It’s that time of year again, and unfortunately, it’s time to think about storing your boat for the season. While it’s always a bit more fun to pull out your boat than to put it away, properly winterizing your MerCruiser sterndrive engine avoids engine damage and will facilitate a smooth start to the beginning of next season.
According to Mercury Marine service guidelines, winterizing a MerCruiser engine is essential for preventing freeze damage and corrosion during extended off-season storage.
While there are several things you can do before storing your boat to get a jump on next season—like changing your oil and changing your fuel-water separating filter—there are six highly recommended steps.
Winterizing a MerCruiser sterndrive involves:
- Flush your MerCruiser engine
- Stabilize your fuel system
- Drain water and fill engine with antifreeze
- Fog your MerCruiser sterndrive engine
- Change lower unit oil
- Disconnect your battery
Follow these six steps to prevent freeze damage and ensure your boat is ready for next season.
Quick summary: how to winterize a MerCruiser inboard outboard
- Flush the engine with water or antifreeze
- Add fuel stabilizer and run the engine
- Fully drain water and add antifreeze
- Spray fogging oil into the carburetor until the engine stalls
- Change gear oil and check for water contamination
- Disconnect and clean your battery
How to flush a MerCruiser engine before winterizing
Flushing your MerCruiser engine is the first step in winterizing your inboard outboard boat motor. Use a garden hose or motor flusher to remove debris that causes corrosion. Freshwater or salt buildup left in the system can lead to serious winter damage if not flushed properly.
Flush with the Engine Not Running (using a hose port)
You’ll need:
- Garden hose
- Water supply
If your engine has a hose adapter, simply screw the hose into the port, turn on the water and let the water run for about 5 minutes.
Finally, tilt the engine all the way up, then trim it all of the way down to make sure every drop of water drains from the engine.
**Caution** Do not run the engine when flushing while using a hose port. This method is solely designed for flushing while your engine is entirely off.
Flushing Your MerCruiser Sterndrive with the Engine Running
You’ll need:
- Motor flusher
- Garden hose
- Water supply
To flush your outboard with the engine running, you will use a motor flusher. We recommend a Quicksilver pin motor flusher, rather than earmuffs. Earmuffs have a tendency to fall off when flushing a sterndrive, but the pin flusher configuration stays very secure.
Screw your garden hose onto the motor flusher, then slide the motor flusher over the engine’s water intakes. You must be sure that the earmuffs are completely covering the water intakes. Using this flushing method, you will turn on the engine. But first, make sure that you have a good water supply.
Start the engine and make sure that water is flowing from the back of the engine. Assure that your engine is not overheating by holding your hand under the flow of water. It should not be uncomfortable to hold your hand in the stream of water. If the water is excessively hot, water is not flowing correctly, and you are overheating your engine—turn off the engine, reposition the flusher, and start again.
Stabilize Your MerCruiser’s Fuel System
You’ll need:
- Fuel stabilizer suitable for long-term storage
Because it is so common, many of us use ethanol-enriched gasoline much of the time. We recommend using a fuel stabilizer all of the time when running ethanol-enriched gasoline. Read our Ethanol Gasoline In Outboard Engines article to learn why. Additionally, it is vital to use a fuel stabilizer like Mercury Quickstor when your boat will be sitting unused for any more than two or three months.
Add the amount of fuel stabilizer directed on the product’s packing to your fuel—for instance, 1 ounce of Mercury Quickstor for every 5 gallons of gasoline.
Run the engine with stabilized fuel for 10-15 minutes to thoroughly circulate the stabilized fuel throughout your fuel system.
Drain the Engine and Add Antifreeze
You’ll need:
- 3-4 gallons of antifreeze
- Winterization kit or hose
- Motor flusher
- Garden hose
- Water supply
Wondering how to winterize a boat with antifreeze? Make sure the antifreeze reaches the exhaust—this confirms the manifolds and engine block are protected from freezing.
Adding antifreeze and fogging the engine have to be done consecutively with the engine running. Start with draining the engine and adding antifreeze.
Remove all of the drain plugs to get all of the water out, so reference your service manual for the drain plug locations. Some newer MerCruiser models have a quick drain system, so again, check your owner’s manual.
Poke around all of the drain holes with a small screwdriver to make sure that there is no debris blocking water from draining.
**Caution** Inadvertently leaving water in the engine is a very expensive mistake. Any water left inside poses a freezing risk which can crack the block or manifold.
Once the engine is drained, put the plugs back in.
Secure your motor flusher to the intake on the drive and turn on the water.
Attach a hose to the antifreeze source, then run the engine while making sure the antifreeze is being consumed.
When the antifreeze goes through the engine, it actually comes out the exhaust, so when you start seeing antifreeze being exhibited out the exhaust, you know that the block and manifolds are full.
Fogging Your Engine
Fogging is an essential part of how to winterize an inboard outboard motor and helps protect internal engine components from moisture and corrosion during storage.
You’ll need:
- Fogging oil
So, what is fogging oil or fogging fluid anyway? Fogging oil is a wax-based oil that coats the engine and prevents corrosion. As you work through this process, you will soon understand why this process is called fogging the engine, as there will be plenty of white smoke.
You will perform this step directly after adding antifreeze, so keep your engine running.
Remove the flame arrester covering the carburetor.
Rev the engine to a bit above idle, and with the engine running, spray the fogging oil down into the carburetor.
Continuously spray fogging oil into your engine’s carburetor until the engine stalls. Don’t spray too much oil at once and kill the engine immediately, but after a continuous spray, the engine will eventually stall. This is precisely what you want, and the ultimate stalling indicates that the engine is thoroughly covered with fogging oil.
**Note** This fogging process is for carbureted engines only. EFI engines have a slightly different fogging procedure.
Change Lower Unit Oil
You’ll need:
- Sufficient quantity of lower unit oil / gear lube oil
- Oil pump
- Drain pan
- Large flat head screwdriver
- A few rags
We strongly recommend changing your lower unit oil before storage to make sure there is no water in the lower unit’s oil. Any water in the oil could freeze, expand, and crack your lower unit. Also, changing lower unit oil means one less thing to do at the beginning of next season.
First, you’ll need to know how much lower unit oil / gear lube you need to fill your lower unit – check your owner’s manual.
Locate the lower and upper lower unit oil drain holes. With your drain pan in position, first, remove the lower drain screw. The oil will start to drip out into the drain pan.
Next, remove the upper drain screw—oil will drain out quickly. Allow the oil to drain completely.
It is essential that you pay close attention to your lower unit’s oil. If the oil is a milky color, there is water in the lower unit, and you will have significant problems down the road. If this is the case, go directly to your marine mechanic to diagnose the problem and hopefully fix the issue before it causes more damage.
With the oil completely drained, screw in your pump fitting and attach the pump. Remove the cap of your bottle of lower unit oil and screw the pump onto the bottle.
Pump until oil begins flowing out of the upper drain hole.
Insert your upper screw plug, hand tighten, then tighten securely with your flathead screwdriver.
With the lower screw in your hand, remove the oil pump fitting. Quickly insert and tighten the lower screw.
Wipe everything down and properly dispose of your old oil lower unit oil.
**Caution**
There must be no water in your lower unit oil. If there is water in the lower unit oil and your boat is stored in the cold, the water in the oil will freeze and crack your lower unit, which is definitely a bad thing.
Disconnect Your Battery
You’ll need:
- Battery terminal cleaner and protector
Disconnect your battery and spray a marine battery terminal protector on your battery terminals.
Frequently asked questions about winterizing a MerCruiser inboard outboard motor
How do you winterize an inboard outboard boat motor?
To winterize an inboard outboard boat motor, flush the engine, stabilize the fuel, drain all water, fill the engine with antifreeze, fog the motor, change the lower unit oil, and disconnect the battery. This protects internal components from freezing, corrosion, and fuel degradation.
Can I use antifreeze to winterize my MerCruiser 5.0?
Yes, you should run antifreeze through the cooling system after fully draining all water. Use marine-grade antifreeze and run the engine until antifreeze exits the exhaust.
What happens if I don’t winterize my inboard outboard engine?
Failure to winterize can lead to cracked engine blocks, frozen manifolds, fuel system damage, and battery failure. Repairs are often costly and avoidable with basic winterization steps.
Do I need to winterize a MerCruiser 3.0 or 5.0?
Yes. Even if stored indoors, winterizing protects your inboard outboard engine from hidden water pockets that can freeze and damage internal parts.
How do I winterize a boat with an inboard outboard motor?
Start by flushing the engine, then stabilize the fuel, drain all water, run antifreeze, fog the engine, change lower unit oil, and disconnect the battery.
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