One of the most important steps in extending the trouble-free life of your Mercruiser engine is to be sure that it has been properly serviced and prepared for cold weather during the winter. Water left in the engine can cause major damage to the engine block and the manifold system. Below are some things you should do each winter. The importance of a service manual cannot be overstated. The Mercruiser's manual has a very good and detailed section on winterization procedures. We strongly recommend that your MerCruiser is winterized by an authorized MerCruiser dealer. Damage caused by freezing or improper winterization is NOT covered by the limited factory warranty

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1. The cooling system is the main area of concern.  All inboard and sterndrive engines have petcocks that are either turned or removed in order to drain water from the block, exhaust manifolds, and hoses. These petcocks can become clogged with debris. You can poke a wire into the drains to remove the debris and allow water to drain. Temporarily remove the water hoses that feed the block and manifolds, and pour antifreeze (red RV is recommended) into them until it runs out of the petcocks.
   
   The drain system on your engine is NOT intended to replace full winterization. It is for extending your boating season by allowing the cooling system to be drained after use if there is a risk of light freezing weather and you will be boating the next day. For full extended winter storage, after completely draining the cooling system, Mercury MerCruiser recommends the use of propylene glycol antifreeze (with a rust inhibitor for marine engines) in the sea water section of the cooling system.
    
2. The engine should be fogged to ensure that the engine's inner workings are not exposed to condensation and rust over the winter. Most carburetor engines can be fogged by slowly pouring a little 20-weight oil or engine storage fogging oil down the throat of the carburetor while the engine runs at a fast idle, then rapidly pouring the rest down the throat to "choke" off the engine.  On newer engines, follow the service manual. Typically, it involves pouring a small amount of outboard oil into the water separating fuel filter, then running the engine to let it ingest the oil. After the fogging is done, change the engine oil and filter
    
3. The fuel system must have fuel stabilizer added for proper winterization.  The process of conditioning the fuel is the same for any internal combustion engine. Add plenty of fuel stabilizer so that you will not be  leaving gummy varnish in the carburetors, EFI injectors and the rest of the fuel system by the time winter is over. Too much leaves a bigger dent in your wallet when you purchase it, but saves big repair bills later. If the boat has a water-separating fuel filter, change the canister now before you start treating the fuel. This will ensure that any water in the system is removed. The conditioner is mixed with the fuel in the boat's tank before it's run for the last time, so that treated fuel ends up throughout the fuel system, not just in the tank.
    
4. The drive gear lubricant must be changed. This will ensure that there is no water in the lower unit that can freeze and cause serious damage. To ensure that the winterization has been done properly, the outdrive should be removed so that the universal joint can be greased and the gimbal bearing checked for wear. Most boaters will leave this to the dealer, and it can be done every second season to save a bit of cash .
    
5. The drive should be touched up if paint is missing, and the speedometer  and cooling ports must be probed with a fine wire to be sure there's no water trapped inside. This is also a good time to check the prop; if it's damaged, send it out for refinishing while the boat is being stored for the winter.
    
6. Lastly, coat the electrical system and engine block with a fine mist of silicone spray. This will keep everything protected during the winter.