Fogging a Boat Engine for Winter Storage

Fogging a boat engine means coating the inside of the engine with fogging oil before storage, so the cylinder walls, piston rings and bearings are not left bare to moisture all winter. In the general method Discover Boating describes, fogging oil is sprayed into the air intake of a warm, running engine until it stumbles or stalls, then into each cylinder through the spark plug holes. Many modern engines do it differently: some fuel-injected four-strokes take a little oil through the plug holes with the engine off, some direct-injection outboards fog themselves in a winterization mode, and some manuals do not ask for fogging at all. The method in your owner’s manual is the one to use.

What fogging oil does

Discover Boating, the boat industry’s consumer site, explains the problem: while an engine sits, its oil drains down to the oil pan and leaves the cylinder walls, piston rings and crankshaft bearings exposed to moisture. Fogging oil adds a protective layer that stays on metal longer than engine oil. One direct-injection outboard maker’s guide puts the cause in one line: “Temperature and humidity changes during storage period can cause corrosion of internal engine components.”

Fogging protects the inside of the engine against rust. It does nothing for the fuel, the cooling water or the gearcase; those are separate steps in a lay-up, covered in how to winterize an outboard motor.

When to fog, and when the manual says otherwise

Discover Boating suggests fogging when the boat will sit for more than 60 days, and notes that in places with temperature swings, surface rust can form in as little as three weeks. Those are general figures; the engine maker’s manual is the authority for your engine.

Read the manual’s storage section before you buy anything, and check for three cases where a generic can and the general method below are the wrong approach:

• The engine has its own winterization mode. Discover Boating notes that some modern direct-injection engines have automatic fogging modes, and that modern outboards may use a fuel additive or an electronic “winterize” mode instead of fogging.
• The manual uses oil through the spark plug holes instead. One fuel-injected four-stroke manual does exactly that, with the engine off.
• The manual names a specific fogging product or entry point. On fuel-injected engines, Discover Boating warns against spraying oil onto sensitive sensors such as the mass air flow sensor.

Fog only after the fuel is treated and has been run through the engine; see boat fuel over winter: stabilizer, ethanol and the tank.

How to fog an engine: the general method

This is Discover Boating’s general method; its winterizing guide says older carbureted and two-stroke outboards may need fogging oil, while modern ones may use a fuel additive or a winterize mode instead. Check each step against your manual, which may change the order, the amount or the entry point.

1. Stabilize and run the fuel. Treated fuel should already be through the fuel system before the engine is fogged.
2. Warm the engine on water. Run it in the water or on the flushing device the manual allows until it is warm. Discover Boating lists fogging a cold engine as a common mistake.
3. Spray into the intake while it runs. Spray fogging oil into the air intake or throttle body, at the entry point the manual shows. What to look for: the engine stumbles and smokes white; on many engines, keep spraying until it stalls.
4. Turn the ignition off.
5. Oil each cylinder. Remove the spark plugs and spray fogging oil into each cylinder.
6. Turn the engine over a few times without starting it, to spread the oil on the cylinder walls.
7. Refit the spark plugs and their caps, and anything you removed to reach the intake.

Fuel-injected four-stroke outboards: where makers part ways

This is the case owners search for most, and the sources do not agree. Each position below comes from a maker’s own document, attributed by kind, and none applies automatically to your engine.

• Oil through the plug holes, engine off. One outboard maker’s owner’s manual for a fuel-injected four-stroke drains the fuel system and then says: “Remove the spark plugs and put a teaspoon of engine oil or spray storage oil into the combustion chamber through the spark plug holes,” and “Turn the starter motor several turns to lubricate inside the cylinder.”
• A fogging oil made for these engines, with water supplied by flush muffs or a flush bag. Another outboard maker sells one fogging oil for “most modern fuel-injected four-strokes” and a different one for its two-stroke fuel-injected and carbureted outboards. That maker’s blog says: “The flush muff and flush bag methods are the only ones approved for fogging the engine for storage.” Muffs fit over the lower-unit inlets on models with one intake.
• Silent on fogging. A third outboard maker’s storage pages do not mention fogging and send owners to the owner’s manual for model-specific storage.

Discover Boating’s advice for fuel-injected engines applies whichever method the manual uses: keep oil off sensors such as the mass air flow sensor, and consult the owner’s manual for the correct entry point. The Virginia wildlife agency’s winterizing tips recommend having a brand-certified mechanic check the rig, especially a fuel-injected four-stroke.

Direct-injection outboards that fog themselves

One direct-injection two-stroke outboard has a long-term storage mode instead of hand-sprayed fogging; the operator’s guide says the engine is “fogged” and extra oil coats its internal parts. In outline, from that guide:

1. Add the maker’s fuel conditioner and fill the tank; an empty tank is one of the listed reasons the engine “will not winterize”.
2. Put the outboard in neutral. In the water, the water intake screens must be completely submerged; on a trailer, remove the propeller and attach a garden hose to the flushing port.
3. Start the engine at fast idle and work the fast idle lever in the timed sequence the guide gives (at least 15 seconds, then another 15 seconds, on its side-mount remote control).
4. The engine accelerates, fogs itself, and shuts itself off, after one to two minutes. If the engine exceeds 3000 RPM, the guide says, it may have been shifted into gear: turn the key off at once and start the procedure again.
5. Do not start it again before storage. Leave it vertical until the powerhead has drained.

The sequence differs by control type and model, so use your guide’s version. The same guide notes that excess oil may appear on the skeg after this procedure, which is normal.

Inboards and sterndrives

On a gasoline inboard, one inboard engine maker’s winterization sheet fogs through the throttle body or carburetor: with the engine at operating temperature, lift the flame arrestor, “Pump or spray 4-6 ounces of oil into the throat of the carburetor or throttle body. Shut off the engine. Reattach the flame arrestor.” The same sheet warns: “Running the engine with the flame arrestor removed increases the possibility of fire or explosion if an engine backfire should occur and gasoline fumes are present.” Keep the time without it as short as the job allows, and never leave the flame arrestor off; Coast Guard minimum equipment rules, as summarized by the BoatUS Foundation, require an approved backfire flame arrestor on each carburetor of an inboard gasoline engine.

For sterndrives, Discover Boating says to warm the engine before fogging. Draining or filling the cooling system, the outdrive and the rest of the lay-up are in how to winterize an inboard or sterndrive.

The methods side by side

Water to the engine while it runs

Any method that runs the engine needs water to it: in the water, in a test tank, or on the flushing device your manual approves. Makers disagree about running on hose devices. One maker approves flush muffs and a flush bag for fogging. Another maker’s manual says not to run the engine on its hose-connector flushing attachment at all, and runs it only in water or in a test tank with the propeller removed and the water level over 4 inches above the anti-ventilation plate. Find what your manual allows before you start the engine.

Safety

• Outdoors only. Exhaust contains carbon monoxide. One outboard maker’s manual says to never start or operate the engine indoors or in any space that is not well ventilated; the CDC calls carbon monoxide a colorless, odorless gas that can poison or kill. Fogging makes the engine smoke on top of its exhaust, so a garage or shed is the wrong place.
• Fire. Discover Boating says to fog in a well-ventilated area, keep a fire extinguisher nearby, and allow no open flames or sparks. Fogging oil is sprayed into a running engine, and fuel is present.
• The boat. Discover Boating also says to support the boat properly before starting the engine; on a trailer, check it is secure and the motor is clear to run.
• Moving parts. Keep hands, hose and spray tube clear of the propeller and belts, and remove the propeller when your manual’s procedure says to.

Common mistakes

• Fogging a cold engine (Discover Boating).
• Using a household spray lubricant or standard motor oil instead of fogging oil (Discover Boating).
• Spraying oil onto an air flow sensor on a fuel-injected engine instead of the entry point in the manual.
• Hand-fogging an engine whose manual uses a built-in mode or plug-hole oil.
• Starting the engine again after fogging “just to check”: one direct-injection maker’s guide says not to start it again after winterization is completed.
• Running it without water, even briefly.

Smoke and oil at the spring start

Expect it. One direct-injection outboard maker’s guide says an engine that was auto-winterized “will emit a puff of smoke on the first start of the season”, and that oil seen on the skeg after the storage procedure is normal; its pre-season check warns not to confuse that engine oil with gearcase lubricant, which is thick and clear. One four-stroke manual’s first-start routine warms the engine for 3 minutes in neutral, runs it for 5 minutes at the slowest speed, then for 10 minutes at half throttle, so the oil used for storage is circulated out. Smoke that keeps coming after that, or a rough idle that does not clear, is a reason to see a mechanic. The full spring routine is in getting a boat ready in spring after winter storage.