How to Fully Winterize Inboard Boat Motors in 4 Steps

How to Fully Winterize Inboard Boat Motors in 4 Steps

It's the end of boating season and time to reluctantly start strapping down those winter boat covers. Then, you'll be moping around winter boat shows to fight the winter boater's blues. Before you get to that, though, there's one crucial thing you have to do:

Winterize your inboard boat motor!

If you've got an inboard or a sterndrive (inboard/outboard) boat motor, then winterizing is an absolute must-do. This process prevents gunk from building up, keeps condensation out of the fuel tank and just overall keeps your engine healthy. I've already covered winterizing pontoon boats, which usually have outboard motors.

For those of you with powerboats, here are the steps to winterize your inboard boat motors. I've removed a few steps from the entire boat winterization process here, like removing the battery and keeping it on a trickle charge.

This post focuses specifically on winterizing motors.

How to Fully Winterize Inboard Boat Motors in 4 Steps

1. Prepare Your Inboard Boat Motor for Winterizing

Fill the tank with gas.

Why do you want the tank filled with gas over winter, you might ask? The answer: This decreases the chance of getting air into the tank. Air is bad for the tank because it brings moisture and, with low temperatures over winter, condensation!

If you don't fill up your tank, you'll wind up with water inside, which can cause severe problems for your engine.

Add the fuel stabilizer (or whatever your boat's Owner's Manual suggests).

I suggest using STAR BRITE EZ-To-Start Fuel Additive (check price on Amazon). What does fuel stabilizer do?

Stabilizer prevents buildup, while your boat sits for long periods, not only in your fuel lines but also in your fuel injectors and carburetors.

Warm the engine. You can do this one of two ways: Run the engine while the boat's in the water or run the engine with an adaptor or motor muffs and a hose running the water supply.

2. Add Antifreeze and Spraying Fogging Oils

After your motor is cleaned and prepared as described above, the first real winterization step is applying antifreeze. The second step is spraying fogging oils into the engine.

Adding Antifreeze

Adding antifreeze to the engine prevents any water condensation or otherwise from freezing. Freezing can be harmful because it means expansion, causing stress and eventually cracks. The last thing you need is to find a milky consistency in your oil and to have to call your mechanic.

Don't contaminate where you play. Purchase a non-toxic propylene glycol antifreeze (check price on Amazon here). Avoid ethylene-based antifreeze, as ethylene can release toxins into the water. 

Adding Antifreeze to Engine Blocks

You'll want to follow your boat manufacturer's specifications to drain the engine block. Once the engine block is drained of coolant, then you add the propylene glycol antifreeze.

Adding Antifreeze to Sterndrives

When you hear the word sterndrive, it's important to know what it means. Essentially, a sterndrive is an inboard engine with an outboard drive system. So a sterndrive has to be treated like an inboard engine, but with a few additional steps to protect the lower parts.

To drain the lines, leave the sterndrive stored in a down position. Using your fresh water source (boat hose and faucet), allow your boat's engine to come to temperature. This allows the thermostat to breathe, letting both the coolant and fuel stabilizer circulate through lines, carburetors and injectors.

To make it easy, boaters can purchase DIY kits like this Camco Winterizer Kit (check price on Amazon here).

Spraying Fogging Oil into the Engine

After applying the antifreeze to your engine, and while it's still warm, the next step is to spray fogging oil into the engine.

Spraying fogging oils into inboard and sterndrive engines is slightly different than spraying it into outboard engines. I suggest the STA-BIL fogging oil (check price on Amazon). For more information, check out this Gold Eagle's article explaining the product's proper usage.

Continue the process by filling a five-gallon bucket (or the convenient DIY kit mentioned above) with non-toxic antifreeze. Closing the intake seacock, transfer the hose to the bucket.

Letting the boat engine idle, keep watch for the bucket's liquid to reach low levels and the exhaust to discharge the antifreeze. Time it for 30 seconds or until the bucket is nearly dry. Then proceed to fog the carburetor.

Generously spray the fogging oil.

Engines with high horsepower may sputter, and low horsepower engines may even stall out but that's okay. You should see white smoke and, when the antifreeze bucket is finally dry, turn off the engine and re-secure the hose to the seacock.

Alternatively, you can go around removing each individual spark plug. With spark plugs removed, you can then spray fogging oils directly into the engine's combustion chamber.

Featured Boat Care Product


 Motor Muffs 

Check Price on Amazon - Better Boat's motor muffs flush excess fuel, saltwater and contaminants from inboard, outboard and jet engines. It connects to a standard garden hose to flush engines after use or while winterizing.  

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

Some boaters tend to leave this task until their spring boat inspection. But I say do it now while your engine is already warm. Draining and changing the oil is easier when it's warm.

Your engine and transmission oil could be contaminated with dirt and particles now, which can harm it during winter if not dealt with properly. Using a continuous water supply with your boat still in the water or using a standard garden hose, start the engine and monitor the fuel filter for leaks.

If a leak is found, turn off the engine and make proper repairs. Check the installed filter once again. Restart the engine. Keep it at an idle RPM and allow it to reach regular temperatures.

The trick here is to not over-cool the engine. It's better to cool it off gradually over time.

When using a combination hose and adaptor, don't run the faucet at a full-on blast. Cut the faucet off so it's a steady flow.

You want to make sure to let the engine run for 15 minutes at a minimum. This will allow the fuel stabilizer to enter the fuel system. But if this proves difficult or you're in a hurry, at least add an oil stabilizer.

4. Do a Final Inspection

Almost there! While your hands are dirty, you might as well follow these routine checks.

Check Exhaust System for Corrosion

Inspect your exhaust system by disassembling it from the water lift muffler. In general, look it over for anything suspicious like corrosion and buildup. Make sure the raw water injection hose hasn't become obstructed in any way.

Inspect Hoses and Hose Clamps

Oftentimes, hoses and hose clamps can become damaged. Check them over to be sure they're not cracked, shredded or rusty. If they are, be sure to replace them now before you forget come spring!

Seal Off Cracks from Air Intake

If you find any openings, seal them up! Cracks and openings allow the build-up of damp air and water condensation. Double-check your air inlet and exhaust outlets. You can use simple tape to secure them. Here are some last-minute things to mention, just so you don't forget basic boat winterizing steps: 
  • Take care of the battery. Remove and keep it indoors at home, or leave it installed and hook it up to a trickle charger.
  • Invest in theft prevention. Stay vigilant and stop by to check on your boat every once in a while.

What are you waiting for? If winter is looming, or if you're expecting some unseasonably cold weather in your area, it's time to winterize your inboard boat motor. Come next spring, you'll find your boat in tip-top shape.

After a quick spring boat inspection and some de-winterization steps, all you have to do is grab the cooler and take the family out for some fun on the water.