How to Winterize a Sterndrive (MerCruiser, Volvo Penta and Similar)

Add marine fuel stabilizer to fresh fuel. If you have an older vented tank, fill it to about 95% to limit condensation; newer sealed tanks do not need to be filled.

Some fuel-injected MerCruiser models (MPI) call for a special storage fuel mix run from a portable tank. Follow your manual if yours does. Carbureted and older injected engines usually do not need this.

Attach flushing muffs over the drive's water intakes (some drives also need a side intake taped or covered; see your manual). Turn the water on fully, then start the engine and run it at fast idle until it reaches normal temperature so the thermostat opens. This also warms the oil.

Watch the temperature gauge the whole time. If it climbs, shut down.

Shut down and pump or drain the warm oil. Replace the filter (lightly oil the new gasket and make sure the old one came off) and refill to the level in your manual with marine oil. Old oil holds acids and moisture that attack bearings over winter, so do this even if you used the boat only a few hours.

Swap the filter, wipe up all spilled fuel, and check for leaks on the next run. Keep the blower running whenever fuel is open in the engine compartment.

Carbureted engines are usually fogged by spraying fogging oil into the carburetor at the end of the last run until the engine stalls or smokes heavily. Many fuel-injected engines should not be fogged through the intake; they use the storage fuel mix or another method. Follow your manual.

Shut the engine off. If the boat is in the water, close the seacock (if equipped). Then open every drain your manual lists. Common locations:

• Both sides of the engine block
• Each exhaust manifold
• Low points of raw-water hoses (and the hose to the seawater pump)
• Power steering or fuel cooler, and the heat exchanger on closed-cooled engines
• The seawater pump itself, on some engines

Poke a stiff wire into each drain hole to break up rust flakes; a clogged hole can look drained when it is not. Let it all drain until it stops completely. Tilt or trim the drive down.

Reinstall the drains. Then get propylene glycol through the entire raw-water path, from the pump all the way to the exhaust:

• Gravity-feed kit: fills the system from a hose connection without running the engine. Follow the kit and engine instructions.
• Bucket method: hook the muffs or raw-water intake hose to a bucket of antifreeze, start the engine and run it at idle until antifreeze comes out of the exhaust for 30 seconds or more. You need enough antifreeze; several gallons is typical. Catch what comes out so it does not go in the water.

The thermostat must be open (engine warm) so antifreeze reaches every passage.

Drain first, then add antifreeze. If you skip the drain and just run antifreeze through a block still full of water, the two mix and the protection can end up far weaker than the jug says. If you are unsure, catch a sample at the exhaust and check it with your tester.

With the engine cold, open the coolant cap slowly and test the mix with an antifreeze tester. It should protect below the coldest temperature you expect. If it is rusty, dirty, or past its change interval, replace it. Do not go richer than your manual says; too much concentrate actually protects worse.

Lower the drive, remove the bottom drain and top vent screws, and drain into a clean pan. Check the oil: creamy or milky lube or water drops mean a seal is leaking and needs a pro. Refill from the bottom until it flows out the vent. If your boat has a gear lube reservoir in the engine compartment, fill it to its mark. Full steps are in the lower unit oil guide.

• Remove the prop, clear any fishing line, grease the shaft and reinstall (torque per manual).
• Inspect the rubber bellows (boots) for cracks; cracked bellows let water into the boat and the gimbal bearing. Replacement is a pro job on most drives. While you are in there, look over the exposed hoses and clamps too; winter is the cheap time to replace them.
• Replace anodes that are about half gone.
• Grease the fittings your manual lists. Engine coupler lubrication is often listed in the annual service.

Charge it fully, disconnect the negative cable first, and store it cool and dry on a maintenance charger if you can.

Leave the drive trimmed all the way down so rain and snow cannot collect inside it. Mercury adds a second reason: a drive stored tilted up for months can permanently deform the rubber bellows, which then fail and let water in. If the boat stays in the water over winter, the drive should also be down so the anodes stay submerged and keep working. Hang a tag at the helm: "Winterized – antifreeze in engine" with the date. Ventilate the cover or shrink wrap so moisture does not build up.