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Q: I use 87 octane in my car, why can't I use it in my outboard?
This stuff's not fit to use in your lawn mower, let alone a $12,000 high performance outboard motor. The low octane and lack of detergent additives just leave you open to motor failures from detonation, gum and carbon formation, coked up rings and piston scuffing. Your automotive engine is a 4 stroke with totally different lubrication and fuel usage requirements.

Q: Can I use oxygenated gasoline in my boat?
A major oil company survey of the manufacturers of engines for non-automotive uses -- boats, garden tools, chain saws and snowmobiles -- indicates that oxygenated gasoline will perform satisfactorily in most later-model engines. However, some manufacturers expressed concerns about its use in older engines. The owner's manual is the most authoritative source of information about the fuel requirements of your equipment. If your equipment is older and the manual does not mention oxygenated gasoline, consult an authorized dealer.

Oxygenated gasoline will perform satisfactorily in most engines under normal conditions of storage and use. However, you should be aware of the potential problems -- two involving the gasoline itself and several involving gasoline/equipment interactions. Follow the suggested precautions if they apply to your situation.

   Main Concerns are:

  • Phase Separation
    Gasoline oxygenated with alcohol readily takes up water when it is present. The water may be condensed out of humid air or be a contaminant in the fuel system. Dissolved water will not interfere with engine operation. But if enough water is present, gasoline oxygenated with alcohol will separate into two liquid phases: a top phase which is almost all gasoline (and oil, in the case of 2-cycle fuel) and a bottom phase of water and alcohol. (Phase separation is not a problem with gasoline oxygenated with MTBE.) Phase separation may make the engine difficult or impossible to start. To solve the problem, replace the separated mixture with fresh gasoline.

    The situation with the greatest potential for damage is phase separation in the fuel tank of a 2-cycle engine without an oil sump. If the engine is able to start and run on the alcohol/water phase, it won't be lubricated -- almost all the added engine oil will be in the gasoline phase.

    To minimize the chance of phase separation, keep water out of your equipment's fuel system and practice good fuel management. You can eliminate the possibility of water pickup when your equipment is not in use if you keep the fuel tank full and close the tank vent.

  • Deterioration During Storage
    If a gasoline is of poor quality or if the storage conditions are adverse, gasoline can oxidize and form gums over the period of several months. These changes can increase engine deposits and filter plugging. There are reasons to suspect that oxygenated gasoline may form gums more readily than conventional gasoline, but actual evidence is limited. A safeguard recommended by some manufacturers is the use of name-brand gasoline from companies recognized for the quality of their products. Another safeguard is good fuel management.

  • Leaning Effect

    Compared to a conventional gasoline, oxygenated gasoline results in a leaner air-fuel mixture. This causes some engines to run rough. Also, engines may run hotter on lean mixtures, which, in the extreme, can result in engine damage. If the engine in your equipment is running rough and it has an adjustable carburetor, check that the adjustment is proper for the fuel you are using. Either contact a knowledgeable serviceman or follow the manufacturer's instructions in your owner's manual.

    Some manufacturers--Arctco, Bombardier, Outboard Marine, and Polaris--indicate that certain of their engines may require larger carburetor jets to compensate for the leaning effect of oxygenated gasoline. Consult your authorized dealer or serviceman about the manufacturer's recommendations for your engine model and year.

  • Materials Compatibility

    Deterioration of elastomers and plastic parts is not expected to be a problem with current production engines. For older engines, some problems have occurred with elastomeric parts (fuel pump diaphragms, needle valve tips and seats, fuel lines, gaskets, etc.) when they were first exposed to oxygenated gasoline. Some manufacturers recommend that you frequently inspect the fuel system for leaks and for deteriorating elastomeric parts when you operate an older engine on oxygenated gasoline. There is also the possibility of leaks after you change back to conventional gasoline: Seals that were swelled by oxygenated gasoline may shrink. (The same mechanism is responsible for leaks cause by large changes in the composition of conventional gasoline.) Replacement fuel system parts have been engineered to be compatible with oxygenated gasoline.

  • Metal Corrosion

    Some manufacturers report corrosion of metallic fuel system parts when gasoline oxygenated with alcohol was used in older engines. Except for periods of prolonged storage, this does not appear to be a concern with late model engines. Corrosion problems may be aggravated by the phase separation of a gasoline oxygenated with alcohol. The alcohol/water phase tends to be more corrosive than the oxygenated gasoline itself.

  • Solvency

    Oxygenated gasoline may loosen deposits from fuel system surfaces because it is a better solvent than conventional gasoline. The suspended solids can plug the fuel filter and carburetor passages. This is not a common problem. It is most likely to occur when older equipment is first fueled with an oxygenated gasoline. Clean the fuel tank and fuel system and replace the fuel filter; then refuel with fresh fuel.

Practice Good Fuel Management
  • Match your purchases to your consumption.
  • Don't buy more than you'll use in one to two months.
  • Store gasoline in a tightly-closed container in a cool, dry place.
Vapor Lock And Difficulty Restarting A Hot Engine

"Vapor lock" is loss of power or engine shutdown due to gasoline vaporization in the fuel system. Vaporization prevents the fuel pump from delivering sufficient gasoline to the engine. Factors favoring vapor lock are high ambient temperatures which accompany high power output. Sometimes vaporization occurs after the engine has been intentionally stopped because of the cessation of mechanical cooling and fuel flow. In this case, the hot engine will be difficult to start.

Vapor lock and hot restarting have been reoccurring problems for engine manufacturers. Some manufacturers are concerned that oxygenated gasoline will aggravate these problems because adding either ethanol or MTBE to gasoline increases volatility.

The volatility of gasoline sold in each area of the country is tailored for the expected ambient temperature range. Buying smaller amounts of gasoline more frequently make it more likely you will have a gasoline of the correct volatility.

Your owner's manual may suggest additional ways to avoid vapor lock. Remember that some situations lead to vapor lock more often than others. One is running an engine at full power for an extended time on an unseasonably warm day. If vapor lock does occur, it will probably be necessary to allow the engine to cool before it can be restarted.

What Octane gasoline should I use for my outboard?
I have an older outboard with higher compression. Should I burn PREMIUM gas?

NO! Here's why: the octane ratings are composed of 2 components, research octane and motor octane (RXM/2) divided by 2. The RESEARCH octane is the quality of the base stock, the MOTOR octane is derived from additives. The oil company will never tell you the ratios. Problem is the highest octanes are achieved by MOTOR octane additives, which will just gum up the pistons in a 2 cycle engine. Base stock is generally the same. The MID-GRADE gas has the detergent additives needed to clean your motor and sufficient octane for MOST motors. If you have a MERCURY, FORCE or OMC motor from the early 70's thru mid-80's refer to technical bulletins from the manufacturers for timing changes and replacement head gaskets to lower compression to use today's gasoline.


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The information provided on these pages is correct to the best of my knowledge, however the MasterTech makes no warranty, express or implied, regarding the use of, results of, or liability created from, application of this data. This information is disseminated in good faith, however MasterTech assumes NO LIABILITY whatsoever in regard to this service. The information, software, products, and services published on this web site may include inaccuracies or typographical errors. Changes are periodically added to the information herein. Mastertech may make improvements to this site at any time. Parts ordered from this website may or may not be in dealer stock at the time of order. Thank you for reading.