How to Winterize a Boat Motor – Steps to Winterize Inboard and Outboard Motor

I already wrote about how to winterize a pontoon boat previously, so I will not go over those winterization steps. Instead, I will concentrate on how to winterize a boat motor in this article because I think it deserves a deeper look at the process.

Winterizing your motor is a critical step in boat ownership. Engines are the heart and soul of every boat, and you have to keep them in working condition if you want to enjoy your boat for the years to come.

Not winterizing your boat motor properly can be very costly. The damage caused by water freezing inside your motor can be measured in thousands of dollars, so take the necessary steps to prevent that from happening.

How to Winterize a Boat Motor

As mentioned above, winterization is a critical process in boat maintenance, so do not overlook these winterization steps. Read the article multiple times if you have to. Take notes or print it out and take it with you when you decide to winterize your boat.

Because inboard and outboard engines are different, I will detail the step-by-step process of proper winterizing for both of them.

Winterization checklist for outboard motor includes:

  • Freshwater Flush
  • Drain Fuel Line and Carburetor
  • Fog The Carburetor Intake
  • Drain cooling passages
  • Lubricate Necessary Surfaces
  • Drain And Refill Gear Case
  • Fill Up Your Fuel Tank
  • Clean and Lubricate the Propeller Shaft
  • Store The Motor Upright During Winter

Winterization checklist for inboard motor includes:

  • Change Engines Coolant
  • Change Engine And Transmission Oil
  • Fill Up Your Fuel Tank
  • Fog The Carburetor Intake
  • Flush Raw-Water Circuit
  • Drain Muffler Canister
  • Grease Control Cables
  • Coat Unpainted Areas With Anti-Corrosion Spray

How to Winterize Outboard Motor

How to Winterize Outboard Motor

Freshwater Flush

Make sure you pressure wash your engine to remove any dirt or salt. As we all know, salt is the worst enemy of boats, cleaning it off completely is essential.

You can either use an extension for your garden hose or buy a pressure washer on Amazon; either works fine. I recommend using Westinghouse WPX2700 Pressure Washer, not that expensive yet powerful machine that does the job right the first time.

Drain Fuel Line and Carburetor

Disconnect the fuel line from the engine while the engine is still running. This will empty the fuel delivery component, and the engine will stop working. This is important to prevent any gums formation or clogging of lines inside the boat engine.

Fog The Carburetor Intake

Just before the engine stops working due to lack of fuel, spray fogging oil into the carburetor. This will help protect the internal surfaces of the engine cylinders and carburetors from corrosion.

When you see that the engine is starting to have a hard time running, that is the best time to add extra fogging oil to ensure that every surface is covered thoroughly.

My choice of fogging oil is Gold Eagle 22003 STA-BIL Rust Stopper. It prevents rust and corrosion, is water-resistant, and very long-lasting.

Drain Cooling Passages

The first step you want to take is to disconnect the battery from the engine.

With the motor in the upright position, remove the flush attachment and the flush tank engine. Let the water drain out.

Empty the powerhead and intermediate housing by opening and draining the plugs. You should probably consult your owner’s manual here.

Remove spark plugs and cover the inside with the fogging oil. It is helpful to rotate the flywheel a few times to help spread the oil inside cylinder walls.

This is not the wrong time to check your spark plugs, so you might want to consider doing that before putting them back in.

Don’t Forget To Lubricate

Consult your owner’s manual once more to ensure that all the surfaces needing lubrication are adequately covered with oil and heavy grease.

Drain And Refill Gear Case

Fill the oil tank back up to the prescribed limit shown on your owner’s manual. Doing this will prevent condensation from forming inside the tank and damaging your motor in the long run.

Fill Up Your Fuel Tank

You can either empty your fuel tank or fill the tank up to about 95% capacity.

I like to fill the tank for two reasons.

  • First, I don’t have to deal with draining the engine. I don’t find that particularly convenient.
  • Second, you want to fill the tank up to 95% to avoid and internal condensation inside the tank. This happens because gasoline contains ethanol, and water may cause water phase separation.

What this means is that water will begin to remove the ethanol from the gasoline. It will, in turn, create a leaner combustion mixture between water and ethanol (higher air to fuel ratio). With more air comes stronger combustion and higher temperature, which can damage the engine.

It is vital to limit the space inside the talk to about 5% to limit the amount of condensation that can be created.

Also, make sure that you use a gas stabilizer to avoid the formation of passage-clogging gums.

Clean and Lubricate the Propeller Shaft

The propeller is what keeps your boat running smoothly. A well-taken care propeller will ensure the highest level of performance and efficiency out on the water.

Inspect the propeller visually, check for small cracks and corrosion. Also, check for wear and tear. Just because the propeller’s blades look the same, it doesn’t mean that they are not worn away and in need of replacement.

If you are not taking the propeller home to prevent it from being stolen, make sure you clean it thoroughly and lubricate it well to prevent corrosion.

Store The Motor Upright During Winter

Lastly, make sure your motor is stored upright. By laying the engine to its side, you risk leftover water draining to places where it should not go. Keeping the outboard motor in its natural upright position is the best way to go.

You can quickly build something to allow you proper motor storage or but it on Amazon. I own Leadaallway Outboard Boat Motor Stand. It’s simple, cheap, convenient, and makes the job a breeze.

How to Winterize Inboard Motor (Sterndrive)

How to Winterize Inboard Engine

Change Engines Coolant

This only applies to freshwater engines. Over time anti-corrosion properties of coolant start fading away, so it is crucial to replace it annually to protect the inside of your inboard engine.

Change Engine And Transmission Oil

When changing the oil, it’s important to take the boat for a ride first. What this does is it warms up the oil, which separates contaminants off the engine walls, so when you drain the engine, you drain the pollutants.

And don’t forget to change the oil filter while you are at it.

Fuel Tank – Gas/Diesel

If you don’t want to drain the motor, fill it up the same way you would with the outboard engine for both gas and diesel engine. The reason is the same as for the outboard engine described above.

On the other hand, if you have a diesel engine, add some biocide to prevent bacteria growth.

Use a stabilizer for the gas engine.

If you have a diesel engine, you don’t have to use a fuel stabilizer unless you expect to run the engine during the winter. You don’t need the fuel stabilized because once the weather gets warmer, the paraffin that precipitates out will be absorbed by the fuel.

Fuel Tank – Gas Engine

Empty the engine by shutting down the fuel supply and letting the engine run until it stops.

Fogging The Carburetor Intake – Gas Engine

You can use the same steps to winterize an outboard boat engine as described above. Steps are nearly identical, but you should still check your owners manual to make sure you are not missing something.

Flush Raw-Water Circuit

Use a freshwater flush connector if you have it.

If not, close the intake seacock. After that, on the outlet side of the raw-water pump, disconnect the hose. Also, disconnect the water cooling discharge hose from the manifold. Rinse it thoroughly with fresh water to remove any deposits and salt.

Drain Muffler Canister

In this case, less is better, less moisture that is. Try to minimize moisture in every way possible to reduce the chance of corrosion.

Grease Control Cables

Grease is your friend when working with the mechanical stuff, so use it.

Remove control cables from housing and grease them up.

If that is not an option, tape an oil-filled bag around the high end of the housing, let it sit there for a while so the oil can work its way down the cables.

Coat Unpainted Areas With Anti-Corrosion Spray

You should use anti-corrosion spray on all unpainted surfaces of the engine.


There you have it, folks, my own guide to how to winterize a boat motor. These are the simplest steps you can take to winterize your boat motor and keep it on the water for years to come.

If you find it a bit overwhelming, it’s best to pay someone to do that for you. I’ve checked with a few places that do this type of work, and they charge between $300 and $500 on average. Of course, the price will depend on where you live and where you take your boat, but it is a small price to pay to keep your boat in working condition.

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