Fogging an Outboard Engine: What It Is, Why It Matters, and How to Do It Right

If you own a boat with an outboard motor, you’ve probably heard the term fogging—usually in the context of winterization or long-term storage. Fogging an outboard engine is one of those maintenance tasks that sounds optional until corrosion, hard starts, or internal engine damage prove otherwise.

This guide breaks down what fogging an outboard engine is, why it exists, when it’s necessary, and how to do it correctly, even if you’re a newer boat owner.

Disclaimer: The information provided in this article is for general educational purposes only and is not intended as professional mechanical, legal, or insurance advice. Boat maintenance procedures, manufacturer recommendations, and insurance coverage options can vary based on vessel type, engine manufacturer, location, and individual policy terms. Always consult your owner’s manual, a qualified marine professional, or your insurance provider for guidance specific to your situation. Coverage availability and requirements may differ by policy and carrier.

What Does “Fogging” an Outboard Engine Mean?

Fogging an outboard engine is the process of spraying a specialized fogging oil into the engine’s air intake (and sometimes directly into the cylinders) to coat internal metal components.

What Fogging Oil Does

Fogging oil is designed to:

• Displace moisture
• Prevent internal corrosion
• Coat cylinder walls, pistons, and bearings
• Protect metal surfaces during storage

Unlike regular lubricants, fogging oil is intentionally thick and sticky, allowing it to cling to engine parts for months at a time.

Why Fogging an Outboard Engine Exists

Outboard engines are especially vulnerable to corrosion because:

• They operate in humid environments
• Saltwater accelerates oxidation
• Internal components are exposed to air once the engine stops running

What Happens If You Don’t Fog

When an engine sits unused:

• Moisture can settle inside cylinders
• Bare metal surfaces begin to oxidize
• Pistons and rings may stick
• Engines may crank poorly—or not at all—after storage

Fogging exists to prevent internal damage you can’t see until it’s too late.

When Should You Fog an Outboard Engine?

Fogging isn’t required after every outing, but it is strongly recommended in specific situations.

You Should Fog Your Engine If:

• The boat will sit unused for 30+ days
• You’re storing the boat for the offseason
• You run your boat in saltwater
• You’re winterizing the engine
• The boat will be stored in a humid environment

Even in warmer climates, long periods of inactivity can still allow internal corrosion to form.

How to Fog an Outboard Engine (Step-by-Step)

There are a few variations depending on engine type and manufacturer, but this method works for most modern outboards. Always be sure to check the details for your specific model of engine for what is required.

What You’ll Need

• Marine fogging oil (aerosol)
• Basic hand tools (if removing spark plugs)
• Fuel stabilizer (recommended)
• Owner’s manual (always helpful)

Step 1: Warm Up the Engine

Run the engine until it reaches normal operating temperature. This:

• Evaporates existing moisture
• Helps fogging oil spread evenly

If you’re on land, use flushing muffs and a water hose.

Step 2: Locate the Air Intake

Most outboards allow fogging through:

• The air intake
• Throttle body
• Carburetor throat (older engines)

Your manual will confirm the best access point.

Step 3: Spray Fogging Oil While the Engine Is Running

With the engine running at idle:

• Spray fogging oil steadily into the intake
• Expect the engine to smoke heavily
• Continue spraying until the engine begins to stumble

This ensures oil coats internal components while they’re moving.

Step 4: Stall the Engine With Fogging Oil

For maximum protection:

• Spray until the engine stalls and shuts off
• This leaves a heavy protective coating inside the cylinders

Step 5 (Optional but Recommended): Fog Through Spark Plug Holes

For long-term storage:

• Remove spark plugs
• Spray fogging oil directly into each cylinder
• Rotate the engine by hand if possible
• Reinstall plugs (do not overtighten)

Two-Stroke vs Four-Stroke Fogging Differences

Two-Stroke Outboards

• Often simpler to fog
• Already oil-mixed, but still benefit from fogging
• Carbureted models are especially vulnerable to corrosion

Four-Stroke Outboards

• More internal components
• Fogging is still important despite oil systems
• Follow manufacturer guidance closely

Some newer four-strokes have manufacturer-specific fogging procedures, so always double-check.

Common Fogging Mistakes to Avoid

❌ Fogging a Cold Engine

Oil won’t distribute evenly and moisture may remain trapped.

❌ Using Regular Lubricants

Only use marine fogging oil, not WD-40 or engine oil.

❌ Skipping Fuel Stabilization

Fogging protects internals—fuel stabilizer protects the fuel system.

❌ Forgetting About Recommissioning

Expect rough starts and smoke when restarting. This is normal.

Does Fogging Affect Boat Insurance?

Fogging is considered routine maintenance, not a repair. Damage caused by corrosion or long-term neglect may not be treated the same as sudden accidental damage under a boat insurance policy.

Regular maintenance, including fogging, can help reduce the risk of preventable engine issues that may fall outside typical coverage scenarios.

How Often Should You Fog an Outboard Engine?

• Seasonal storage: Every offseason
• Saltwater boats: Anytime the engine will sit unused
• Infrequent use: Every 1–3 months if idle

If in doubt, fogging more often is safer than skipping it.

Final Thoughts: Is Fogging Worth It?

Fogging an outboard engine is a low-cost, low-effort maintenance step that can help protect one of the most expensive components on your boat. Whether you’re storing for winter, taking a break from boating, or just planning ahead, fogging helps preserve engine longevity and reliability.