How to Winterize Your Inboard Boat Engine Like a Pro

When boating season ends, your inboard engine is left with water in the cooling system, moisture in the oil, and fuel in the tank. If ignored over the winter, these can cause expensive damage. Freezing water can crack the block or manifolds, acids in old oil can corrode bearings, and untreated fuel can separate and damage injectors or carburetors.

Winterizing removes or protects these fluids, coats internal parts against corrosion, and keeps the engine ready to run in spring.

Note: Expect to spend 2–3 hours and $50–$150 in materials. In return, you get peace of mind knowing your engine is safe from freeze cracks, rust, and fuel problems, no matter how brutal the winter.

Why Winterize Your Inboard Boat Engine?

When an engine is left full of water, old oil, and untreated fuel, the conditions are perfect for freezing, corrosion, and chemical breakdown. Often, these issues remain silent during storage, manifesting only when you attempt to start the engine in the spring.

Here are the main risks you eliminate by winterizing properly:

Freeze Damage: Water left inside the cooling passages will freeze when temperatures drop. As it turns to ice, it expands and can crack the engine block, manifolds, risers, or coolers. Even one night of hard freezing can cause damage that costs thousands to repair.
Corrosion Protection: Old oil contains moisture and acids that slowly eat away at internal metal surfaces like bearings, journals, and cylinder walls. Engines that have been in salt water are especially vulnerable because salt deposits can keep attracting moisture.
Fuel System Preservation: Ethanol-blended gasoline (E10) absorbs water from the air in the tank. Over time, this can cause phase separation, where a layer of ethanol-water mix settles at the bottom. This corrosive layer is often the first thing drawn into the engine at spring startup, damaging injectors, carburetors, and pumps.
Condensation Control: As temperatures rise and fall, condensation forms inside the fuel tank, cooling system, and gearcase. This repeated moisture exposure leads to rust, pitting, and varnish buildup that can shorten the life of many components.

Essential Winterization Supplies You'll Need

Having the right materials on hand before you begin will make the process faster, cleaner, and more effective.

Each item plays a specific role in protecting your engine through months of storage:

Marine-Grade Antifreeze: Made with USP-grade propylene glycol, this antifreeze protects against freezing while also leaving corrosion inhibitors inside cooling passages. Choose –50°F for milder climates or –100°F for harsh winters. Most inboard engines need 2–4 gallons, depending on size and whether they have a closed or raw-water cooling system.
Fogging Oil: A corrosion-blocking spray used to coat internal engine surfaces, such as cylinder walls and valves, with a protective film. Products like Star Brite Professional Grade Fogging Oil cost around $10 per can and help prevent rust from forming while the engine sits.
Fuel Stabilizer: Keeps gasoline fresh, prevents varnish buildup, and reduces the risk of phase separation in ethanol-blended fuels. Add the stabilizer to the tank before your final run of the season so the treated fuel circulates through the system. Popular options like STA-BIL cost under $15.
Fresh Engine Oil and Filter: Used oil holds moisture and contaminants that can corrode engine parts during storage. Changing the oil and filter after your last outing removes these harmful deposits. Expect to spend $15–$30 depending on your engine’s requirements.
Basic Tools and Setup: A 5-gallon bucket, short length of garden hose, hand tools for removing drains and clamps, and safety gear such as gloves and eye protection. These make it easier to control antifreeze flow, open drains, and work safely.

Starting with the correct supplies ensures each winterization step works as intended and helps you avoid missing a critical task.

Step-by-Step Inboard Engine Winterization

Step 1: Add Fuel Stabilizer and Fill the Tank

Gasoline starts to break down within weeks, and ethanol-blended fuel (E10) can absorb moisture from the air. Over winter, this can cause phase separation, leaving a corrosive layer that damages injectors, carburetors, and pumps. Treating the fuel before storage keeps it stable and prevents these problems.

Add marine fuel stabilizer to the tank, following the manufacturer’s instructions (commonly about 1 ounce per gallon).
Fill the tank to about 95% full. This reduces the empty space above the fuel, limiting the amount of moist air that can enter and condense, while leaving room for thermal expansion.
Run the engine for 5–10 minutes at normal operating temperature. This ensures stabilized fuel reaches every part of the system, including fuel lines, injectors, and carburetor passages.

Pro Tip: Always add stabilizer before your final run of the season. That way, the treated fuel works through the system under load, not just at idle.

Step 2: Warm Up the Engine

Before you can drain the cooling system, the thermostat needs to be open so water can flow freely out of all passages.

Running the engine to operating temperature also finishes circulating the fuel stabilizer from Step 1 through the entire fuel system.

Start the engine and let it idle until it reaches normal operating temperature. Most engines will be ready in 5–10 minutes.
Confirm the thermostat is open. You’ll know because the temperature gauge will climb to its normal range and stay steady, and water will be moving freely through the exhaust.
Once warm, shut the engine down and prepare to drain the cooling system.

Why this matters: If you try to drain the engine while the thermostat is closed, some cooling passages will still hold water. That trapped water can freeze and cause cracks in the block, manifolds, or risers.

Step 3: Flush the Cooling System

Before you drain the engine, it’s important to rinse out any debris, silt, or salt left inside the cooling passages. Flushing ensures that contaminants aren’t left behind to cause corrosion during storage.

This step is essential for saltwater boats, where salt deposits can rapidly eat away at internal metal surfaces.

So, connect a garden hose to the raw-water intake. This may involve attaching a flushing adapter or connecting directly to the sea strainer. Next, start the engine and let it idle while fresh water flows through the system. Keep an eye on the exhaust. The water should run clear, without sediment or discoloration.

Flush for at least 5–10 minutes to ensure the fresh water has completely replaced the old water in the system. Then finally shut down the engine and get ready to drain the cooling system.

Pro Tip: If your boat has been in salt water, consider using a marine engine flush solution in the hose water. These products help dissolve salt deposits more thoroughly than water alone.

Step 4: Drain the Cooling System

Once the system has been flushed, the next step is to remove all remaining water from the engine and exhaust components.

Any water left behind can freeze, expand, and crack the block, manifolds, risers, or coolers.

Locate all drains. These may be petcocks or threaded plugs on the block, exhaust manifolds, heat exchanger (if equipped), oil cooler, and transmission cooler. Your engine manual will show the exact locations.
Open each drain and allow water to flow out freely. Keep a small bucket or pan under each opening to collect the water.
Clear any clogs if water does not drain. Rust, scale, or debris can block passages. Remove the petcock or plug and gently clear the opening with a small tool, stiff wire, or plastic zip tie.
Check for hidden pockets. Some engines have low spots in hoses or coolers where water collects. Tilt the boat or disconnect hoses if necessary to fully drain these areas.
Use a vacuum pump or low-pressure compressed air to help clear stubborn pockets of water. Work gently to avoid damaging hoses or fittings.

Tip: Keep track of all drain plugs and petcocks you remove. Place them in a labeled container so they can be reinstalled before adding antifreeze in the next steps.

Step 5: Add Marine Antifreeze

Draining the cooling system removes most of the water, but small amounts can still remain in low spots, hoses, or components you can’t fully empty. If that trapped water freezes, it can still crack a block or manifold. Circulating marine antifreeze through the system pushes out any remaining water, fills every passage, and leaves behind corrosion inhibitors that protect metal surfaces during storage.

The most reliable way to do this on an inboard engine is the bucket method:

Prepare the antifreeze. Use undiluted, non-toxic marine antifreeze (propylene glycol) with a freeze protection rating that matches your climate, typically –50°F for mild winters or –100°F for severe conditions. A standard inboard will use 2–5 gallons, so have extra ready.
Shut off the water intake. If your boat is still in the water, close the intake seacock to prevent backflow.
Disconnect the raw-water intake hose from the seacock and place it into a clean 5-gallon bucket filled with antifreeze. Make sure the hose stays submerged to avoid drawing in air.
Start the engine and let it idle. The raw-water pump will pull antifreeze through the entire cooling system.
Watch the exhaust outlets. When you see a steady stream of solid antifreeze color (usually pink) coming from both sides for at least 30 seconds, you know the system is fully protected.
Top up the bucket as needed so the pump never runs dry. Drawing in air can leave parts of the system unprotected.

Once you’ve confirmed antifreeze is flowing cleanly from the exhaust, shut down the engine. At this point, every part of the raw-water side is protected from freezing and corrosion until spring.

Step 6: Apply Fogging Oil

With the cooling system protected, the next priority is to prevent corrosion inside the engine. Fogging oil coats cylinder walls, piston rings, and valve components with a protective film that stays in place for months.

Use the carburetor intake method. When your antifreeze bucket from Step 5 is nearly empty, start spraying fogging oil into the carburetor air intake. Keep spraying until you see thick white smoke from the exhaust.

Lower-horsepower engines may stall at this point; higher-horsepower models might sputter but keep running. Once you’ve sprayed a generous amount, shut down the engine.

Alternatively, use the spark plug method. Remove each spark plug, spray fogging oil directly into the combustion chamber for several seconds, then reinstall the plugs. This approach gives more targeted coverage but takes extra time.

Either method works; the carburetor intake method is faster, while the spark plug method allows a more even coat inside each cylinder.

Step 7: Change Engine Oil and Filter

Why do this now, instead of waiting until spring? Because the old, dirty oil contains contaminants and acids that will harm the engine during the lay-up. Clean oil is a must for the health of your engine.

For best results, run the engine for a few minutes to lower the oil's viscosity and allow it to pick up impurities. Shut off the engine, change the oil, and replace the filter.

Change transmission fluid too, for the same reasons. Oil change pumps like the West Marine Flat Tank Oil Changer make restricted access oil changes easy.

Step 8: Seal and Protect

Finally, wrap duct tape around the flame arrester and cover the top of the engine with plastic after it has cooled. This helps prevent corrosion from forming on internal engine parts. Attach a tag somewhere as a reminder about the duct tape, which will prevent the engine from starting next spring.

Cover all engine openings including:

Air intakes
Crankcase breathers
Exhaust outlets
Transmission vents

Create a checklist of sealed openings so you remember to uncover them during spring commissioning.

Additional Winterization Tasks

With the main engine winterization complete, it’s worth taking a few extra steps to protect related components and ensure everything is in top condition for next season.

These checks only take a short time but can prevent headaches later.

Inspect and Maintain

Exhaust System: Disconnect the exhaust from the water lift muffler or exhaust manifold and inspect for carbon buildup, scale, or corrosion. Remove the raw-water injection hose from its nipple and check for blockages caused by debris or mineral deposits.
Hoses and Clamps: Examine all hoses for softness, bulges, cracks, or other signs of wear. Pay particular attention to the hot side of the exhaust and cooling systems, where heat accelerates deterioration. Make sure all hose clamps are tight and free of corrosion.
Grease Points: Lubricate all grease fittings on the engine. Apply marine grease to exposed moving parts to protect against rust and ensure smooth operation in spring.

Storage Considerations

Battery Care: Disconnect the battery to prevent slow discharge. Charge it fully before storage and recharge it at least twice over the winter to maintain capacity.
Ventilation: Store the boat in a dry, well-ventilated area. Reducing moisture in the storage space lowers the risk of rust, corrosion, and mold growth on the engine and surrounding components.

Common Winterization Mistakes to Avoid

Even if you’ve been around boats for years, it’s easy to miss something during winterization. A small oversight now can turn into a big, expensive problem in spring.

Here are the mistakes you don’t want to make and how to avoid them.

Using the Wrong Antifreeze: Never pour automotive antifreeze into your boat’s cooling system. It’s highly toxic and not designed for marine use. Instead, stick with non-toxic, marine-grade propylene glycol. It’s safer for the environment and still gives you the freeze protection you need.
Skipping the Fuel Stabilizer: Leaving untreated fuel in your tank is asking for trouble. Over the winter, it can break down, form varnish, or separate if it contains ethanol. That gunk will be the first thing your engine pulls in at spring startup. Adding stabilizer now keeps your fuel fresh and your fuel system clean.
Forgetting to Fog the Engine: Moisture inside your engine will cause rust, and once that happens, you can’t reverse it. Fogging oil puts a protective coating on cylinder walls, pistons, and valves so they stay corrosion-free during storage.
Not Draining Completely: Even a few ounces of water trapped in the wrong place can freeze, expand, and crack expensive parts like the block or manifolds. Open every drain, check every low point, and make sure there’s nothing left that can freeze.

If you take your time, follow each step, and avoid these common pitfalls, you’ll save yourself a lot of frustration and possibly thousands in repairs when you’re ready to hit the water again.

Professional vs. DIY Winterization

You have two options when it comes to winterizing your inboard engine: pay a professional or do it yourself. Both can protect your boat, but the right choice depends on your comfort level, budget, and schedule.

Hiring a Professional

Taking your boat to a marina or service shop typically costs $100–$200. You’re paying for their experience, tools, and time, but also for the convenience of dropping it off and knowing it’s done right.

For some owners, especially those with little mechanical experience or limited time, that peace of mind is worth the price.

Doing It Yourself

DIY winterization costs about $50–$150 in materials, plus the time you spend. The upside is you’ll save money and gain valuable knowledge about your engine’s systems.

Many steps are straightforward once you know the process, and tools like the Camco 65501 DIY Boat Winterizer flushing system (around $40) make it easier and can be used for years.

The bottom line: Professional winterization makes sense if you’re short on time or unsure about the process. DIY is a great option if you want to save money, learn your boat inside and out, and have full control over the work being done.

Either way, skipping winterization altogether can lead to costly damage that can easily run $500–$2,000 to repair.

Spring Preparation Tips

Winterization protects your engine through the cold months, but you still need to undo those steps correctly when boating season returns. Rushing this process or forgetting something can undo all your winter work.

Here’s how to bring your boat back into service the right way:

Unseal the Engine Completely. Remove all tape, plastic, and covers from air intakes, exhaust outlets, crankcase breathers, and any other openings you sealed. Refer to the checklist you made in the fall so you don’t miss anything.
Check Every Fluid. Verify engine oil, transmission fluid, and coolant levels (if your engine is closed-cooled). Top up with marine-grade fluids as recommended in your owner’s manual.
Inspect for Winter Damage. Look over hoses, clamps, wiring, and fittings for cracks, corrosion, or looseness. If your boat was stored outdoors, check for signs of water intrusion or rodent activity.
Flush the Cooling System. Connect a garden hose or use your flushing adapter to run fresh water through the system until all traces of antifreeze are gone. This ensures clean cooling water will circulate when you run the engine.
Test All Systems at the Dock. Before you head out, make sure bilge pumps, navigation lights, electronics, steering, and throttle controls are working properly. Address any issues while you’re still tied up.

Protect Your Investment Year-Round

Winterizing is insurance for your engine’s health and your boat’s value. A few hours of work now can prevent thousands in repairs later, keep your systems reliable, and make spring commissioning quick and stress-free.

The key is doing it before the cold sets in. Start early, when you can work in comfortable conditions and have time to spot and fix any issues. Gather your supplies — marine antifreeze, fogging oil, fuel stabilizer — and follow the process step by step.

Treat winterization as part of your regular maintenance routine, not an optional extra. It’s one of the simplest ways to make sure your inboard engine lasts for many more seasons and starts right up when you’re ready to get back on the water.

by RV Trader Team

How to Winterize Your Inboard Boat Engine Like a Pro: Complete Step-by-Step Guide