How to Fully Winterize Inboard Boat Motors in 4 Steps

At a glance

Winterize an inboard by following the manual for fuel care, stabilizer, an oil and filter change, draining the raw water system, antifreeze, fogging, and a final inspection. Drain points, amounts and methods differ by engine.

• Change the oil and filter while the engine is warm.
• Use propylene glycol antifreeze made for marine or RV use on the raw water side.
• Finish by checking the exhaust, hoses and clamps, and sealing air intake openings.

Do Inboard Motors Need to Be Winterized?1. Prepare Your Inboard Boat Motor for Winterizing2. Change the Engine's Oil and Replace the Oil Filter3. Drain the Engine, Add Antifreeze and Fog the Engine4. Do a Final InspectionFrequently Asked Questions

The right tool for this job

Boat and RV Winterizing Kit

Make seasonal winterizing more organized with a gravity flow antifreeze tank and fittings in one kit.

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The boating season is winding down, and it is time to reluctantly start strapping down those winter boat covers. Then comes the moping around winter boat shows to fight the winter boater's blues. Before that, though, there is one crucial job to finish.

Winterize your inboard boat motor.

To winterize an inboard boat motor, follow the storage guidance from the engine, boat and fuel tank manufacturers for the fuel tank, add fuel stabilizer, and run the engine briefly so the treated fuel reaches the fuel system. Change the oil and filter while the engine is warm. Then drain the raw water cooling system as the manual directs, protect it with propylene glycol antifreeze if the manual calls for that, fog the engine using the method the manual allows, and inspect the exhaust, hoses and clamps. Drain points, antifreeze amounts and fogging methods differ from engine to engine, so the manufacturer's manual always comes first.

If you have an inboard or a sterndrive (inboard/outboard) boat motor, winterizing is an absolute must in cold climates. The process helps limit gunk buildup, helps keep condensation out of the fuel tank, and protects the engine's cooling passages from freezing damage.

This post focuses specifically on winterizing the motor. Other parts of the job, like removing the battery or keeping it on a trickle charger, are covered in the full boat winterizing guide and the boat winterizing checklist. Owners of boats with outboards can follow the outboard winterizing steps instead. Sterndrive owners will also find extra detail in the guide to winterizing an inboard outboard motor.

Do Inboard Motors Need to Be Winterized?

Yes, if the boat will sit anywhere temperatures can drop to freezing. Water left in the engine block, exhaust manifolds, risers, hoses and pumps expands as it freezes, and that can crack castings and split hoses. Even in milder climates, long storage is a good reason to add fuel stabilizer, change the oil and fog the engine where the manual allows. Check the manufacturer's manual for the cold weather steps that apply to your engine.

1. Prepare Your Inboard Boat Motor for Winterizing

Follow storage guidance for the fuel tank.

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De-Salt Concentrate Salt Remover and Flusher

Break down salt on engines, hulls, gear, and equipment before it sits through storage or another week at the dock.

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Many boaters store the boat with a fuller fuel tank over winter. A fuller tank leaves less room for air, and air brings moisture that can turn into condensation when temperatures drop. Water inside the tank can cause severe problems for your engine. The right level depends on the tank material, fuel type, ventilation and manufacturer guidance, so follow the engine, boat and fuel tank manufacturer's recommendations. If a fuller tank is advised, never fill beyond the rated safe level, and leave room for the fuel to expand.

Add fuel stabilizer (or whatever your boat's owner's manual suggests).

Choose a marine fuel stabilizer and follow the label directions. Adding it before topping off the tank helps it mix. What does a fuel stabilizer do? It helps protect your boat's fuel lines, injectors and carburetors while the fuel sits for long periods.

Flush the engine if you boat in salt water. Salt left in the cooling system can sit through storage. The De-Salt Concentrate Salt Remover and Flusher works as an engine flush with a compatible flushing system. For the full routine, see how to flush a boat engine the right way.

Warm the engine and circulate the stabilizer. You can do this one of two ways. Run the engine while the boat is in the water, or run it on a fresh water supply from a boat hose through the flushing connection your manufacturer specifies. Some drives allow motor muffs or an adaptor and some do not, so confirm what the manual permits and that the fitting matches your drive's water intakes before you start.

For sterndrives, leave the drive in the down position and supply fresh water the way the manual describes. As the engine warms, the cooling thermostat opens where the engine has one, so cooling water or coolant can circulate. Running the engine also draws the treated fuel through the fuel lines, carburetor or injectors. These are two separate systems, and both are being prepared at once.

Before starting, confirm strong water flow at the flushing connection. Run the engine at idle only, and never run it without a water supply, because dry running can quickly damage the raw water pump impeller. Watch the temperature gauge and the water coming out of the exhaust, and shut the engine down immediately if either looks abnormal. Run it only long enough to reach normal operating temperature and circulate the treated fuel, following the stabilizer label and the engine manual.

2. Change the Engine's Oil and Replace the Oil Filter

Some boaters leave this task until their spring boat inspection. It is better to do it now while the engine is already warm, because warm oil drains more easily. Used oil can hold acids, dirt and particles that sit against engine parts all winter, so fresh oil and a new filter go in before storage.

Replace the fuel filter or fuel water separator as the manual directs. Check the transmission fluid and the sterndrive gear lube too. A milky look can mean water has gotten in, and that needs attention before winter.

After the new filters are on, start the engine with a continuous water supply as the manual allows and check around the oil filter and fuel filter for leaks. If you find one, turn off the engine and make proper repairs. Then restart the engine, keep it at an idle RPM and let it reach regular operating temperature, then let it cool down gradually. Set the water flow exactly as specified by the engine manufacturer and the flushing adapter manufacturer.

3. Drain the Engine, Add Antifreeze and Fog the Engine

The next steps are draining and protecting the raw water cooling system and fogging the engine. The exact procedure comes from your engine manufacturer, because cooling layouts differ.

Why Antifreeze Matters

Correctly selected and properly used antifreeze provides freeze or burst protection for any water left in the engine, according to its label rating and the engine manual. Unprotected water that freezes expands, which causes stress and, eventually, cracks. The last thing you need is to find a milky consistency in your oil and have to call your mechanic.

Use a propylene glycol antifreeze made for marine or RV use, and avoid automotive ethylene glycol in the raw water side unless your manual specifically says otherwise.

Adding Antifreeze to Engine Blocks

Follow your engine manufacturer's instructions to drain the engine block. Drain points vary by engine, and they often include block drains, manifolds, risers, coolers, pumps and low spots in hoses. Open and clear every drain the manual specifies so water actually runs out, then add the propylene glycol antifreeze the way your manual describes.

If your engine has a closed cooling system, the fresh water loop uses its own coolant. Have that coolant tested for freeze protection that suits your lowest winter temperatures, and top it off as the manual directs.

How Much Antifreeze Does an Inboard Need?

There is no single amount that fits every engine. The quantity depends on the engine size, the hoses, the heat exchanger and any other water lines it feeds, so check the manual and get the amount it specifies. Keep extra on hand so the supply never runs dry during the procedure. Antifreeze coming out of the exhaust alone does not prove every freeze-sensitive passage is protected, because some cooling layouts can bypass the block, so complete every drain and step the manual lists.

Adding Antifreeze to Sterndrives

A sterndrive is an inboard engine with an outboard-style drive. It gets treated like an inboard engine, with a few additional steps to protect the lower parts. Leave the sterndrive in the down position so water can drain out, and follow the manual for the drive's drain points.

To make the job easier on the right setup, the Better Boat Boat and RV Winterizing Kit gives you a gravity flow antifreeze tank and fittings in one kit. It is made for engines with the raw water pump mounted inside the outdrive, using the muff type flushing method. It is not suitable for front engine mounted raw water pump systems, so confirm your pump location in the manual first.

Running Antifreeze Through the Engine

The manual shows how to connect the antifreeze supply for your engine, and the setup differs by design. On a direct inboard with the raw water pump on the engine, the manual explains how to isolate the seacock and feed antifreeze to the pump inlet, including how any strainer and backflow concerns are handled. On a sterndrive with the pump inside the outdrive, the muff type method may apply if the engine and drive manuals permit it, and that is the setup the kit above is made for. The gravity kit should not be used for a bucket and seacock style setup on a front mounted pump.

Whichever setup applies, run the engine at idle only, keep the antifreeze supply from running out so the pump never pulls in air, and watch the temperature gauge. Shut the engine down when the manual says the procedure is complete, and finish every drain and step it lists. Do not treat exhaust color or flow as proof of complete protection.

Spraying Fogging Oil into the Engine

Fogging oil helps coat internal engine surfaces for storage. Choose a marine fogging oil and follow the label. Methods differ between inboard and sterndrive engines and outboards, and the engine manual controls which one applies. For more detail, see this Gold Eagle article explaining proper use of fogging oils.

On a carbureted engine whose manual permits intake fogging, the oil is sprayed into the carburetor air intake as the label directs. Intake fogging is done only while the engine is running with the approved cooling water or antifreeze supply connected. Never restart the engine without that supply. The manual determines how fogging is coordinated with the antifreeze step and with shutdown. Fuel injected, catalyst equipped and sensor equipped engines may prohibit intake fogging or require a different procedure, such as a specific fogging port, a treated fuel mixture or cylinder fogging. Follow the manual for your engine.

Another option on many engines is to remove each spark plug and spray fogging oil directly into the cylinders. Follow the manual for turning the engine over afterward and for reinstalling the plugs. When the procedure is done, reconnect the hose to the seacock fitting and make sure the seacock is returned to the position the manual describes.

4. Do a Final Inspection

Almost there. While your hands are dirty, run through these routine checks.

Check the Exhaust System for Corrosion

Inspect the exhaust system at the water lift muffler. Look it over for anything suspicious, like corrosion and buildup. Make sure the raw water injection hose has not become obstructed in any way.

Inspect Hoses and Hose Clamps

Hoses and hose clamps often become damaged. Check them over to make sure they are not cracked, shredded or rusty. If they are, replace them now before you forget come spring.

Seal Off Openings at the Air Intake

If you find any openings, seal them up. Cracks and openings allow damp air and condensation to build up. Double-check your air inlet and exhaust outlets, and cover them with simple tape if needed. Leave a note at the helm so the tape comes off before the first start in spring.

Here are a few last-minute reminders so you do not forget basic boat winterizing steps.

• Take care of the battery. Remove it and store it in a cool, dry, ventilated spot away from flames and sparks, or leave it installed and hook it up to a charger approved for your battery type. Follow the battery manufacturer's instructions.
• Consider pests. Whether you keep your boat on the lift or in storage, plan accordingly. Remove food, clean up crumbs and close off gaps and openings.
• Invest in a quality winter boat cover. Boat cover support poles are a helpful addition under the cover. Moisture inside a covered boat is covered just below. There is more information on covers and poles for winter coverage here, and more on storage in the winter boat storage guide.
• Invest in theft prevention. Stay vigilant and stop by to check on your boat every once in a while.

Keep Moisture Down While the Boat Is Stored

A boat that sits covered or closed up for months traps air in the cabin, console and lockers. As the temperature swings between day and night, moisture in that air condenses on cool surfaces, and with little airflow under a cover it tends to linger. That trapped dampness is where the stale, musty smell many owners notice in spring comes from.

Moisture absorber containers are a simple way to handle it, with no power needed. Place containers in the enclosed spaces that stay closed, such as the cabin, console and lockers, on a flat, level spot where they will not tip. The Boat Dehumidifier Container uses moisture absorbing pellets to trap excess moisture and activated charcoal to help reduce musty odor, unscented and with a spill resistant mesh. The Refillable Dehumidifier Container also absorbs excess moisture in closed spaces and uses activated charcoal to help reduce musty odor, and when its pellets have dissolved you drop in a new refill bag instead of replacing the whole container. Check the containers whenever you visit the boat over the winter.

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Boat Dehumidifier Container

Moisture absorbing pellets trap excess moisture in small enclosed spaces, and activated charcoal helps reduce musty odor. Unscented, with a spill resistant mesh.

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Better Boat pick

Refillable Dehumidifier Container

Refillable container that absorbs excess moisture and uses activated charcoal to help reduce musty odor. Replace the refill bag when the pellets are spent.

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If winter is looming, or if unseasonably cold weather is on the way, it is time to winterize your inboard boat motor. Come next spring, the boat will be ready to go. After a quick spring boat inspection and some de-winterization steps, all that is left is grabbing the cooler and taking the family out for some fun on the water. Wondering about expenses? The cost of winterizing a boat is covered separately.

Frequently Asked Questions

Do inboard motors need to be winterized?

Yes, whenever the boat will sit where temperatures can reach freezing. Trapped water can crack the block, manifolds and hoses. Check the manufacturer's manual for the steps your engine needs.

How much antifreeze does it take to winterize an inboard boat motor?

The amount depends on the engine and its cooling system, so no single number fits every boat. The manual lists the exact procedure and the drains that must be opened. Antifreeze reaching the exhaust does not prove every passage is protected.

How long can you run a boat on muffs?

Only as long as the manufacturer allows, and only with a strong, steady water supply. Not every drive allows muffs, so check the manual first. Keep the engine at idle, watch the temperature gauge and the exhaust water, and shut it off immediately if either looks abnormal. Running without water can quickly damage the impeller.

Can you winterize a boat without starting it?

Only if the manufacturer provides a procedure for it. Some cooling systems can be protected by draining and following the manual's steps without running the engine. The other storage steps still need to be completed, including fuel system care, the oil and filter change, transmission and drive checks, and fogging where the manual allows.

Should the fuel tank be full or empty for winter?

Follow the engine, boat and fuel tank manufacturer's storage guidance. A fuller tank with fuel stabilizer added is common advice because less air means less condensation, but never fill past the rated safe level, and leave room for expansion.

What kind of antifreeze should be used on an inboard?

Marine or RV propylene glycol antifreeze is the usual choice for the raw water side. Closed cooling systems use their own coolant, so follow the engine manual for both.

Keep reading

How to Winterize an Inboard Outboard Motor: Easy Steps to Protect Your BoatWinter Is Coming: How to Winterize Your Boat Like a ProfessionalHow to Winterize a Boat with a Cover