Frequently Asked Questions
- MerCruiser Corrosion
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Fuel & Fuel Systems
What is phase separation, and how do I deal with it?
If significant amounts of water are present in a fuel tank with gasoline that contains ethanol, the water will be drawn into the fuel until the saturation point is reached for the three-component mixture of water + gasoline + ethanol. Beyond this level of water, phase separation could cause most of the ethanol and water to separate from the bulk fuel and drop to the bottom of the tank, leaving gasoline with a significantly reduced level of ethanol in the upper phase. If the lower phase of water and ethanol is large enough to reach the fuel inlet, it could be pumped directly to the engine and cause significant problems. Even if the ethanol water phase at the bottom of the tank is not drawn into the fuel inlet, the reduced ethanol level of the fuel reduces the octane rating by as much as 3 octane numbers, which could result in engine problems.
The level at which phase separation can occur is determined by a number of variables, including the amount of ethanol, the composition of the fuel, the temperature of the environment and the presence of contaminants. It is very important (A) that the system is inspected for significant quantities of water in the tank before using gasoline with ethanol and (B) to limit exposure of the fuel tank to excess water. If phase separation has occurred, it is necessary to completely remove all free water from the system and replace the fuel before continuing operation. Otherwise, engine problems could occur.
Is an additive available that can prevent phase separation?
There is no practical additive that can prevent phase separation from occurring. The only practical solution is to keep water from accumulating in the tank in the first place.
Should I add an additional fine-micron filter to the system to prevent debris from entering the engine?
The addition of another filter to the system will create another possible flow restriction that can starve the engine of fuel. Mercury already provides the appropriate level of filtration to protect the engine from debris.
Are older fuel lines prone to failure from ethanol-based fuels? What about gaskets?
During the 1980s, many rubber components for use in fuel systems were developed to withstand exposure to fuels containing ethanol. If rubber components in a fuel system are suspected to be of this vintage or older it may be advisable to replace them with newer ethanol-safe components before using fuels containing ethanol. Check with the manufacturer for advice or frequently inspect these fuel-system components for signs of swelling or deterioration and replace if problems are noted.
Can ethanol-blended fuels affect the performance of traditional carbureted two-stroke outboards?
Two-stroke outboards should experience little or no decrease in performance due to gasoline fuels containing up to 10-percent ethanol when operated according to Mercury's standard recommendations. When gasoline with ethanol is used for the first time after a fuel changeover from MTBE, the tank must be completely free of water prior to introduction of gasoline with ethanol. Otherwise, phase separation could occur that could cause filter plugging or damage to the engine. (It is probably better for a boat owner to fill the fuel tanks with ethanol fuel for the first time when the tank is low on fuel, but that is not critical. There should be no difficulties if the tank is clean and free from water. If the tank is not free from water, a partial load of fuel will more easily phase separate because with less ethanol in place it takes less water to cause phase separation. The important thing for boaters to concern themselves with is the presence of water in their tanks.)
If an engine is a 1990 or older model, frequent inspections of all fuel-system components are advised to identify any signs of leakage, softening, hardening, swelling or corrosion. If any sign of leakage or deterioration is observed, replacement of the affected components is required before further operation.
How does ethanol affect my fiberglass fuel tank?
Fiberglass tanks manufactured prior to 1991 may not be compatible with gasoline containing ethanol. It has been reported that, in the presence of ethanol, some resins may be drawn out of fiberglass and carried into the engine where severe damage could occur. If an older fiberglass tank is used, check with the manufacturer to determine if gasoline with ethanol can be safely used.
Ethanol is replacing MTBE in my region - what should I do?
Before gasoline with ethanol is introduced to your fuel tank, ask your boat manufacturer if any special precautions should be considered with the use of fuel containing ethanol. Check for the presence of water in the fuel tank. If any is found, remove all water and dry the tank completely. As a precaution, it is advisable to carry a few extra filters in case filter plugging becomes a problem during boating.
Are there any additives that can allow the phase-separated mixture to remix when added to the fuel tank?
No, the only way to avoid further problems is to remove the water, dispose of the depleted fuel, clean the tank and start with a fresh, dry load of fuel.
Is there a simple solution to water condensation in the tank as a result of ethanol?
It is best to maintain a full tank of fuel when the engine is not in use. This will reduce the void space above the fuel and will reduce the flow of air in and out of the tank with changes in temperature. This will reduce condensation on the internal walls of the tank and will limit exposure of the ethanol in the fuel to humidity and condensation.
What should be done when storing boats with ethanol-blended fuels for extended periods?
Follow the instructions for normal storage preparation found in the Warranty Information and Operation and Maintenance manuals. When preparing to store a boat for extended periods of two months or more, it is best to completely remove all fuel from the tank. If it is difficult or not possible to remove the fuel, maintaining a full tank of fuel with a fuel stabilizer added to provide fuel stability and corrosion protection is recommended. It is best to add the stabilizer and fuel treatment to the tank at the recommended dosage, run the engine for 10 minutes to allow the system to be cleaned, shut off the fuel valve to interrupt the fuel supply and allow the engine to run until it stops, and top off the tank until it’s full to reduce the amount of exchange with the air that might bring in condensation. Do not cap the tank vent and do not fill with fuel to the point of overflowing. Some extra space should be maintained in the tank to allow for expansion and contraction of the fuel with temperature changes. A partially full tank is not recommended because the void space above the fuel allows air movement that can bring in water through condensation as the air temperature moves up and down. This condensation could potentially become a problem.
Mercury Marine Quickstor can help maintain fuel systems in storage. Quickstor contains oxidation inhibitors to reduce oxidation and gum formation, metal-chelating agents to protect metal components from corrosion, and water-absorbing agents to reduce the presence of free water.
Will the use of fuels containing ethanol void my engine warranty?
Fuels containing up to 10 percent ethanol are considered acceptable for use in Mercury engines. Fuels containing higher levels of ethanol, such as E15 gasoline, are not considered acceptable for use. The use of fuels containing more than 10 percent ethanol can void the warranty.
Mercury offers an alcohol tester (See image below) to find the percentage of ethanol/alcohol in gasoline.
How can I order the Mercury ethanol test kit?
You can order a Mercury ethanol test kit (part number 879172T28) from a Mercury Authorized Dealer. Click here to find a dealer near you.