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Fuel basics

Caring for the fuel in your Mercury Marine engine and properly maintaining your fuel system can help ensure many seasons of reliable service.

Basic fuel care for boaters

Protect your engine

Mercury Marine recommends following these basic precautions to protect your engine:

buy petrol from known and trusted sources whenever possible. A busy fuel dock or filling station is likely to dispense fresh fuel.  

Keep your fuel tanks clean and check them periodically for water and sediment. 

Check your fuel filter often. Fuel filters help prevent fuel contaminants from entering your engine.

A pro tip How to check for water in fuel

There are several methods to check for the presence of water or phase separation in your fuel tank. One simple way is to siphon or drain a few ounces of fuel into a clear glass container and let it to sit for about 30 minutes to allow the fuel to settle. Because water is denser than petrol, it will form a clear bubble or layer at the bottom of the container. Phase separation will be evident by the formation of an opaque, gelatinous layer below the petrol. Another method is to apply water-finding paste to a wooden stick or dowel and submerge it down to the bottom of the fuel tank. The paste will change colour if it comes in contact with water.

Use of ethanol-blended fuel

All current Mercury petrol engines are designed to operate safely on fuel with a maximum of 10 percent ethanol (known as E10). Avoid using fuel with more than 10 percent ethanol (such as E15 or E85) in your Mercury engine. The metal, rubber and plastic components in the engine and fuel system are not designed to tolerate the corrosive effects of an ethanol blend higher than 10 percent. When possible, Mercury recommends using petrol containing no ethanol. Use a tool such as the Quicksilver Fuel Tester (PN 91-879172T28) to accurately measure the percentage of ethanol in your fuel if you have doubts.

Phase separation

Ethanol in petrol attracts water and can absorb moisture from the atmosphere or marine environment through the fuel tank vent system. In time, it is possible for the ethanol to absorb enough moisture that water, which is heavier than petrol, will drop to the bottom of the fuel tank. This is called phase separation. If this water/ ethanol phase reaches the fuel inlet it can be drawn into the engine and cause poor performance or even damage. The fuel remaining in the tank will then have a lower octane rating, which can also damage the engine.

To avoid phase separation

Purchase ethanol-free gasoline when possible.

Keep the fuel tank filled between outings or before long-term or offseason storage, especially if your boat is stored outdoors or if you have an older boat with a fuel system that is vented to the atmosphere. This limits the amount of moisture-laden air that is in the tank and can help reduce the formation of condensation in the tank. Newer boats with non-vented, low evaporative emissions fuel systems are less susceptible to the buildup of condensation and can usually be safely stored with low fuel levels.

Mercury fuel care system

The fuel in your boat requires special care to keep your engine running at peak performance. In as little as two weeks, fuel components can begin to oxidise, forming a gum-like substance that settles in fuel lines and tanks, carburettors and injectors. When the engine tries to burn this fuel, deposits can form in the combustion chamber. Over time, these deposits build up and reduce the engine’s performance. Older engines with carburettors are especially prone to problems caused by these deposits, which can clog small jets and passages in the carburettor.


Use this complete fuel treatment when you fill your tank with new fuel. Quickare maximises engine performance with all grades of petrol.

• It helps prevent corrosion, gum and varnish build-up.

• It helps control moisture and the corrosion problems associated with ethanol-blended fuel.

• It cleans existing varnish and gum deposits from fuel lines, tanks, carburettors and injectors, intake valves, and spark plugs.


Use this deep-cleaning fuel treatment periodically during the boating season. Quickleen contains aggressive cleaners that quickly and thoroughly remove carbon deposits from carburettors and injectors, intake valves, spark plugs, piston crowns and cylinder heads.

• It prevents engine knocking and piston seize-up.

• It extends spark plug life.


Use this additive before short or long-term (offseason) storage. Quickstor stabilises fuel for up to a year and keeps fuel ready for use when it is time to put your boat back on the water. For best results, always treat fresh fuel before storage.

• This helps prevent regular and ethanol-blended fuel from breaking down and oxidising.

• It reduces fuel system corrosion.

• It helps keep gum and varnish from forming in the fuel and settling in fuel lines and tanks, carburettors and injectors.

• It keeps injectors and carburettors lubricated.

Water separating fuel filters

A water separating fuel filter is designed to protect your engine from water that enters the fuel system. Most current Mercury Marine engines are equipped with a water separating filter mounted on the engine. Some boats may be rigged with a remote water separating fuel filter located between the fuel tank and the engine.


The fuel filter on your engine should be inspected or replaced per the maintenance schedule in your owner’s manual, usually every 100 hours or annually. In most cases changing the filter is a job you can do yourself. Under certain conditions it is possible for filters to suddenly become clogged, so it is a good idea to keep an extra filter on board should it need immediate replacement. If this problem persists, it may be a sign of a more significant fuel-related issue that should be referred to your Mercury authorised dealer.

Remote fuel filters

Any fuel filter has a limited capacity to capture water. One way to add filtering capacity is by installing a remote fuel filter that can separate water from the fuel before it reaches the on-engine filter. A remote fuel filter contains a water separating element within a metal or plastic housing that has a greater capacity than most engine-mounted filters. When the remote filter reaches its water capacity the element is easy to change, even when the boat is at sea. Always keep spare filter elements on board. A remote filter is required for some large Mercury outboard engines, but can be good insurance for any boat, especially those that cruise far from their home port or head offshore for fishing.


It is important that an accessory remote fuel filter does not restrict the flow of fuel to your engine. Your Mercury authorised dealer can help you select and install the best filter kit for your boat and engine.

Treating fuel for storage

When preparing your boat for offseason or long-term storage, it is important to store it with fresh, preferably ethanol-free, fuel that has been treated with a marine-rated fuel stabiliser such as Mercury Quickstor Fuel Stabiliser.

If you have an older boat with a vented fuel tank, it is a good idea to fill the tank with fresh fuel to reduce the chances of condensation forming in the tank. Stop when the tank is about 95 percent full because extreme temperature changes over the winter can cause the fuel to expand, potentially forcing petrol out of the vent.

Fuel storage

The fuel tanks on most newer boats with low evaporative emissions fuel systems do not freely vent to the atmosphere and are not as susceptible to moisture from the air. These tanks can be safely stored with a low fuel level, but any fuel remaining in the tank should be treated with stabiliser. If you are unsure whether or not your fuel tank is vented, consult your dealer.

Stabilised fuel

If your boat uses a portable fuel tank, use it to run fresh, stabilised fuel into the engine. Any remaining fuel can be consumed in your car or other power equipment.

Add the appropriate amount of stabiliser based on the instructions on the bottle for the volume of fuel in your tank. To circulate the treated fuel through the entire fuel system, run the engine for about 10 minutes, either in the water or while connected to a garden hose (follow the owner’s manual instructions if using a hose). Finally, replace the fuel filter. Now your fuel system is ready to go next season.

Next topics

Maintenance: What's in it for you?

Boating is an activity that fuels pride of ownership, and for many owners a strong do-it-yourself attitude. Knowing you have covered the basics of maintenance gives you confidence that your boat is in tip-top shape every time you leave the dock.

Fuel basics

Using the correct fuel in your Mercury Marine® engine and properly maintaining your fuel system can help ensure many seasons of reliable service. For more guidance on fuel requirements for your engine, see the fuel section of your Mercury® operation and maintenance manual.

Preventing corrosion

Your engine lives and works in the water, where it is constantly exposed to elements that can cause corrosion.

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