The Truth About Marine Fuel Stabilizers: Why They Matter, How They Work, and the Best Ones to Use for Winter Storage

When boating season winds down and you start preparing for off-season storage or winterization, there’s a long list of tasks to knock out—oil changes, lower-unit inspection, removing gear, cleaning the interior, charging batteries, and more. But at Cope Marine, there’s one step we see boat owners skip more than any other:

➡ Treating your fuel with a quality marine fuel stabilizer before storing your boat.

Skipping this simple, inexpensive step is one of the fastest ways to end up with:

• Phase-separated fuel

• Corroded components

• Gummed-up injectors

• Hard-start problems

• Expensive spring repairs

Our service team in O’Fallon, IL and Branson West, MO sees the same pattern every year—boats come out of winter storage with:

• Varnished injectors

• Stuck carburetors

• Corroded fuel pumps

• Rust inside tanks

• Engines that won’t start

And nearly all of it could have been prevented by using a fuel stabilizer.

If you store your boat for winter or go more than 30 days between outings, this guide is essential reading.

What Exactly Is a Marine Fuel Stabilizer?

A marine fuel stabilizer is a multi-function additive formulated to keep gasoline fresh and protect your fuel system during periods of non-use. In marine environments—where moisture, ethanol, and long storage times cause fuel to break down quickly—stabilizers are essential.

A quality marine fuel stabilizer helps:

✔ Prevent oxidation (fuel breakdown)

✔ Reduce fuel evaporation

✔ Resist moisture absorption

✔ Prevent phase separation in ethanol-blended fuel (E10+)

✔ Protect metal components from internal corrosion

Most stabilizers include antioxidants, lubricants, corrosion inhibitors, and moisture-control agents.

Think of a fuel stabilizer as cheap but powerful insurance:

• Keeps fuel fresh

• Protects your entire fuel system

• Prevents rust, corrosion, and buildup

For a few dollars, you potentially save hundreds—sometimes thousands—in repairs.

Why Boats Need Fuel Stabilizer (Especially for Winter Storage)

Boats are far more vulnerable to fuel problems than cars. That’s because:

✔ Boats sit unused for long periods

✔ They have vented fuel tanks that pull in moisture

✔ Temperature swings cause condensation

✔ Most boaters use E10 fuel, which absorbs water

Two issues do the most damage:

1. Fuel Degradation — Fuel Goes Bad Within 30 Days

Gasoline starts to degrade fast, especially in marine environments. Four main factors destroy fuel quality:

Once fuel breaks down, it forms:

• Sticky varnish

• Deposits

• Gumming in injectors and carburetors

This leads to rough idling, poor performance, and hard starts in the spring.

What Is Phase Separation?

If you use E10 gasoline, this part matters.

Ethanol is hygroscopic—it absorbs moisture until it can’t hold any more. Then it separates into two layers:

• A gasoline layer on top

• A corrosive ethanol/water layer at the bottom—where your fuel pickup sits

When your engine pulls from this layer, expect:

• Misfires

• Stalling

• No-start conditions

• Internal corrosion

• Major engine damage

Phase-separated fuel cannot be “fixed.” Once it happens, the fuel must be discarded, and the tank often has to be cleaned.

A stabilizer is your best protection.

2. Corrosion: The Silent Killer of Marine Fuel Systems

When ethanol absorbs water, it forms acetic acid—which is corrosive. Over winter, corrosion slowly eats away components inside:

• Fuel tanks

• Injectors

• Carburetors

• Pumps

• Fuel lines

We’ve seen fuel pumps destroyed simply from sitting with untreated E10 fuel.

A marine fuel stabilizer stops this damage long before it starts.

When Should You Use Fuel Stabilizer?

✔ Add stabilizer at every fill-up during the season

✔ Always treat fuel before winter storage

✔ Treat fuel before any downtime longer than 30 days

This is one of the most affordable, effective ways to protect your boat.

How to Use a Marine Fuel Stabilizer during winterization (Step-by-Step)

Using stabilizer correctly is key:

Step 1: Add stabilizer to the tank

Measure the correct amount based on the manufacturer's treat rate.

Step 2: Fill the tank with fresh fuel

This mixes the stabilizer thoroughly and reduces airspace where condensation forms.

Step 3: Run the engine 5–10 minutes

This pulls treated fuel into:

• Fuel rails

• Injectors

• Carburetors

• Fuel lines

Now your entire fuel system—not just the tank—is protected through winter.

Not all fuel stabilizers are the same. When choosing one:

✔ Choose realistic claims

Avoid products promising:
❌ “removes ethanol”
❌ “reverses bad fuel”
❌ “restores separated fuel”

No additive can fix phase-separated fuel.

✔ Check the treat rate

This tells you how far the bottle goes:

✔ Buy from trusted marine brands

Marine environments require marine-grade additives.

Recommended Marine Fuel Stabilizers (Carried at Cope Marine)

We stock and trust these proven, marine-grade stabilizers that all protect against phase separation:

✅ Mercury Quickstor

OEM-approved for Mercury and MerCruiser engines, providing dependable protection against phase separation and corrosion.

✅ Yamalube Fuel Stabilizer & Conditioner PLUS

OEM-approved for Yamaha outboards, providing reliable protection against phase separation and corrosion.

✅ Star brite Star Tron Enzyme Fuel Treatment

Uses advanced enzyme technology to prevent phase separation.

✅ Evinrude Johnson 2+4 Fuel Conditioner

Ideal for Evinrude/Johnson and many older engines, providing reliable protection against phase separation and corrosion.

✅ STA-BIL 360° Marine

Perfect for long-term storage, preventing phase separation and providing corrosion protection.

✅ K100 Fuel Treatment

Delivers strong moisture control for reliable phase-separation protection and thorough fuel system cleaning.

FAQs About Marine Fuel Stabilizers

Is fuel stabilizer really necessary?
Yes — especially for boats stored longer than 30 days.

Can it fix already bad or separated fuel?
No. Once fuel has degraded or phase-separated, it must be removed.

Will stabilizer harm my engine?
No. When used correctly, stabilizers protect and improve engine life.

Final Thoughts from Cope Marine

Using a marine fuel stabilizer is one of the simplest and most cost-effective steps you can take to protect your investment. A few ounces today prevent:

• Clogged injectors

• Carburetor varnish

• Corrosion damage

• Hard-start issues

• Costly repairs in the spring

When in doubt — stabilize it.

If you’d like help winterizing your boat, or want us to treat and store your boat for you, Cope Marine is here to make off-season care easy and worry-free.

Your boat will thank you when spring arrives.