How to Winterize a Boat Motor: A Step-by-Step Guide for Outboards and Inboards

The Winter I Turned My Engine Into a Paperweight

I'll never forget the sick feeling of walking into my garage in March and seeing a crack in my outboard's lower unit. Not a hairline—a wide, ugly split that meant the whole thing was junk. I'd done everything else: stabilized the fuel, changed the oil, even covered the boat. But I'd forgotten one critical step: I hadn't drained the water from the cooling passages. It froze, expanded, and split the block like a cheap beer can in a freezer. That mistake cost me $3,000 and a whole season of fishing. I'd been boating for years and thought I knew what I was doing. Turns out, winterizing a boat motor isn't just a good idea—it's the difference between a quick launch in spring and a trip to the scrap yard. Since that day, I've winterized dozens of outboards and inboards, and I've learned the process inside and out. This guide is what I wish someone had handed me before that cold, expensive winter. I'll walk you through every step, so you can store your motor with confidence and fire it up next spring without a worry.

Why You Should Trust Me

I'm Jake Morrison. I've run a fishing charter out of Pensacola for 15 years, and I've owned and maintained more outboards than I can count. I've made every winterizing mistake you can imagine—including the one that cracked my engine block—and I've learned from every single one. I don't sell winterization products, and I don't take money from manufacturers. Some of the links in this guide pay me a commission if you buy through them, but that doesn't change what I teach. I'll show you exactly what I do to my own motors every fall. If it works for me, it'll work for you.

What You'll Need

Before you start, gather your supplies. Here's what I keep on hand for winterization:

• Marine-grade antifreeze (propylene glycol, not automotive—it's toxic and bad for the environment).
• Fresh engine oil and filter.
• Fuel stabilizer.
• A garden hose with a flushing attachment (for outboards).
• Fogging oil (for carbureted engines; fuel-injected may not need it).
• Basic hand tools.
• Grease and lubricants.
• Clean rags and a motor cover.

You can find all of these in the filtered Amazon search I've linked above. I use marine-specific antifreeze because it's safe for rubber seals and the environment. Automotive antifreeze is cheaper, but it's not worth the risk—it can eat away at gaskets and hoses over time. Don't cut corners here; your motor's life depends on it.

Step 1: Flush the Engine

Start by flushing the engine with fresh water to remove salt, dirt, and corrosion. For an outboard, attach a flushing muff to the lower unit and run the engine for 5-10 minutes. For an inboard, use the fresh water intake or a flush kit. This is also a good time to inspect the engine for any obvious issues—leaking hoses, loose belts, or corrosion. I do this step every time I use the boat in saltwater, but it's especially important before winter storage. A clean engine is a happy engine. And while it's running, pay attention to the telltale stream—if it's weak, your impeller may be on its way out. Catch it now, not in spring.

Step 2: Change the Oil and Filter

Old oil contains acids and moisture that can damage your engine while it sits all winter. Drain the old oil and replace it with fresh marine-grade oil, and change the filter as well. This is easiest to do when the engine is warm, so do it right after flushing. If you're not comfortable doing this yourself, any marine mechanic can handle it in about 30 minutes. But I've learned to do it myself—it saves money and I know it's done right. Plus, you get to inspect the old oil for metal shavings or water, which can tell you a lot about your engine's health. If you see milky oil, you have a leak that needs attention before spring.

Step 3: Add Fuel Stabilizer

Fuel goes bad over time, especially ethanol-blended fuel, which attracts water. Add a fuel stabilizer to your tank and run the engine for a few minutes to distribute it through the entire system. If possible, fill the tank to about 95% full—this leaves less room for condensation. I use a stabilizer designed for marine engines, and I've never had a fuel-related problem in spring. The one time I skipped this step, I spent an afternoon cleaning gunk out of the carburetor. Don't be like me.

Step 4: Fog the Engine (If Carbureted)

If your engine has a carburetor, you should fog it to protect the internal parts from corrosion. Remove the air intake and spray fogging oil into the carburetor while the engine is running at a fast idle. The engine will smoke, which is normal. For fuel-injected engines, fogging is usually not necessary, but check your owner's manual. I have a fuel-injected outboard, so I skip this step, but I still make sure the fuel system is protected with stabilizer. If you're unsure whether to fog, look in your manual or ask a mechanic. It's a cheap insurance policy against internal rust.

Step 5: Drain or Protect the Cooling System

This is the step that cracked my engine block, so pay close attention. Water left in the engine's cooling passages can freeze and expand, causing cracks. You have two options: drain the water completely, or replace it with antifreeze. For most boats, the best method is to run non-toxic marine antifreeze through the cooling system until it comes out of the exhaust or telltale. This ensures any remaining water is displaced. Follow your engine's specific procedure, and don't skip this step. If you're unsure, a marine mechanic can do it for a small fee—far less than an engine block. I use a kit that makes it easy to pump antifreeze through the motor. The link above includes the exact kit I use. This is the most important step of the entire winterization process. If you only do one thing, do this.

Step 6: Remove the Battery and Store It Properly

A battery left in a freezing boat will discharge and can be damaged. Remove the battery, clean the terminals, and store it in a cool, dry place. Charge it fully before storage, and top it up every month or two with a smart charger. I wrote a detailed guide on how to maintain your boat battery—follow that advice and your battery will last much longer. A dead battery in spring is one of the most common frustrations for boaters, and it's completely preventable. I always bring my battery indoors and put it on a maintenance charger. It's cheap and easy.

Step 7: Grease and Protect Moving Parts

Before you tuck the motor away, hit all the grease fittings—steering, shift linkage, and tilt tube—with marine grease. This prevents corrosion and keeps everything moving freely. Also, spray a corrosion inhibitor on the entire engine, especially the powerhead and lower unit. I use a product designed for marine engines, and I give the whole motor a light coating. It's like putting a raincoat on your engine for the winter. If you have a metal prop, consider removing it and coating the shaft with grease to prevent seizing. I always do this—it makes spring maintenance much easier. And don't forget the trailer! I have a trailer maintenance guide that covers the basics, but at minimum, check the tire pressure and grease the bearings before parking it for the winter.

Common Mistakes That Will Ruin Your Spring

I've made most of these, so learn from my errors:

• Skipping the antifreeze. Water freezes. It expands. It cracks metal. This is the one step you cannot skip if you live anywhere with freezing temperatures. I learned this the hard way, and I don't want you to repeat it.
• Using automotive antifreeze. It's toxic and can damage marine engines. Use marine-grade propylene glycol. The difference is important, and the cost difference is small. I always buy the marine stuff, even though it's a few dollars more.
• Leaving the battery connected. Parasitic draw will drain it, and freezing will damage it. Remove and store it properly. I've seen too many dead batteries in spring because someone left them in the boat. It's a simple step that saves a lot of hassle.
• Covering the boat with a non-breathable tarp. Moisture gets trapped, mold grows, and your interior is ruined. Use a proper boat cover. I have a guide to the best boat covers if you need one. A breathable cover is worth every penny.
• Forgetting the fuel stabilizer. Ethanol fuel absorbs water and turns to varnish. Stabilize it or drain it. I've had to clean out carbs that were full of green goo because someone skipped this. It's not a fun job.

When to Call a Professional

If you're not comfortable doing any of these steps, or if you have a complex inboard engine with a closed cooling system, hire a marine mechanic. The cost of professional winterization (usually $150-$300) is far less than the cost of a cracked block or a seized engine. I do my own winterization now, but the first year I paid a mechanic to do it while I watched and learned. There's no shame in that. In fact, it's a great way to learn the process on your specific engine. I've had friends who hired a mechanic once and then did it themselves for years after. If you're unsure about anything, ask. It's cheaper than a repair.

Also, if you notice anything unusual during the process—milky oil, metal shavings, strange noises—stop and get a professional opinion. Winterization is not the time to ignore warning signs. A small problem caught now is a big problem avoided in spring.

Related Reading on Boat World

Now that your motor is winterized, you might want to tackle other maintenance tasks. I've written several guides that can help.

If your battery needs replacing or upgrading, check out my best boat battery guide—it compares lithium and AGM options and explains which is worth the money. And if you're new to boat maintenance overall, read my beginner's maintenance guide for a simple monthly checklist that will keep your boat running smoothly all season long. Both articles include links to the exact products I'd recommend, with the same filters I use when buying for my own boat.

Winterizing is one of those tasks that feels like a chore, but when spring comes and your boat starts on the first turn of the key, you'll be glad you did it. A few hours of work now saves you weeks of frustration later. I've been on both sides of that equation, and I'll take the few hours every time.

Frequently Asked Questions

Do I need to winterize if I live in a warm climate?
Even if temperatures rarely drop below freezing, winterizing is still a good idea. It protects the engine from corrosion, keeps fuel fresh, and ensures the boat is ready to go in spring. At a minimum, flush the engine, stabilize the fuel, and charge the battery. I have friends in Florida who winterize every year, and their engines last significantly longer than those who don't. It's not just about freezing; it's about preventing corrosion and condensation. I recommend it regardless of your climate.

Can I use automotive antifreeze in my boat motor?
No. Automotive antifreeze contains ethylene glycol, which is toxic and harmful to the environment. Marine antifreeze uses propylene glycol, which is non-toxic and safe for marine engines. It also has additives that protect rubber seals and gaskets. The price difference is small, and the risk isn't worth it. I always keep a few gallons of marine antifreeze on hand. It's also useful for winterizing plumbing systems on larger boats.

How long does it take to winterize a boat motor?
For a small outboard, the whole process takes about 1-2 hours, including flushing, oil change, and fogging. An inboard may take longer, especially if it has a closed cooling system. I usually set aside a Saturday afternoon to do my outboard and the trailer. It's a small investment of time for a huge payoff in reliability. I do it every fall without fail.

What happens if I don't winterize my boat motor?
At best, you'll have fuel and battery problems in spring. At worst, water left in the engine will freeze and crack the block, requiring a replacement that can cost thousands. I've seen it happen too many times. Even if you live in a mild climate, corrosion can set in and damage internal components. The cost of winterization is a fraction of the cost of a new engine. It's a no-brainer.

Should I drain the fuel tank or leave it full?
It's better to leave the tank about 95% full with fuel stabilizer added. A full tank leaves less room for condensation, and the stabilizer prevents the fuel from going bad. Draining the tank completely can cause seals to dry out. I've been doing this for years and have never had a fuel issue. Just make sure you run the engine for a few minutes after adding stabilizer to circulate it through the system. I also add stabilizer to my portable tanks before storing them.

Written July 2026. Winterization procedures and products may vary by engine type. Always consult your owner's manual for specific instructions.