In order to winterize your sterndrive inboard-outboard boat you will begin by filling the fuel tank then treating with a fuel treatment like Star Tron, Star Brite Ez-Store / Ez-Start or Stabil marine fuel treatment. Many experts agree that new ethanol gas blends “suck” water vapor from any air in the fuel tank. By filling the tank completely you remove most of the air and water vapor from the tank. Also treating the fuel prevents the gas from gumming up the fuel lines and carburetor over time. This is a great time to replace any fuel filters you may have. This step will help ensure a smooth running engine for your spring commissioning!
Next you will want to flush the engine and bring it up to operating temperature and add antifreeze to the cooling system. You will need a set of engine flush muffs, a winterizing kit or a 5 gallon bucket with a bulkhead / spigot installed and short hose or garden hose to attach to the muffs. You will also need a can of fogging oil if you have a carbureted engine. See our post on making your own winterization bucket.
It is very important to bring the engine up to operating temperature while flushing! Otherwise antifreeze will not fill the block and result in freeze damage to the engine block!
Start by attaching the muffs over the inlet holes on the bottom of the sterndrive and attaching the garden hose. Remove the spark arrestor from the carburetor and set it in a safe location. Turn on the garden hose and verify good water flow to the outdrive. Once you have verified water is going to the outdrive start the engine and allow it to reach operating temperature. This is the temperature that the thermostat opens and allows water to circulate through the engine block. You may notice while watching the temperature gauge at the helm that the needle starts dropping, this is when the thermostat opens and lets cooler water circulate. Once the engine is at operational temperature disconnect the garden hose from the muffs and connect your winterizing kit or 5 gallon bucket to the muffs. Fill the kit or Bucket with marine antifreeze, place it higher than the engine, turn on the valve to let the antifreeze flow to the muffs and start the engine. You should see the antifreeze being “sucked” out of the container and through the cooling system. After seeing antifreeze coming out of the boats exhaust system for 20-30 seconds you can begin to fog the carburetor. To fog the engine spay the fogging oil directly into the carburetor until the engine bogs down and stops. (some high horse power engines will not stop and will sputter and miss instead.) Fuel injected engines typically do not need to be fogged. In either case check your owners manual to see specific instructions. Replace the spark arrestor.
I recommend using marine antifreeze that is rated for -100 degrees F and has anti corrosion protection. Some people like to use a cheaper antifreeze that has -50 or -25 degree protection but regardless of how you fill and flush the engine there is always a possibility of a water pocket in the cooling system that will dilute the antifreeze you put into the engine. By using -100 degree antifreeze you are protecting yourself from minor dilution.
We will continue the winterization process in part 2 discussing how to change the engine oil and what tools are needed.