How to Winterize a 4-Stroke Outboard Engine

The end of the boating season means it's time to winterize your boat's outboard motor(s). As soon as your outboard is done running for the year, winterizing is a necessary maintenance process to protect it against corrosion while in storage.

 Watch the video above or follow the steps below to winterize a 4-stroke outboard motor.

Tools and Parts - Four-stroke Outboard Winterization

  • Ratchet and extensions
  • Socket set
  • Screwdrivers
  • Torque wrench


Mercury 4-stroke engine oil

Mercury outboard gearcase lube

Winterizing a 4-Stroke Outboard: Oils and Lubricants

The first steps in the winterization process are just part of your outboard's regular maintenance schedule: changing the lower unit and engine oils.

4-stroke outboard winterization supplies

Step 1. Drain and replace the oil in the lower unit. 

How to winterize a 4-stroke outboard engine

Watch our outboard lower unit oil change video below to see how it's done.

Step 2. Drain and replace the oil in the powerhead, and replace the oil filter.

How to winterize a 4-stroke outboard motor

Watch our outboard engine oil change video below to see how it's done.

Fuel oxidizes and goes stale when left sitting for weeks or months at a time. The methanol content within the fuel also attracts water molecules, which causes corrosion. Adding fuel stabilizer prevents the fuel from breaking down and going stale. The fuel stabilizer also absorbs water molecules attracted by the methanol content.

Winterizing a 4-Stroke Outboard: Fuel System

Step 1. Add fuel stabilizer to the tank, then run the motor for a minute to draw the stabilized fuel through all the fuel lines and into the fuel system.

How to winterize a 4-stroke outboard fuel stabilizer

NOTE: Follow the manufacturer's guidelines on the packaging to prevent over or under-saturating the fuel.

 Step 2. Fill the fuel tank to the brim to prevent condensation from forming within it.

Mercury Quickstor fuel stabilizer

Mercury outboard fogging oil

Any exposed metal surfaces on an outboard are prone to corrosion while the motor isn't in use. This includes the piston heads and cylinder walls within the combustion chambers. Spraying them with fogging oil provides a protective coating over them.

Winterizing a 4-Stroke Outboard: Combustion Chambers

Step 1. Remove the spark plugs, and spray the fogging oil directly into the engine's combustion chambers. Spray the oil for a couple of seconds into each spark plug hole.

How to winterize a 4-stroke outboard spark plugs

How to winterize a 4-stroke outboard engine spark plugs

How to winterize a 4-stroke outboard motor spark plugs

How to winterize a 4-stroke outboard motor fogging oil

Step 2. Install the spark plugs back into the engine, and torque them to the manufacturer's specs provided in the outboard's owner's manual. 

4-stroke outboard winterization

PRO TIP: Now is a great time to replace the spark plugs with new ones as part of the outboard's routine maintenance schedule.

4-stroke outboard engine winterization

4-stroke outboard motor winterization

Having protected the inside of the engine against corrosion, it's now time to protect the outside of the motor. Spraying the powerhead with corrosion guard provides a thin oil coating that protects any external metal parts or surfaces from rusting.

Winterizing a 4-Stroke Outboard: Powerhead

Step 1. Spray corrosion guard on every bit of the outboard, with the exception of any belts, anodes and zincs, which should not be sprayed.

Winterize a 4-stroke outboard corrosion guard spray

The battery might not be part of the outboard, but if you don't winterize it too, it'll be completely dead when it comes time to use it again. Batteries need to be cycled (the repeated process of charging and discharging) to remain healthy. A battery left idle for months loses its charge and can be completely ruined.

Winterizing a 4-Stroke Outboard: Battery

Step 1. Remove the battery from the boat, clean the terminals and protect them with a coating of dielectric grease.

Step 2. Preserve the battery with a battery tender. The tender will keep a battery's power level optimized for months on end, so it'll still be in perfect health when boating season rolls around again.

Mercury corrosion guard spray

Battery Tender Plus

Got a 2-stroke outboard instead of a 4-stroke? The winterization process for an outboard is the same or similar for all outboard engine types, but the video above is specific to 2-stroke engines. Watch the video below to see how to winterize a 2-stroke Mercury outboard. 




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