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Inboard Motor Winterizing

Winterizing your inboard motor is an important part of protecting your boat during the winter months. With a properly winterized engine, your fear during the cold February nights will be put at ease.

The Why of Winterizing

Cold temperatures can wreak havoc on inboard motors. Residual water can freeze, causing engine block cracks. Additionally, idle periods can allow fuel to degrade or internal components to corrode. Winterization mitigates these risks.

Step 1: Fill the Fuel Tank

Fill your fuel tanks. Fuel tanks that are not full allow for condensation to form on the tank walls during the winter months, and this condensation will introduce water into your fuel supply. We recommend the fuel tanks be kept at least 3/4 of the way full.

Step 2: Fuel Stabilizer:

Once the tanks are filled, add fuel stabilizer to your gasoline or diesel fuel tank. The amount added is based on the tank size, so be sure to read the directions and add the proper amount. Also be sure to use the proper fuel stabilizer for your fuel type, as gasoline and diesel fuel stabilizers are very different in both their chemical makeup and effect.

Gasoline: Fuel stabilizer prevents gumming or varnishing of the fuel through the engine system, prevents rust by displacing water, and helps prevent the separation of ethanol in the gasoline. Fuel stabilizer, such as Sta-Bil, is compatible with both ethanol and non-ethanol gasoline, and stays effective for roughly 2 years.

Diesel: Diesel stabilizer works primary by preventing bacterial and algae growth in your fuel tank. These growths, if not properly prevented, will cause sludge to form in you fuel tank, which will clog your filters and cause major issues while boating. These stabilizers also work by displacing water and provide additional lubrication of the engine components during the winter months.

Step 2: Engine Flushing

Once the fuel system is stabilized, we recommend running the engine for an extended period of time, using freshwater to flush the cooling system. By running the engine, the stabilized fuel will have the chance to run through the fuel system and also allow for easy removal of the engine oil.

Step 3: Oil Change

Old oil can accumulate moisture and contaminants and will become acidic after a season of use. Before storing:

  • Drain Old Oil: Run the engine to warm it up, then drain. If a drain pump is not accessible, a vacuum oil pump can be used through the dip stick to remove the oil from the engine block.

  • Replace Filter: Always pair an oil change with a new filter. We date the filters with a permanent marker to prevent any future confusion.

  • Add Fresh Oil: Refill with fresh oil up to the recommended level. Be sure to use the oil weight and type specified by the manufacturer to insure proper operation of the motor.

Step 4: Engine Fogging (Gasoline Only)

Once your engine has been thoroughly flushed with antifreeze and up to temperature, we recommend using a fogging spray to protect your engine internals. Fogging spray works by coating the inside of your engine and cylinders with a thin film of oil, prevent moisture in the air from causing corrosion.

  • Carbureted Engines: With the engine running, spray directly into the carburetor until the engine stalls. This ensures the oil reaches and coats internal components.

  • Fuel-Injected Engines: For these, access the spark plugs and spray directly into the cylinders.

Step 5: Cooling System Protection

  • Drain and Protect: Completely drain the water from the cooling system. MerCruiser Engines have blue drain plugs on the block that allow for all water to be drained. Afterward, circulate antifreeze designed for marine engines. Draining of the block is a required, as antifreeze may dilute with the existing water in the engine block. The antifreeze will additionally help with internal engine corrosion.

  • Diesel motors do not have drain plugs on their engine block. We recommended to flush the block with antifreeze by feeding the antifreeze into the water strainer while the motor is running. The water pump will consume the antifreeze quickly, so be sure to have plenty of antifreeze on hand and opened to prevent any airlocks from forming in your motors cooling system.

Step 6: Checking and Replacing Antifreeze

Make sure the antifreeze in your boat’s system is fresh and properly mixed. At Sea and Land Yacht Works, we also test your antifreeze by using a tester to check its Specific Gravity. Over time, antifreeze in your engine can become acidic, and pickup rust and scale in your cooling system. These containments will decrease the antifreeze freezing and boiling range, potentially causing winter freezing issues, and during boat use, cause the engine to run hot or overheat. Engine manufacturers often list the recommended lifespan of antifreeze in their engines, and we recommend strictly following those timelines to prevent damage and issues to your engine.

Step 7: Check Belts and Hoses

Inspect hoses for cracks or signs of wear and make note to ensure they are fixed before the springs boating season. The winter will give you plenty of time to make sure you have these parts before arriving back to your boat for springs commissioning.

Step 8: A look at the Lower Unit

  • Drain and Refill: Drain the gear oil from the lower unit, looking for any milky substance (indicative of water intrusion). Refill with fresh gear oil.

Step 9: General Protection

  • Mufflers: Drain any water from exhaust mufflers.

  • Propeller: Examine the propeller for damages and make necessary replacements.

  • Cutlass Bearing: If the motor is shaft driven, now is a good time to check your cutlass bearing for play. Any movement necessitates replacement.


Winterizing your inboard gasoline motor or diesel requires is an extremely important process to prolong the life span of your motor, and avoid costly repairs. After all, as any seasoned boater will tell you, a well-maintained boat is a happy boat!


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