Winterizing Your Outboard - Draining Cooling Systems

As outboard motors have become more technically advanced, the cooling motors have become more complex.  In the old 2-stroke days, cooling systems were pretty basic.  Water pumped through passages in the block and cylinder heads, then it was dumped back into the river or ocean.

Modern outboards, in addition to water flowing through the block and heads, cooling water is sent to cooling passages in the VST, fuel rails, rectifier/regulators, and additional oil cooling passages on the block.  Usually, the cooling water is routed through small diameter hoses between components.

After running a motor for the last time before freezing weather arrives, whether during winterizing or, if not winterizing, after the last time the motor is used, it is very important to allow enough time for all the water to drain out of all passages.  This is particularly important if a boat has been operated in muddy and sandy conditions where mud and sand may have entered the cooling system.  The mud and sand can collect in hoses and passages, restricting the flow of water as the system is draining.

To allow complete draining, the motor must be left in a vertical position long enough to allow all water to drain.  This is particularly important on models where sections of the cooling system may be lower than the water passages in the block and midsection when the motor is tilted up.  Also, if you suspect ingested mud or sand could be restricting water passages, it may be necessary to blow compressed air through sections of the cooling system to open restricted passages.  Also, tilting up a motor before it is completely drained may allow water to remain in the exhaust passage in the lower unit where it can freeze, cracking the lower unit housing.