Outboard Annual Maintenance & Winterization Checklists

Periodic maintenance is essential to the health and wellbeing of your outboard motor, especially when winterizing it for an extended period of off-season storage. 

The average outboard motor is run for about 100 hours per year, which is why annual maintenance is commonly known as 100-hour service. Even if you haven’t used your boat for 100 hours, you should still perform a complete annual maintenance service on the outboard(s). And if you did or do use your boat all year, it’s a good idea to do the 100-hour maintenance at least twice per year. Here’s our checklist for doing a comprehensive 100-hour maintenance/winterization service on your outboard motor(s).

Outboard annual maintenance and winterization

Annual Maintenance or 100-Hour Maintenance Checklist

Engine Oil. Drain and replace the engine oil (4-stroke motors) to ensure the motor is well lubricated and its moving parts encounter less wear.

Gearcase Oil. Drain and replace the lower unit/gearcase oil. As with the engine oil, new gearcase oil ensures proper lubrication and reduces wear to the transmission parts.

Outboard annual maintenance gearcase oil

Spark Plugs. Change the spark plugs. New plugs perform better and provide better combustion, stable engine temperature and improved fuel economy. 

Water Pump. Replace the impeller to prevent water pump failure. While replacing the impeller, go ahead and inspect the water pump housing/canister and the lower plate, and replace them if they show any signs of wear.

Outboard annual maintenance water pump impeller

Thermostat. It’s not necessary to replace a thermostat every year, but it’s also a part that can’t be repaired. Make note of when the thermostat was last changed, and consult your outboard’s owner manual to see the manufacturer’s recommendations on when to replace the thermostat. If it’s time or close enough to it to change out the thermostat, now would be a good time to take care of that. 

Propeller. Check the propeller blades for damage. Minor scrapes and blemishes can be repaired with a file, while serious dents and deformations will require the propeller to be replaced.

Outboard annual maintenance propeller

Grease Fittings. Re-pack any zerk/grease fittings with marine grease. Typically zerks are found on trim and tilt mechanisms and steering systems.

Cooling System. Flush the cooling system. Sand and silt buildup can cause blockages in the cooling system, so flush it thoroughly with fresh water. If you run your outboard in saltwater, flush the cooling system with salt remover to remove salt deposits that can cause corrosion, then flush the system with fresh water.

Outboard annual maintenance cooling system

Leak Down Test. This is often overlooked at the annual/100 hour service, but it’s an important test that tells you the condition of the engine’s cylinders.

Annual Winterization Checklist

Winterization is preparing an outboard for an extended period of inactivity, typically for winter storage. The winterization process should be applied to any period of two or more months without using an outboard.

Cooling System. Having flushed the cooling system with fresh water (and salt remover if it applies), flush the system with marine antifreeze. This prevents any moisture left in the system from freezing during the winter, which could damage the outboard and even crack the cylinder block.

Engine/Gearcase Oil. Always drain and replace both the engine and gearcase oils (4-stroke motors) even if they were recently changed, because any contaminants in used oil can corrode the engine.

Outboard winterization oil change

Fuel System. Add fuel stabilizer to the fuel tank and run the outboard for a couple of minutes to ensure stabilized fuel is drawn into the carbs or the fuel rails and injectors. Stabilizer prevents fuel from oxidizing and absorbing water molecules. Then fill the fuel tank to brim so there’s no room for condensation to form within the tank.

Outboard winterization fuel system

Cylinders. Spray fogging oil into each cylinder. This oil coats the inside of the cylinders and piston heads, protecting them against corrosion. With the spark plugs removed, now is the ideal time to replace them.

Powerhead. Coat the powerhead with an anti-corrosion spray or corrosion guard to form an oily barrier that protects all the metal surfaces.

Outboard winterization powerhead

Battery. Remove the battery from the boat and connect it to a battery tender. A tender keeps the battery well-preserved and prevents it from draining.

Propeller. Lightly coat the prop and prop shaft in marine grease to fend off corrosion.

External. Wash the cowling, driveshaft housing and lower unit, then give them at least two coats of marine wax. Touch up any scratches or chips in the paintwork and replace decals that are missing or faded.

Outboard engine winterization cowling

Cover. Finally, protect the outboard with a cover to protect it against condensation and prevent rodents from making their winter nests under the cowling.


Why Boats.net?

Commercial Discounts

Special discounts for companies in the marine industry


Government Sales

Discounts for federal and most state and municipal agencies