It does not KEEP the water out Saying keeping the water out implies water cannot get into the fuel due to ethanol being mixed in the fuel. To say it takes the water out of the fuel would be more correct.
Ethanol is hygroscopic it absorbs water (does not reach out an take water just absorbs it); gasoline is hydrophobic it repels water. If there is moisture in the air, the ethanol will absorb it to allow it to vaporize and burn with the E10 mixture. If no ethanol water will remain separate from the fuel and will not be burned.
Generally less water gets into the modern closed fuel system unlike many years ago when it was not closed. And depending on the humidity levels of where you live is a big factor. Being in the mountains here, humidity tends to be very low at almost all time. No moisture in the air to condensate inside the fuel tank.
And I have fuel cans that have two year old non-ethanol fuel in that have no water in the fuel. One reason why almost all small engine manufactures highly recommend to use non-ethanol fuel for their engines. And that I have proven from experience with small engines and fuel cans with two year old fuel with just a small shot of sea foam to keep stabalized over the years.
This ethanol problem of absorbing water is terrible for fuel stored in a non-closed system, even in a state with generally low humidity. When I first moved here in 2014, I had terrible time with winter starting of generators, blowers, etc. and the local small engine power sports company told me to use the non-ethanol fuel and since then, have NEVER had a problem and never had to drain a carb between seasons and the small engines start up perfectly.
All my friends out here including the few neighbors in this area all use the same non-ethanol fuel in their small engines and their gasoline engine vehicles in the winter. Thus the reason I went to it.
This is a debate many have, but so far, with many mornings well below zero, I have had NO problem staring with non-ethanol fuel in the tank. And it almost instantly starts. Admittedly, this is my first gasoline powered vehicle in many years; having had and still have diesel powered PU trucks.
Also, With the presence of ethanol, the volatility (vapor pressure) of gasoline increases. The increased volatility means it takes more energy for ethanol to vaporize. Cold temperatures make this harder. So the colder the weather, the more time and energy it takes for ethanol to vaporize.
There is a decrease in fuel mileage with non-ethanol fuel, I will accept that. However, from experience of those I know, non-ethanol is the best for me in this Tucson; especially, where I live.
It's a debate that can go on forever, with those loving E10-E15 saying it is better than the non-ethanol fuel, but where I live, until proven different, I will go with the non-ethanol in the winter, E10 in the summer months and to be direct, I sure as **** trust the knowledge of people I know who have been using non-ethanol for years in their vehicles to info received by complete strangers on the internet.
And yes I know what the ingredients of HEET as well as other advertised dry gas additives are. And when I was a kid growing up in MI we did use Heet or other brands to take the water out of the fuel in our open fuel systems in our carbureted engines in the winter. Also used starter fluid a lot too. But MI is a humid state.