Gone Outdoors

How to Winterize a 305 Boat Engine

by Will Charpentier

Mercury Marine's 5.0-liter Mercruiser engine is a Chevrolet 305 engine--the same block used in Chevrolet trucks, with many of the same parts, such as the water pump, and many of the same maintenance practices. The biggest difference is that you "winterize" a boat engine when you lay your boat up at the end of the season. When you winterize a boat engine, you're preparing it for its "winter nap" during the non-boating part of the year.

1. Set an oil recovery pan beneath the engine oil drain plug. Remove the engine oil drain plug with an adjustable wrench and drain the engine oil into the oil recovery pan. Wrap a strap wrench around the engine oil filter after the oil has drained completely and turn the filter counterclockwise to remove it. Thread a new oil filter onto the receiver for the oil filter and tighten the filter hand-tight by turning it clockwise.

2. Unscrew the locking ring at the top of the clear plastic fuel/water separating filter. Remove the filter element from the clear plastic bowl and dump the bowl into the oil recovery pan. Set a new filter core in place, fill the plastic bowl with fuel, and replace the clear plastic bowl on the separator, threading the locking ring onto the bowl. Replace the old engine oil with new oil.

3. Set additional oil collection pans under the coolant drains on the block and beneath the drains at the bottom of the exhaust manifolds. Open the drains by removing the plugs from the drains with a screwdriver and allow the condensation to drain from the block and exhaust manifolds.

4. Replace the drain plugs, disconnect the hoses that run from the water pump to the thermostat housing. Unscrew the thermostat housing from the cooling system.

5. Slowly pour low-toxicity antifreeze with rust inhibitors into the raw water inlet. The system capacity may vary slightly between model years, but is specified in the motor's owner's manual. Add fuel stabilizer to your fuel tank and start the motor. Run the motor until a mist of antifreeze comes out of the cooling system where the thermostat housing has been removed. Shut down the motor. Screw a new thermostat housing into place and reconnect the thermostat hoses. Fog the surface of the engine with marine fogging oil, available at boat-supply houses.

Items you will need
  • Oil recovery pans
  • Adjustable wrench
  • Strap wrench
  • Oil filter
  • Fuel/water separator filter element
  • Fuel
  • Motor oil
  • Screwdriver
  • Low-toxicity antifreeze with rust inhibitors
  • Fuel stabilizer
  • Thermostat housing
  • Marine fogging oil

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