How to Winterize Inboard Boat Engine: I/O and Stern Drive Preparation
Your inboard engine doesn’t care about your budget—it’ll crack if you skip winterization. One hard freeze is all it takes to turn coolant passages into expensive scrap metal. But here’s the good news: winterizing an inboard or stern drive isn’t rocket science. You just need the right steps, some basic supplies, and about two hours. Let’s get your engine ready for winter without paying a mechanic $300 to do it.
Why Winterizing Your Inboard Actually Matters
Water expands when it freezes. Everyone knows this, right? But inside your engine, that expansion doesn’t just push things aside—it cracks engine blocks, destroys heat exchangers, and splits exhaust manifolds. A single freeze can cause $2,000-$5,000 in damage, and insurance won’t always cover neglect.
The Difference Between I/O and Inboard Systems
Inboard/outboard (stern drive) systems have components both inside and outside your hull. The engine sits inside, but the drive unit hangs out back like an outboard. This means more spots where water hides. Straight inboards keep everything internal, but they’ve still got cooling systems, exhaust routes, and transmission coolers that need attention.
Both types need winterization, but stern drives require extra work on that drive unit. We’ll cover both scenarios so you’re set either way.
What You’ll Need Before Starting
Don’t halfway through the job and realize you’re missing something. Here’s your shopping list.
Essential Supplies
- Marine antifreeze (propylene glycol, NOT automotive antifreeze) – 3-5 gallons
- Engine fogging oil – 1-2 cans
- Gear lube for lower unit (stern drives only)
- Fuel stabilizer
- Oil and filter for oil change
- Basic tools: wrenches, screwdrivers, pliers
- Bucket or large container
- Garden hose or winterizing flush kit
Pro tip: Buy the pink RV/marine antifreeze, not the green automotive stuff. Automotive antifreeze is toxic and will destroy your engine’s rubber components. Marine antifreeze is non-toxic and safe for your cooling system.
Optional But Helpful Items
- Shop vacuum for sucking out stubborn water
- Pump for circulating antifreeze
- Spare hose clamps
- Corrosion inhibitor spray
- Rags and cleaning supplies
Step-by-Step Winterization Process
Alright, let’s do this. Work methodically, and don’t skip steps because you’re in a hurry. Winter doesn’t care about your schedule.
Step 1: Run the Engine and Change the Oil
Start your engine and let it warm up to operating temperature. Warm oil flows easier and carries more contaminants out with it. While it’s running, add fuel stabilizer to your tank—use the amount recommended on the bottle for your tank size.
Once warm, shut down and change your oil and filter immediately. Used oil contains acids and moisture that’ll corrode internal parts all winter. Fresh oil provides a protective coating instead.
Step 2: Flush and Drain the Cooling System
This is where things get real. Your cooling system holds water in multiple places, and you need to evacuate all of it.
For raw water cooled systems (water comes straight from the lake):
- Disconnect the raw water intake hose
- Place it in a bucket of marine antifreeze
- Start the engine and let it draw antifreeze through until you see pink coming from the exhaust
- Shut down immediately
For closed cooling systems (like a car’s radiator):
- Drain the coolant from the engine block drain plugs
- Open the petcocks on the manifolds if equipped
- Remove the raw water pump cover and drain it
- Refill with fresh 50/50 antifreeze mix
Step 3: Protect the Engine Internals
While the engine’s still accessible, pull the spark plugs or injectors. Spray fogging oil into each cylinder—about 2-3 seconds per cylinder. This coats everything in a protective film that prevents rust.
Have someone bump the starter (with the kill switch engaged) to distribute the fogging oil. You’ll see smoke, which is normal. That’s the fogging oil doing its job.
Step 4: Winterize the Stern Drive Unit
If you’ve got a stern drive, the lower unit needs attention. It’s got its own oil that can trap water.
| Task | Why It Matters | Time Required |
|---|---|---|
| Drain lower unit gear oil | Water contamination shows up as milky oil | 10 minutes |
| Inspect for metal particles | Signs of bearing or gear wear | 5 minutes |
| Refill with fresh gear lube | Protects gears during storage | 10 minutes |
| Grease all fittings | Prevents corrosion on pivot points | 5 minutes |
To drain the lower unit: Remove both the upper and lower drain plugs. Let it drain completely into a container. If the oil looks milky or has water droplets, you’ve got seal issues to address before next season.
Refill from the bottom up using a pump. When oil appears at the top hole, you’re full. Replace the top plug first, then the bottom.
Step 5: Treat the Fuel System
Stale fuel gums up carburetors and injectors. You’ve already added stabilizer, but now run the engine long enough to circulate treated fuel through the entire system—about 10 minutes should do it.
Some folks drain their tanks completely, but modern ethanol fuel actually stores better with a full tank. A full tank prevents condensation from forming on the tank walls.
Step 6: Final Touches and Checks
Almost done. Now for the finishing details that separate amateurs from pros.
Spray fogging oil into the carburetor or throttle body while the engine runs. This coats the intake tract and intake valves. The engine will smoke heavily and probably stall—that’s fine.
Remove your boat’s battery and store it somewhere above freezing. Batteries on concrete floors in cold garages go dead faster. Trickle charge it monthly if possible.
Wipe down the engine with a light coat of oil or corrosion inhibitor. This prevents surface rust on unpainted parts.
Common Mistakes That’ll Cost You
I’ve seen people make these errors, and they always regret it.
Using Automotive Antifreeze
Seriously, don’t. Marine antifreeze costs a bit more, but automotive antifreeze contains ethylene glycol that’s toxic, corrosive to aluminum, and incompatible with rubber seals. It’ll cost you way more in repairs than you saved on antifreeze.
Not Running Antifreeze Long Enough
You need to see pink antifreeze coming from your exhaust. If you shut down too early, water pockets remain in the system. Those pockets freeze and crack components.
Forgetting Hidden Water Spots
Water hides in weird places: shower sumps, livewell pumps, washdown systems, and air conditioning heat exchangers. Anything with a water line needs winterizing or draining.
Skipping the Transmission Cooler
Many inboards have transmission oil coolers that use raw water. These need draining or antifreeze treatment just like your engine. Check your owner’s manual for the drain locations.
When to Call a Professional
Look, I’m all for DIY boat maintenance. But some situations call for expert help.
If you’ve never winterized before, consider having a mechanic do it once while you watch and ask questions. That hands-on learning is worth the service fee.
If your engine has complicated systems like turbochargers, multiple heat exchangers, or you’re not confident about any step, get help. A $200 service call beats a $3,000 repair bill.
And if you discover milky oil in the lower unit or other problems during winterization, address them now with professional help. Don’t store a boat with known issues.
Dewinterizing Next Spring
When spring fever hits, you can’t just turn the key and go. You’ll need to reverse this process.
Reinstall the battery, check all fluid levels, and change your lower unit oil again if you found water last fall. Inspect all hoses and belts for winter damage. Run the engine on muffs or in the water, checking for leaks and proper operation.
Take it slow the first time out. Listen for unusual sounds, watch your temperature gauge, and test everything before you head offshore.
FAQ
Q: Can I just use RV antifreeze in my engine?
A: Yes, that’s exactly what marine antifreeze is. RV and marine antifreeze are the same product—propylene glycol based and non-toxic. Just avoid automotive antifreeze.
Q: How much antifreeze do I actually need?
A: Most inboard engines need 3-4 gallons to fully flush the cooling system. Stern drives might need 4-5 gallons since they have more components. Buy extra—leftover antifreeze keeps until next year.
Q: Do I need to fog my fuel-injected engine?
A: Yes. Fogging oil protects cylinder walls regardless of how fuel enters. Pull the injectors or spray it through the throttle body. Either way works.
Q: What if I don’t have winterization drain plugs on my engine?
A: Some engines don’t have convenient drains. You’ll need to circulate antifreeze through the cooling system using a pump or by drawing it through the raw water intake. Check your owner’s manual for the recommended method.
Q: Is one freeze really enough to crack my engine block?
A: Absolutely. Water expands about 9% when it freezes, which creates thousands of pounds of pressure. Cast iron and aluminum can’t handle that kind of force. Even one night below 32°F can cause catastrophic damage if water’s still in there.
