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With E10 Ethanol Blend Fuels now at all the gas pumps,

your fuel will be contaminated in the spring,

whether or not there's condensation...


I can't understand why the "full or empty tank" and "condensation" debates still continue...   


With ethanol fuels, you have much bigger water contamination issues to worry about than a little condensation.


With E10 gas, it's best to leave the tank empty,

or be prepared to discard fuel that will be water contaminated, BAD AND LEAN GAS, by the spring.



* Ethanol (an alcohol fuel) has an amazing ability to absorb water. 


Unlike conventional gasoline, E10 fuel, undergoes phase separation and water contamination in a very short period of time, even under perfect temperatures and humidity.


Research studies have proven, that under all atmospheric conditions,

E10 will phase separate within 100 days.


We're now told, to use E10 gasoline within 15-90 days, in boats and marine engines.

(In a  rich water environment ethanol has a better opportunity to absorb water).


Most marine engine manufacturers recommend E10 gas be used in less than 30 days. (Check your owners manual).



* Conventional gasoline can dissolve up to 150 parts per million (ppm) water.


The situation is different for gasoline oxygenated with 10% volume ethanol.


The E10 gasoline-alcohol blend can dissolve much more water, at about 6000–7000 ppm.


When this blend is cooled, both the water and some of the ethanol become insoluble.

The result is two layers of liquid:

An upper ethanol-deficient gasoline layer and a lower ethanol-rich (up to 75% ethanol) water layer.

The process is called phase separation and it occurs because ethanol is completely soluble in water but only marginally soluble in hydrocarbons...


After phase separation, the gasoline layer will have a lower octane number -

Octane drops about 3 points -


Therefore, if you started with 87 octane, after phase separation, you'd be running your engine (unsafely) on about an 84 octane.


Since winters generally last close or beyond 100 days, fuel stored in the tank is contaminated/phase separated by the spring.




Besides, the risk of "condensation" in a fuel tank during the winter has always been very low, (maybe a few capfuls, even with non-alcohol gas).


I think the full tank supporters' state, "leaving less air in the tank, decreases the possibility of air attracting water from the atmosphere (condensation) when temp changes from cold to hot."


If a rapid change in temp truly did cause a high risk of water contamination, (from condensation), you'd see sweating on the tank every time you had a cold morning, followed by a sunny/hotter afternoon - But you don't.


Ok, so under all temps and conditions,



E10 is already absorbing water, from every possible source available,

so why would a little more water absorption from "condensation" make a difference?


Do the "full tank supporters", still plan to run this full winter/fall tank of gas in their engines, in the spring?




I certainly would not waste money on new gas now,

when I'm certain I would have to discard of it

(due to water contamination and low octane),

in just a few months down the road.



Are there any other reasons to still debate "full tanks" for storage, with E10?




BTW, If you think you're not filling up with ethanol fuel at a public gas station, you're probably mistaken. 

Labeling pumps, that ethanol was added for oxygenation, is not required by law.


All major gas suppliers (Chevron, Texaco, Mobil, etc.) NOW ADD ETHANOLat the pumps for oxygenation.

(Required by environmental laws passed in 2005-06). 


Add: 1/8/07 Link to Ethanol Handbook State-by-State labeling laws - Some states label pumps, some don't - check your state...

Gail, Assistant, Consultant + Sister of The Outboard Wizard -
Owner of Fuel-Testers Company + MLR Solutions (Health/Science Consulting firm)
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