Preparing a MerCruiser® sterndrive-powered boat for the offseason or long-term storage is an important process that can help protect the engine and drive from damage. Some of the steps are intended specifically to prevent damage that can occur when offseason temperatures are likely to fall below freezing, which is why in those climates it’s common to say a boat has been “winterized.” Even if the boat is kept in a warm climate or heated storage, following the long-term storage guidance outlined in your MerCruiser operation and maintenance manual is important for maintaining performance and reliability. These storage steps usually include annual maintenance that should be completed even if the boat is not going to be stored for the offseason. In the long run, this maintenance can help ensure your engine and boat stay ready for more fun and adventure on the water.

By the Book

Prepping a MerCruiser sterndrive for long-term storage can be a rather involved process. For this reason, most MerCruiser owners choose to have a Mercury Authorized Dealer handle the entire process. For serious do-it-yourself owners, the MerCruiser operation and maintenance manual for your engine and drive will have a maintenance schedule and instructions on how to complete essential tasks. A copy of the Mercury service manual for your engine will have more detailed step-by-step maintenance instructions and is recommended as the best guide for a dedicated do-it-yourself owner. Service manuals for many Mercury engines can be ordered online or through a Mercury Authorized Dealer.

Here, we’ve outlined the basic steps for preparing a MerCruiser 4.5L/6.2L MPI model for storage. Consult the proper manual for detailed instructions for other MerCruiser models.

Annual Service Items

Start by completing the annual/100-hour maintenance schedule as outlined in the operation and maintenance manual. This will usually include the following:

• Change engine oil and filter before long-term storage, even if your seasonal use is not near 100 hours. During operation, engine oil accumulates acidic combustion byproducts that can be harmful to internal engine components if left in the engine during storage.
• Change the sterndrive gear lubricant and inspect the lubricant for water. Any water present could freeze and damage the gearcase casting. If water drains from the gearcase or the lubricant appears cloudy or milky, have the gearcase inspected by a professional technician.
• Replace the water-separating fuel filter.
• If your engine has a freshwater cooling system, check the coolant fluid level and concentration for freeze protection.
• Lubricate the propeller shaft splines and check the prop shaft for fishing line.
• Lubricate the engine coupler.

Additional items may be required at three years/300 hours and five years/500 hours. Tasks on the maintenance checklists labelled as a “dealer item” should be completed by a Mercury Authorized Dealer.

Stabilize Fuel

At the end of the season, try to run your boat fuel tank almost empty and then add fresh fuel (ethanol-free if available) treated with a marine stabilizer product such as Mercury Quickstor® Fuel Stabilizer.

If you have an older boat with a vented fuel tank, it’s a good idea to fill the tank with fresh stabilized fuel to keep condensation from forming in the tank during the offseason. Stop when the tank is about 95 percent full because extreme temperature changes over the winter can cause the fuel to expand, potentially forcing gas out of the vent. The fuel tanks on most newer boats can’t freely vent to the atmosphere and won’t collect moisture from the air. These tanks don’t need to be filled.

Special Fuel Mix

MerCruiser MPI systems must have a special mix of fuel, stabilizer and lubricant completely fill the fuel delivery system. This fuel mix stabilizes the fuel and lubricates the fuel pumps, fuel pressure regulator and fuel injectors, and it reduces internal oxidation of the fuel system metal components. Mercury recommends using a 6-gallon outboard motor remote fuel tank for this mix. To 5 gallons of gas in this tank add 2 quarts of Mercury Premium Plus 2‑Stroke Marine Engine Oil and 1 ounce of Quickstor Fuel Stabilizer. Follow the steps in the manual for your engine regarding adding this mix to the fuel system. MerCruiser engines equipped with older fuel-injection systems or a carburetor do not require this special fuel mixture.

Freeze Protection

Drain the engine seawater cooling system, following the instructions for your engine. Then fill the seawater cooling system with propylene glycol mixed to the manufacturer's recommendation to protect the engine to the lowest temperature to which it will be exposed. This will also prevent the formation of exfoliating rust in the cooling system passages. A dealer will immerse the outdrive in a large tank of antifreeze solution and then run the engine to circulate the antifreeze into the cooling system. A better option for the DIY owner is to use a gravity flow device that connects with a garden hose. It is very important to displace all water from the cooling system. If it freezes that water can cause significant damage.

Your MerCruiser powertrain is now prepared for the offseason and will be ready to go come spring. Always store the boat with the drive tilted down if the boat is kept outdoors to prevent rainwater or snow from collecting in the drive.

For more winterizing tips, see How to Store Your Boat for Winter.