# How to Winterize an Outboard Motor: Maintenance Step-by-Step > To winterize an outboard motor, stabilize the fuel, flush, fog, change the oils, drain water and store it upright. Step list plus a routine service schedule. Watercraft & Outboards By [MARS Engineering & Sales Team](/about/) Published Oct 23, 20267 min read How to winterize an outboard motor comes down to six jobs: stabilize the fuel and run it through the engine, flush the cooling system with fresh water, fog the cylinders to protect them from corrosion, change the engine oil (four-strokes) and gearcase oil, drain all water by storing the motor upright, and grease, protect and disconnect the battery. Done before the first frost or any lay-up longer than about two months, it prevents the three classic spring failures: varnished fuel systems, cracked or corroded water passages and seized or rusted internals. ## Key takeaways - Any outboard that may see freezing temperatures must be fully drained of cooling water, which normally means storing it vertical or tilted down, never tilted up. - Fuel stabilizer only protects the parts of the fuel system it reaches, so run the engine for several minutes after adding it to the tank. - Fogging oil coats cylinder walls and valves; fuel-injected engines often need a specific procedure in the owner's manual rather than spraying into the intake while running. - Milky gear oil at the lay-up change means water is entering the gearcase and the seals should be repaired before the next season. - Many four-stroke service schedules call for engine oil, filter, gear oil and fuel filters every 100 hours or once a year, whichever comes first. ## Do you have to winterize an outboard motor? Yes, if the motor will sit unused for a season or face freezing temperatures. Water left in the cooling passages can freeze and crack castings, and fuel left untreated for months can degrade and leave gum in injectors, carburetors and pumps. Even in warm climates, a boat that will stand for more than a couple of months needs most of the same steps, especially fuel treatment and fogging. Ethanol-blended gasoline is the main reason. E10, approved by most outboard makers, absorbs water, and once it holds more than a small amount, the ethanol and water separate into a corrosive layer at the bottom of the tank. E15 is not approved for marine engines in the US. ## How to winterize an outboard motor: step by step Work through these steps in order. The engine needs to run for some of them, so do them with the motor in the water, in a test tank, or on flush muffs with the water supply on. - **Treat the fuel.** Fill the tank to limit air space (or nearly empty a portable tank), add a marine fuel stabilizer at the dose on the label, and run the engine for roughly 10 minutes so treated fuel reaches the pumps, vapor separator, injectors or carburetor. - **Flush the cooling system.** Run on fresh water through the flush port or muffs until the water stream is clean and the engine reaches operating temperature. This is essential after saltwater, brackish or silty use. - **Fog the engine.** On carbureted motors, spray fogging oil into the air intake at fast idle until the engine smokes heavily, then shut it down. On many fuel-injected four-strokes and direct-injection two-strokes, the manual instead calls for spraying fogging oil into each spark plug hole with the engine off, then turning the flywheel by hand. Follow the procedure for your model. - **Drain carburetors where fitted.** Carbureted motors can be run dry or have their float bowls drained, so no fuel varnishes in the jets. - **Change the engine oil and filter (four-stroke).** Warm the engine first so contaminants are suspended, then drain or extract the oil, replace the filter and refill to the dipstick level with the grade the maker specifies. Fresh oil keeps acidic combustion by-products off bearings during storage. - **Change the gearcase oil.** Remove the lower drain screw and upper vent screw, drain, check the oil and the magnetic drain plug, then refill from the bottom hole until oil comes out of the vent. - **Replace fuel filters and drain water separators.** Fit a new water-separating filter and check engine-mounted filters, as covered in our [outboard fuel filter and tank guide](/blog/outboard-fuel-system-filters-tanks/) . - **Grease and inspect.** Grease the swivel bracket, tilt tube, steering pivot and propeller shaft splines, and check the propeller for damage, fishing line and spun hubs. - **Check anodes.** Replace sacrificial anodes that are badly eroded, commonly once around half is gone. - **Protect the battery.** Disconnect, clean the terminals, charge fully and keep it on a maintainer or recharge periodically. - **Store upright.** Leave the motor vertical or tilted down so all water drains, apply a corrosion inhibitor spray to the powerhead, and fit a breathable cover. ## How do you winterize a two-stroke vs a four-stroke boat motor? The core steps are the same, but oil service and fogging differ. Two-strokes burn their oil with the fuel, so there is no crankcase oil change; four-strokes have a sump that must be changed. | Task | Two-stroke outboard | Four-stroke outboard | | --- | --- | --- | | Engine oil | No crankcase oil; top up the oil injection reservoir or use premix at the correct ratio | Change oil and filter before storage | | Fogging | Usually through the intake while running (carbureted) or per manual (direct injection) | Often through spark plug holes with engine off on EFI models; follow manual | | Fuel | Stabilize; premix fuel degrades, so avoid storing old mix | Stabilize; run to fill the fuel system | | Gear oil | Change and inspect | Change and inspect | | Cooling | Flush and drain | Flush and drain; check thermostat area for salt | For the background on the two engine types, see [two-stroke vs four-stroke outboards](/blog/two-stroke-vs-four-stroke-outboard/) . ## What does routine outboard motor maintenance include? Routine outboard maintenance has three layers: quick checks before and after every trip, an annual or 100-hour service, and less frequent major items. Intervals vary by maker and model, so use the owner's manual as the authority and the table below as a typical pattern. | Interval (typical) | Tasks | | --- | --- | | Before each use | Check engine oil level (four-stroke), fuel and lines for leaks, cut-off switch and lanyard, telltale water stream after start | | After each use | Flush with fresh water after salt or silty water, wipe down, tilt down to drain | | Break-in (first 10–20 hours on many models) | Initial inspection and, where specified, first oil and gear oil change | | Every 100 hours or annually | Engine oil and filter, gear oil, fuel and water-separating filters, spark plug inspection, grease points, anodes, propeller check | | Every 1–3 years or 100–300 hours | Water pump impeller replacement (many dealers do it annually as prevention), thermostat check | | Longer intervals | Timing belt (where fitted) and valve clearance checks on four-strokes, typically at several hundred to 1,000 hours depending on model | Rental and commercial fleets reach 100 hours far sooner than private owners, so they should plan servicing on engine hours rather than the calendar. Our [outboard and PWC fleet maintenance schedule](/blog/outboard-and-pwc-fleet-maintenance-schedule/) covers how to structure that. ## How do I do an outboard oil change correctly? A four-stroke outboard oil change works like a car oil change, with a few marine twists. Run the motor briefly to warm the oil, stand it vertical, and drain through the drain plug or extract through the dipstick tube with a pump. Then replace the oil filter, lightly oiling the new gasket, and refill with the oil grade and quantity in the manual, usually a marine four-stroke oil certified to the NMMA FC-W standard. Check the level after a short run and after the motor has stood for a few minutes. Record the date and engine hours, because warranty claims and resale value both depend on documented service. ## What should I check when recommissioning in spring? Recommissioning reverses the lay-up and catches anything that failed in storage. Most spring problems trace back to the fuel system or the battery. - Reconnect and load-test the battery. - Inspect fuel hoses, primer bulb and clamps; replace any that are cracked or stiff. - Check gear oil level and look for leaks at the lower unit. - Start on muffs or in the water and confirm a strong telltale stream within seconds. - Run at varied speeds before the first full-load trip, and check for alarms. ## When should a fleet replace an outboard instead of servicing it? Replace when repairs become frequent, parts become hard to source, or major components such as the powerhead or gearcase need rebuilding at a cost close to a new motor. Low compression, repeated overheating and severe corrosion are common triggers. MARS supplies replacement outboards from portables such as a [9.9 hp EFI four-stroke](/products/outboard-motors/df9-9bt-efi-4-stroke-portable-outboard-motor/) up to mid-range motors like the [60 hp EFI four-stroke](/products/outboard-motors/mercury-60-efi-fourstroke-outboard-motor-60-hp/) , and each motor is checked before export packing. If you are planning off-season fleet renewals, [send us your engine list](/contact/) . ## Frequently asked questions Do you need to winterize an outboard motor that is stored indoors? Yes, if the building can freeze or the motor will stand for months. Even in a heated space, fuel treatment, fogging and oil changes protect against fuel degradation and internal corrosion. How do you fog an outboard engine? On carbureted motors, spray fogging oil into the intake at fast idle until the engine smokes and stalls or is shut off. On many fuel-injected models, spray it into each spark plug hole with the engine off and turn the flywheel by hand, following the owner's manual. Should an outboard be stored tilted up or down? Store it vertical or tilted down so water drains from the cooling passages and exhaust. Storing tilted up can trap water that freezes or runs back into the powerhead. How often should outboard gear oil be changed? Typically every 100 hours or once a year, and always at winter lay-up. Milky oil indicates water intrusion that needs seal repair. How do you winterize an inboard outboard (sterndrive) boat motor? A sterndrive has a car-type engine block inside the boat, so its block, manifolds and any closed cooling system must be drained or filled with antifreeze as the manual specifies. This is more involved than an outboard, which drains itself when stored upright. M **MARS Engineering & Sales Team** Written by the engineering and sales team at Shanghai Mars Industrial Co., Ltd., who specify, inspect and ship marine and industrial power equipment from Shanghai every week. ## Share - [Share on LinkedIn](https://www.linkedin.com/sharing/share-offsite/?url=https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Foutboard-motor-maintenance-winterizing%2F) - [Share on WhatsApp](https://wa.me/?text=How+to+Winterize+an+Outboard+Motor%3A+Maintenance+Step-by-Step+https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Foutboard-motor-maintenance-winterizing%2F) - [Share by email](mailto:?subject=How+to+Winterize+an+Outboard+Motor%3A+Maintenance+Step-by-Step&body=https%3A%2F%2Fwww.marine-equipment-supply.com%2Fblog%2Foutboard-motor-maintenance-winterizing%2F) **Products mentioned** - [Mercury 60 EFI FourStroke Outboard Motor (60 HP)](/products/outboard-motors/mercury-60-efi-fourstroke-outboard-motor-60-hp/) - [Mercury 115 ELPT FourStroke Outboard Motor (115 HP)](/products/outboard-motors/mercury-115-elpt-fourstroke-outboard-motor-115-hp/) - [DF9.9BT EFI 4-Stroke Portable Outboard Motor](/products/outboard-motors/df9-9bt-efi-4-stroke-portable-outboard-motor/) - [Mercury 20 EFI FourStroke Portable Outboard Motor (20 HP)](/products/outboard-motors/mercury-20-efi-fourstroke-portable-outboard-motor-20-hp/) - [Fuel / Water Separator Filter (FS19517 / FS19599 / FS19591 / FS19735)](/products/engine-parts/fuel-water-separator-filter-fs19517-fs19599-fs19591-fs19735/) ## Need a quotation? Send your model, application and quantity — we reply within 24 hours on working days. [Get a quote](/contact/#inquiry) ## Products from this guide Mercury · Outboard Motors ### [Mercury 60 EFI FourStroke Outboard Motor (60 HP)](/products/outboard-motors/mercury-60-efi-fourstroke-outboard-motor-60-hp/) The Mercury 60 EFI is a mid-size 60 HP four-stroke gasoline outboard motor with electronic fuel injection… View details → Mercury · Outboard Motors ### [Mercury 115 ELPT FourStroke Outboard Motor (115 HP)](/products/outboard-motors/mercury-115-elpt-fourstroke-outboard-motor-115-hp/) The Mercury 115 ELPT is a 115 HP four-stroke gasoline outboard motor built for dependable marine… View details → Suzuki · Outboard Motors ### [DF9.9BT EFI 4-Stroke Portable Outboard Motor](/products/outboard-motors/df9-9bt-efi-4-stroke-portable-outboard-motor/) A compact, portable 9.9 HP four-stroke outboard with EFI and tiller-handle steering, ideal for tenders… View details → Mercury · Outboard Motors ### [Mercury 20 EFI FourStroke Portable Outboard Motor (20 HP)](/products/outboard-motors/mercury-20-efi-fourstroke-portable-outboard-motor-20-hp/) The Mercury 20 EFI is a compact, portable 20 HP four-stroke gasoline outboard motor with electronic fuel… View details → ## Keep reading [All articles](/blog/) Watercraft & Outboards · Oct 23, 2026 ### [Best Outboard Motor for Saltwater: Reliability Criteria to Check](/blog/best-outboard-for-saltwater-reliability/) The best outboard motor for saltwater combines strong corrosion protection, the right anodes, easy flushing and local service and parts. 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