hand tools, a prop wrench, marine grease and a grease gun, corrosion inhibitor

A note on antifreeze: compare products by their stated burst-protection rating and recommended dilution, not just the label claim — the same advertised rating can hide different concentrations (West Marine). How much will you need? The honest answer is: run it through until you see antifreeze exiting the motor’s telltale water discharge (West Marine). That’s the test that matters, not a gallon figure.

Buy early. Yamaha warns that supplies sell out more often than you’d think (Yamaha).

02Flush it, and warm the oil

Connect flushing muffs to a garden hose and place them squarely over both sides of the water intake. Turn on the water, then start the motor. When you’re done, shut the motor off before you shut the water off. That order — water before start, engine before water — is the discipline. Keep it every single time (West Marine).

Let the engine idle for a few minutes on fresh water (West Marine). If you’re changing engine oil on a 4-stroke, do it now, while the engine is warm on the muffs — warm oil drains fully and carries suspended contaminants out with it (West Marine).

Refill with marine-specific 4-stroke oil. Never automotive oil in a marine engine (Mercury, West Marine). Then, Mercury’s dealer team lead suggests restarting briefly after the oil change to circulate fresh oil and check for leaks — with a specific warning: “it’s not uncommon for the oil filter gasket to stick to the engine… I’ve seen engines ruined because the owner didn’t notice engine oil leaking out past a doubled gasket” (Mercury Marine). Look. Twice.

03Treat the fuel — and here the experts differ

This is the first honest conflict. Yamaha says: “Be sure your fuel tank is topped off and treated with Yamaha Fuel Stabilizer before you begin.” Mercury, more detailed, says store with fresh fuel treated with stabilizer, and fill older vented tanks to about 95 percent — enough to prevent condensation, but leaving room for fuel to expand in temperature swings, which can otherwise force gas out the vent. Newer non-venting tanks don’t need to be full at all. Topped off versus 95 percent versus “doesn’t need to be full” — all three positions are from the manufacturers themselves (Yamaha, Mercury). Mercury also recommends starting from a nearly empty tank and adding fresh fuel: “Most of the issues our dealers run into are caused by stale fuel.” Ethanol-free fuel such as REC-90 is preferred where you can get it (Mercury Marine).

Whatever the fill level: add stabilizer at the label ratio, then run the engine long enough — roughly 10 minutes per Mercury, 5–10 per West Marine — so treated fuel fills the whole fuel system, not just the tank.

The run-it-dry question. Old 2-stroke habit says run the engine out of fuel before storage. The sourced modern position splits by engine type: on a carbureted engine, running the carburetors dry is a legitimate final step — West Marine has you disconnect the fuel line at the end of fogging and let the engine die on the remaining fuel, which “prevents varnish deposits from gasoline left in the carburetor bowl.” On an EFI engine, do the opposite. West Marine’s EFI instructions are explicit: do not cut off the fuel supply and wait for the motor to stop. Run it until you see white smoke, then shut it off before your antifreeze supply runs out. Never run an EFI system dry.

04Fog it — and never mix up the cans

Fogging oil coats pistons, rings, and cylinder walls with a protective film against months of still air (West Marine). There are two methods, and you may use both:

Intake fogging (carbureted engines): with the engine running on antifreeze through the winterizing kit, spray fogging oil into the carburetor intake. Small engines will typically stall; larger ones “may cough and sputter but continue running and emit white smoke.” Spray a substantial amount, then pull the fuel line and let it run dry — carbureted engines only, per section 03 (West Marine).

Plugs-out fogging: Mercury’s method — “ideally while the motor is still warm… remove the spark plugs and spray the fogging oil directly into each cylinder, following the directions on the can.” Follow the can; Mercury also markets Storage Seal for exactly this (Mercury Marine).

The EFI answer, plainly: yes, EFI engines get fogged — but the oil goes through the fuel system, not the intake. West Marine: mix fogging oil formulated specifically for EFI motors into a small container of stabilized gasoline and supply it to the motor, using the ratio in your owner’s manual. And the hard warning, worth memorizing: “Do not use fogging oil formulated for carbureted motors in an EFI motor. Carbureted fogging oil contains a wax-like substance that can clog fuel injectors” (West Marine).

DFI 2-strokes (Mercury OptiMax and similar) are the exception to everything above: no fogging oil at all. Instead, squirt one ounce of DFI outboard oil into each cylinder through the spark plug hole — a small oil can with a flexible neck works well (Mercury Marine).

05Change the gear oil — and read it like a chart

Do this now, at lay-up, so you’re not leaving whatever’s in the leg sitting in the gearbox all winter — and so you find water intrusion before spring launch, not after (West Marine).

Trim the engine so the lower drain screw is at the lowest point of the gearcase. Remove the bottom drain plug first, catching the gasket, then the vent screw above it — the oil will gush out. Let it drain fully; West Marine suggests an hour or more. Stuck screws are common — steel in aluminum seizes by galvanic corrosion. Penetrating oil overnight, a manual impact screwdriver, or heat will get them moving (West Marine).

Now read the oil like a chart:

Refill from the bottom up with a pump until oil overflows the vent hole — filling from the bottom eliminates trapped air. Vent screw in first, then swap the pump for the bottom plug quickly. New gaskets or O-rings on both screws, always; West Marine suggests coating the new O-rings with non-metallic marine anti-seize. How much oil? Check your manual — capacities vary by engine.

06Get the water out — or replace it with antifreeze

BoatUS frames the whole principle: winterizing means draining any water aboard or replacing it with enough of the right antifreeze to protect against the coldest temperatures your boat might see. This is the second honest conflict, and here’s how it actually falls out.

Draining only is common practice on self-draining outboards — West Marine says so outright.

Antifreeze pull-through is what West Marine recommends anyway: “Simply draining the cooling water out of the motor is common practice, but we recommend flushing with antifreeze to eliminate any residual water that could freeze and cause damage during a hard freeze.” Notably, no manufacturer in our research mandates it for outboards — and Nordic yards treat it as standard practice: Finland’s Venehallit sells a “glykolin ajo” (glycol run) as a €100 line item alongside a €25 engine freshwater flush (Venehallit). When both a major retailer and a whole region’s boating industry sell it as a service, it’s not exotic.

The sourced method: hook a winterizing kit (a muffs-mounted tank with a valve) to your flush muffs, fill the tank with non-toxic propylene glycol antifreeze, open the valve, restart the engine, and watch the level drop until you see antifreeze exiting the telltale (West Marine). BoatUS describes the bucket variant: a five-gallon bucket of undiluted antifreeze, with the lower unit drawing from the bucket as the engine runs. Keep it at that — draw from the bucket, run, watch the telltale.

Propylene glycol is the right stuff because it has low toxicity and is biodegradable; ethylene glycol is “highly toxic… dangerous to pets and wildlife and makes disposal problematic” (West Marine). Engine coolant is never a substitute.

07Trim it down. Yes, down.

This question generates more forum argument than any other, and the sources are unanimous — so the argument can end.

Trim the engine fully down to its vertical position. Mercury: “the motor should be trimmed down to its vertical position so that any water remaining in the engine can self-drain. If water trapped in the engine freezes, it could cause serious damage.” West Marine: “store the engine in an upright position so any remaining water will drain out.” BoatUS: “Store outdrives and outboards in the down position so water can drain; otherwise ice can crack the housing.”

The “store it up” advice you’ll hear comes from a different situation entirely: when a boat is moored and living, the engine is raised for prop clearance and security. That’s a docked-boat habit, not a lay-up position. For winter, down. Let the leg drain.

08The closing small tasks

• Pull the prop. Check for fishing line wrapped on the shaft, clean and grease it before reinstalling. If the motor stays on the boat, take the prop home — West Marine’s blunt anti-theft advice (West Marine).
• Check the anodes. Replace any more than half dissolved, matched to your water: zinc for saltwater, aluminum for salt or brackish, magnesium for freshwater only (West Marine).
• Grease the pivot points at their zerk fittings, hit the motor with corrosion inhibitor, and touch up paint chips (Mercury, West Marine).
• Check the trim fluid (Mercury).
• The battery: keep it on a regulated charger if shore power is available; otherwise charge it fully and disconnect it at the terminals — better still, remove it to a warm dry place. A discharged lead-acid or AGM battery can freeze (Yamaha).
• Taking the motor off the boat? Removing it for indoor storage is normal practice — yards sell it as a service: Snug Harbor on Lake Michigan charges $50 a season for heated outboard storage plus remove-and-replace labor (Snug Harbor). Store the motor upright so any trapped water keeps draining, and it lives by the same rule as the boat: no freezing temperatures, no surprises.

094-stroke vs 2-stroke vs DFI: what actually differs

The core procedure is identical across all three. The differences are oil and fogging.

10Do you even need to winterize it?

Here’s the mechanism, and it’s not subtle: fresh water expands about 9 percent when it freezes and pushes outward with tremendous force — enough to crack an engine block (BoatUS). In a ten-year analysis of GEICO | BoatU.S. freeze claims, more than three-quarters involved cracks in the engine block or exhaust manifolds “because water remained in the engine or cooling system during a hard freeze” — and the claims came from all over the country, not just snow country. Southern-state claims often involved boats not winterized at all, or a heater that lost power on the coldest night. The frequent outcome, per BoatUS: a complete engine replacement, and most of a season gone.

Indoor storage? Heated indoor storage removes the freeze risk — but not the lay-up. Yamaha, on parking the boat in your garage: “Winterize the engine and any hoses and pumps that carry water — and you’re done.” The engine still gets stabilized fuel, fresh gear oil, and fogging, because fuel degradation and internal corrosion work in any temperature. Boats stored ashore are actually more freeze-exposed than boats left in water, since water retains heat longer than air — so a trailer-stored rig needs winterizing earlier (BoatUS). The operational trigger, wherever the boat sits: before the first hard freeze. The regional calendar is its own article in this series — when should you winterize.

It already froze? The damage signatures are what you’d check in spring: brown froth weeping from a crack in the block, chocolate-milk oil on the dipstick (BoatUS), milky gear oil (West Marine). Any of those means a professional inspection before the engine runs again.

11DIY or dealer — and what it costs, honestly

Mercury is direct about DIY: the storage procedure is in every owner’s manual, and “with a few basic tools and the proper supplies, those inclined to do-it-yourself can usually tackle the task.” Yamaha agrees: “if you’re handy and have the right tools and time, you can do it yourself,” with prepackaged oil-change kits at dealers (Mercury, Yamaha). This guide, minus the antifreeze pull-through kit, is that manual procedure in the order the professionals do it.

What a yard actually charges varies more than the folklore says. Published rates:

What you get for the money varies by tier. A basic package runs the treated fuel through, greases the fittings, checks the lower unit, services the battery, and pulls the prop; mid tiers add the oil, filter, and gear-lube changes; top tiers add water pump, thermostat, and full fuel-service items on a multi-year schedule (Timber Marine, Bluff Creek). The real dealer value, per Mercury, is the second set of eyes — a tech who spots loose transom bolts or a bent propshaft while the cowl is off.

Nordic yards: the engine work à la carte

In Sweden, Mälarbåtar’s heated, dehumidified hall near Mälaren includes haul-out, launch, and hull rinses in the base price, with “motorkonservering” added via their online calculator — and they want your booking by 1 October, since haul-out season runs mid-September to end-October (Mälarbåtar). If a hall package already includes a motor conservation add-on for a modest sum, paying for it while you spend your own afternoon on the boat itself is a reasonable split.

The verdict: a confident DIYer with the manual can match the basic tier in an afternoon. Go to the dealer if the engine is under warranty with complex systems, if you have no access to flush water or a place to run the engine, or — non-negotiably — if the gear oil came out milky.

12Your weekend checklist

• Manual out, supplies bought (correct fogging oil for your engine type)
• Muffs on → water on → engine on, idle a few minutes
• 4-stroke: change engine oil and filter while warm; restart, check for leaks
• Stabilize the fuel (fresh fuel in; ~95% if the tank vents); run 5–10 minutes
• Winterizing kit on; pull antifreeze through until it exits the telltale
• Fog: intake (carb) / plugs-out / EFI fuel-mix / 1 oz-per-cylinder DFI — per your engine
• Carbureted only: disconnect fuel, run the bowls dry
• Drain and refill gear oil; new gaskets; investigate anything milky or gray before spring
• Prop off, check for line, grease, reinstall (or take it home)
• Anodes, grease points, corrosion spray, trim fluid
• Battery: charge, disconnect, ideally move somewhere warm and dry
• Trim the engine down. Walk away.

Prefer it on paper? The free winterization checklist carries the same order of operations for the whole boat.

13FAQ