How to Winterize a Closed System in a Mercruiser

Look up the coolant change intervals, and the type and amount of coolant required, in your engine's manual. Some newer Mercruiser Horizon engines only need a coolant change every five years. Other models need new coolant in the closed-loop side every year or two.

Drain the closed side of the cooling system if you will be changing the coolant this year. If the drain plug is hard to reach, disconnect one of the coolant hoses from the water pump. Several gallons of coolant will gush out; have a couple of buckets ready to catch it.

Changing the coolant ahead of schedule is a good idea if you've been using the boat a lot. Some boaters change it every year; Mercruiser allows up to five years for a Horizon engine in typical light-duty use.

Open all the cooling system seacocks, with the boat out of the water, to drain the raw water side of the system. Remove the raw water drain plug, if your engine has one, from the bottom of the heat exchanger. Open the raw water strainers and remove any debris that has accumulated there.

Disconnect the coolant and raw water hoses from the heat exchanger. Check the inside of the heat exchanger for rust or debris buildup. If it's clogged with rust, order a new heat exchanger. If it's clogged with debris and you can't easily get it clean, take the exchanger to a marine mechanic for inspection and cleaning. If the heat exchanger looks OK, reconnect its hoses.

Open the cover of the raw water pump and check that the rubber impeller is intact. If it looks dry or cracked, replace it. If the impeller is missing blades, their remains are probably stuck in the heat exchanger—check it again for debris, then replace the impeller. Do the same for the closed cooling system pump.

Reconnect the closed cooling system hose or reinstall the drain plug, then fill the coolant reservoir with fresh coolant. Mercruiser Horizon engines require a special long-life coolant; some other Mercruiser engines can take ordinary car engine antifreeze mixed 50:50 with water.

Fill a five-gallon bucket with a 50:50 mix of water and non-toxic propylene glycol plumbing antifreeze. Close the raw water seacock, disconnect the raw water intake hose from its strainer, and stick the hose in the bucket of antifreeze. Have someone run the engine for a few seconds until you see antifreeze starting to trickle out the exhaust pipe, then shut it off. The antifreeze will protect the raw water side of the heat exchanger from damage in below-freezing weather.

Drain any remaining water or antifreeze from the water-lift muffler, if your engine has one. Continue with the rest of the winterizing procedures, such as oil changes, as you would for any other engine.