How to Winterize a Boat Engine – Oil, Fuel, and Antifreeze Guide

Winter is tough on boats, especially here on the West Coast where freezing temperatures and damp conditions can wreak havoc on your motor. If you don’t winterize your engine properly, you risk corrosion, cracked blocks, stale fuel, and costly spring repairs. The good news? With a few key steps and the right products, you can protect your investment and ensure a smooth start-up next season.

This guide covers everything you need to know about winterizing your boat engine — from changing the oil to treating fuel and flushing with antifreeze.

Step 1: Change Your Engine Oil and Filter

Old engine oil holds contaminants that can cause internal corrosion if left sitting all winter. Performing an oil change before storage gives your engine a clean slate.

• Run the engine briefly to warm it up.

• Drain the old oil and replace it with fresh, marine-grade oil.

• Swap in a new oil filter to ensure proper protection.

Step 2: Stabilize the Fuel System

Fuel that sits for months can break down, forming varnish and gum that clog injectors and carburetors. Moisture in the tank can also cause phase separation in ethanol-blended fuels.

• Add a marine fuel stabilizer to your fuel tank.

• Top off the tank to reduce condensation.

• Run the engine for 10–15 minutes to circulate treated fuel throughout the system.

Step 3: Fog the Engine (Optional but Recommended)

Fogging oil coats internal components with a protective layer that prevents rust and corrosion during long storage.

• With the engine running, spray fogging oil into the air intake until it begins to smoke.

• Shut down the engine and spray directly into each cylinder for maximum coverage.

Step 4: Flush and Protect with Marine Antifreeze

Freezing water in your engine or cooling system can cause cracks and serious damage. Protect your system with non-toxic marine antifreeze.

• Connect a flush kit or use a pump system to circulate antifreeze through the cooling passages.

• Run until antifreeze comes out of the exhaust — this ensures the entire system is filled.

• Use only propylene glycol marine antifreeze (pink or -50°C to -100°C formulations).

Step 5: Final Checks Before Storage

• Check belts and hoses for wear.

• Grease fittings and moving parts.

• Disconnect the battery or connect it to a maintainer.

These small steps make sure your engine stays in top shape while your boat is laid up.

Conclusion

Winterizing your boat engine may seem like extra work, but it’s one of the most important maintenance tasks of the year. With fresh oil, stabilized fuel, fogging protection, and antifreeze in your cooling system, your motor will be ready to fire up smoothly when boating season returns.