To winterize a MerCruiser, add fuel stabilizer to the gas tank, and run the engine for one or two minutes. Fog the engine and carburetor according to the manufacturer's instructions. Remove the drain plugs from the raw-water section of the engine, and drain the water from the manifolds, heat exchanger, pump and block. Fill the raw-water side with propylene glycol antifreeze, and change the fuel and oil filters. Change the drive gear lubricant, and remove the batteries.
The fogging procedure varies by MerCruiser model. For older engines, run the engine at a high-speed idle, and pour a small amount fogging oil into the carburetor. Then, pour the oil in a thick stream until the engine chokes. If you have a new MerCruiser, refer to the user's manual for specific instructions.
Fill the tank with ethanol-free gas, and add stabilizer. Alternatively, drain the gas tank to a low level,and add stabilizer that is designed to protect the engine from ethanol.
Some MerCruiser engines feature an air-activating draining system. If your engine has one, connect the hoses and the included pump, and set the pressure to the correct level. Turn the key to crank the engine and remove the water.
After removing the batteries, clean the terminals. Replace the batteries if they are cracked or do not hold a charge.