How to Winterize a Closed Cooling Marine Engine

A closed cooling system protects your marine engine from internal saltwater corrosion, but it doesn’t make your boat winter-proof. If the raw water loop isn’t properly drained or antifreeze isn’t circulated through the right components, freezing temperatures can still crack heat exchangers, manifolds, risers, and water pumps. Winterizing a closed cooling engine is about protecting both systems — coolant and raw water — before the freeze sets in.

What Is a Closed Cooling System?

Closed systems circulate coolant through the engine block and heads. That coolant never leaves the internal loop. But raw water still flows through the heat exchanger, oil cooler, risers, manifolds, and sometimes the transmission cooler. That raw water loop is what needs to be drained or protected during winterization.

Steps to Winterize a Closed Cooling System

1. Change the engine oil and filter Dirty oil holds moisture and contaminants. Change it before layup so the engine sits with clean oil.
2. Check and top off engine coolant Use marine-rated antifreeze with corrosion inhibitors. The coolant loop should be freeze-resistant down to your local lowest temperatures. If in doubt, test it with a hydrometer.
3. Flush and drain the raw water side This includes the seawater strainer, raw water pump, and all hoses leading to the heat exchanger, manifolds, and risers. Open all drain plugs and low-point petcocks. If drains are clogged, remove hoses and clear them manually.
4. Run pink antifreeze through the raw water circuit Use marine non-toxic antifreeze. Either connect a hose to the raw water inlet or use a flush bucket. Run the engine (briefly and carefully) until antifreeze exits from the exhaust. This protects internal passages in the risers and exhaust elbows.
5. Fog the engine if applicable For carbureted or EFI engines with fogging ports, spray fogging oil into the intake to coat the cylinders and valves. This prevents rust from condensation during storage.
6. Inspect and replace pencil anodes Winter is the ideal time to check internal anodes. If they’re more than 50 percent gone, replace them. Never store the engine with bare metal exposed.
7. Disconnect and charge batteries Remove or disconnect batteries and store them on a maintainer indoors. Cold temperatures drain battery life and can cause freezing if discharged.

Don’t Forget These Critical Spots

• Drain plugs on manifolds and risers — often missed, often frozen
• Transmission coolers — these sit low and collect water
• Heat exchanger end caps — check for trapped water in the raw side
• Water lift mufflers — some designs hold water if the boat is not level

Should You Run Antifreeze Through the Closed Loop?

If the coolant is in good condition and tested for the correct freeze point, you don’t need to flush or replace it annually. Top it off with matching coolant and inspect the reservoir. If it’s dirty, rusty, or low on inhibitors, drain and refill the loop according to the manufacturer’s recommendations.

Conclusion

Winterizing a closed cooling marine engine isn’t complicated — but it has to be done thoroughly. Protect the coolant side with the right antifreeze, and flush or fill the raw water loop with marine antifreeze so nothing freezes or cracks. At our shop, we’ve rebuilt too many engines that froze simply because a single hose or drain plug was overlooked. Winter doesn’t forgive shortcuts. Do it right, and your engine will be ready when spring comes.