How to Winterize an Outboard Motor in North Dakota

Winterizing an outboard motor means getting the water out, treating the fuel, coating the inside of the engine with oil, changing the gearcase lube, and storing the motor upright with the battery indoors. In North Dakota that work fits between the last run on Sakakawea and the first hard freeze, a window that closes fast once October nights drop below freezing. Here is the full sequence, in order, and the points where a factory-certified technician belongs in it.

Why a North Dakota winter is different for an outboard

Water expands as it freezes. Any water left inside an outboard's cooling passages, water pump housing, or gearcase turns to ice, and the ice pushes outward on aluminum castings that were never built to stretch. The result shows up in May as a cracked block, a split water pump housing, or a gearcase weeping lube: damage that happened on one November night and sat hidden all winter.

Minot and Bismarck get hard freezes in the fall while the lakes are still open. Sakakawea fishes deep into October, and the same cold snap that puts walleye on the bite can freeze a motor left trimmed up on a trailer overnight. Fall winterization at Vallely runs September through November. The closer to the first hard freeze, the longer the line.

Step 1: Treat the fuel and run it through

Fill the tank most of the way and add marine fuel stabilizer at the dose on the bottle. A fuller tank leaves less air inside, and less air means less condensation forming through a winter of temperature swings. Then run the motor on a hose and muffs, or on the lake during the last trip, long enough for treated fuel to reach the carburetors or injectors. Stabilizer sitting in the tank protects the tank. Stabilizer that has reached the fuel system protects the parts that gum up.

Ethanol-blended gas absorbs moisture over a long storage period and can separate. Non-oxygenated gas, the ethanol-free pump labeled for boats and small engines, takes that variable out of the storage question. Replace the fuel-water separating filter while you are at it so the spring startup pulls through a clean element.

Step 2: Flush, then drain every drop

Flush the cooling system with fresh water on the muffs or through the flush port. Missouri River and Sakakawea water carry silt and sand that settle in the passages, and a flush moves it out before it dries in place. Then shut the motor down and leave it vertical. An outboard drains its cooling water through the leg when it is trimmed all the way down. Trimmed up, water pools in the water pump housing and the lower passages and waits for the first freeze.

Trailer home with the motor tilted if road clearance requires it, then trim it down the moment the boat is parked and leave it there for the winter.

Step 3: Fog the engine

With the motor warm and running on the hose, spray fogging oil into the air intake until the engine stumbles and the exhaust turns white, then shut it off. Pull the spark plugs, spray fogging oil into each cylinder, turn the flywheel by hand a few times to spread the film across the cylinder walls and rings, and reinstall the plugs. That oil film is what stands between bare steel and a winter of damp, cold air inside the powerhead.

Owner manuals differ on where the fogging oil goes, and some four-stroke manufacturers want it through the plug holes only. Follow the manual for your engine.

Step 4: Change the gearcase lube and read it

Put a pan under the lower unit, remove the bottom drain screw, then the top vent screw, and watch what comes out. Fresh lube drains clean and dark. Milky, gray, or creamy lube means water is getting past a seal, often a prop-shaft seal cut by fishing line picked up on Devils Lake or Oahe. Water inside a sealed gearcase freezes solid and cracks the housing. A seal is a service job. A cracked gearcase is a different conversation.

Refill from the bottom hole with the manufacturer's gear lube until it runs out the top vent, install the top screw first, then the bottom, with new gaskets on both. Pull the prop while you are there, clear any line wrapped on the shaft, and grease the splines before it goes back on.

Step 5: Oil, grease, and the battery

Four-stroke outboards get an engine oil and filter change before storage, so a season of acids and combustion byproducts is not sitting on the bearings all winter. Hit every grease fitting: steering, tilt tube, swivel bracket. Spray the powerhead with a corrosion inhibitor and touch up any paint chips on the leg.

Disconnect the battery and bring it inside to a cool, dry spot on a maintainer. A fully charged battery resists freezing. A discharged one freezes at a far higher temperature and comes out of winter ruined. Clean the terminals now rather than in May.

Step 6: Cover and store

Motor vertical, cowl on, a cover that breathes. Pull the hull drain plug so snowmelt runs out instead of sitting in the bilge, and set a mouse deterrent in the boat. Indoor storage, out of the wind and off the snow load, is the easiest winter an outboard can have.

When to hand it to a technician

Everything above is routine maintenance on a healthy motor. Milky gear lube, an overheat alarm during the season, hard starting, a fuel smell, or an electrical gremlin are reasons to have the engine looked at before it sits, since a problem that goes into storage comes out of storage worse.

Vallely's service departments in Minot and Bismarck handle marine winterization, gearcase lube, oil and filter service, battery inspection, and trailer bearing repacks for boats, pontoons, jet boats, and WaveRunners, with factory-certified technicians at both locations. Fall winterization runs September through November, and the spring side of the season starts with Spring Splash, Vallely's launch program, in May.

Service requests for either location go through the service appointment form on vallelymarine.com: https://www.vallelymarine.com/schedule-a-service-appointment--xservice_request

Vallely Sport & Marine. Minot and Bismarck, ND.

Frequently asked questions

When should I winterize an outboard in North Dakota?

Right after the last trip of the season and before the first hard freeze, which in Minot and Bismarck commonly arrives while the lakes are still open in October. A motor left trimmed up on the trailer with water inside can crack in a single cold night. Fall winterization at Vallely runs September through November, and the line is longest right before the freeze.

Does a four-stroke outboard need to be winterized?

Yes. Fuel treatment, draining the cooling water, fogging the cylinders, and fresh gearcase lube apply to every outboard. A four-stroke adds an engine oil and filter change before storage so used oil is not sitting on the bearings all winter. Owner manuals differ on where the fogging oil goes, so follow the one for your engine.

What happens if water freezes inside an outboard motor?

Water expands as it freezes and the aluminum castings around it do not. Water left in the water pump housing or cooling passages can split the housing or crack the block, and water inside the gearcase cracks it from the inside. The damage usually shows up at the first spring start, months after it happened.