How to Winterize a MerCruiser 5.7

Written by david curtis
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How to Winterize a MerCruiser 5.7
Performing winterisation tasks on a Mercruiser 5.7 engine (speed boat image by sheldon gardner from

Winterising a Mercury marine engine is necessary to ensure long life and peak performance of vital engine parts. Freezing temperatures can easily crack an engine block if water is left trapped inside the engine. Routine fall maintenance will have your boat ready to use in the spring. You can purchase necessary supplies through your local marine products supplier. Do it yourself or have the service performed by a marine mechanic.

Skill level:
Moderately Challenging

Things you need

  • Open-end wrench
  • Socket set
  • Flat screwdriver
  • Fogging oil
  • 2 containers
  • Cleaning solvent
  • Compressed air
  • MercRuiser starting tool
  • 2 gallons of RV antifreeze
  • Bearing grease
  • Propeller tool
  • Motor oil and gear lube if necessary

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  1. 1

    Park the boat and trailer on a flat surface. Raise the bow slightly higher than the stern with the trailer jack. Place a large container under the drain plug at the stern and remove the plug. This will capture water draining from the bilge area.

  2. 2

    Pour fuel stabiliser into the fuel tank and fill it with fresh gasoline. This will minimise tank condensation.

  3. 3

    Remove the engine compartment cover and remove the spark arrester with a wrench. Clean the arrester by placing it in a flat container with enough cleaning solvent to cover the air fins. Allow it to soak for 15 minutes and remove it. Spray it with compressed air to dry.

  4. 4

    Lower the stern drive to the operating position. Place a Mercury engine starting tool over the intake ports just above the propeller cone. Attach a garden hose and turn on the water. Start the engine and allow it to warm up to operating temperature. Spray fogging oil directly into the carburettor opening until the engine begins to stall and turn off the engine. Turn off the water and remove the tool.

  5. 5

    Locate the drain plugs on each side of the engine. A mirror and flashlight are helpful in finding the plugs. Older models use brass plugs and later versions use twist-off connectors. Open the drain plugs on older models with a socket wrench and remove the plastic drain plugs by hand on later versions. Allow all of the water to drain from the block. Remove the drain plugs on the exhaust manifolds and allow the water to drain. Replace the drain plugs when all the water has been removed.

  6. 6

    Remove the clamps connecting the upper end of the water hoses with a flat screwdriver. Remove the hoses and fill them with RV antifreeze. Reattach the hoses.

  7. 7

    Remove the dipstick and check the engine oil level. Change or fill the oil to the proper level as needed. Reinstall the spark arrester.

  8. 8

    Remove the oil fill plug on the port side of the propeller cone of the stern drive. Check the lubricant for cleanliness. Remove the vent plug and fill to proper levels if the fluid is low. Check and make sure the lubricant is the proper colour and consistency. Mercury Marine oil is dark blue in colour, with most other brands dark brown. Drain and replace the stern drive lubricant if it appears dirty. Replace the fill and vent plugs.

  9. 9

    Remove the propeller with a prop tool or adjustable wrench and lube the shaft splines with bearing grease. Replace the propeller on the shaft. Leave the drive in the downward position for storage.

  10. 10

    Remove the negative terminal from the battery and give it a full charge.

Tips and warnings

  • If the stern drive lubricant is a milky colour, contact a marine mechanic immediately. This is an indication that water has entered and contaminated the system. Storing the stern drive with contaminated lubricant will severely damage the gears in the drive unit.

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