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What's the scoop on gas-line antifreeze?
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As you use up the gas in your gas tank, air flows in, and often that air is heavy with moisture, even if the air temperature is not very high. When the tank cools afterward, the moisture in this air will condense onto the cool sides of the tank (think how much dew you sometimes find all over your car on a cool summer morning). This water then runs down to the bottom of the tank, since water is more dense than gasoline and won't mix with it. By fall, especially in humid climates, there can be a cup or so of water rolling around in the bottom of your gas tank. When the temperature drops below freezing, that water turns into something like snow mixed with ice crystals. Before it freezes, it can get caught in the intake to your fuel pump and even work its way up front to your fuel filter, causing low fuel pressure and fuel starvation.

If you've made a religious habit, since new, of keeping your tank as full as possible, it's highly unlikely you'll ever need gas-line antifreeze. Keep your tank at least 3/4 full (preferably freshly topped off!!) every time you park the car overnight, unless you have no choice but to leave it low. The less tank exposed, the less air present in the tank, the less water you'll end up with. It takes diligence and is a bit of a PITA, but really pays off in the long run.

What to do if you do have ice in your fuel system? Just add some alcohol to your gas tank. That's all the so-called gas-line antifreezes are, just alcohol! But beware, the term alcohol covers a lot of territory. There are a lot of different kinds of alcohol. The three kinds of interest to your gas tank are ethanol, methanol and isopropanol (also known as isopropyl alcohol). Ethanol is also called grain alcohol; vodka is essentially ethanol and water in a 50/50 mix. No, you don't want to put vodka in your gas tank! You need an anhydrous alcohol, that is to say, an alcohol with as little water already in it as possible. You'd need 180 OP vodka, and that's pretty hard to find! You can buy gas that has ethanol already in it however, and Honda approves the use of gas containing up to 10% ethanol. This gasohol has a good dose of gas-line antifreeze effectively built in. Many oil companies already sell gasoline with ethanol. Sometimes it will say so right on the pump.

Methanol (methyl hydrate) is also called wood alcohol, and since it's quite poisonous and undrinkable you can buy gallon containers of it at many hardware stores because it's used to thin shellac. A cup of it in your gas tank will take care of any water and it's a lot cheaper than the small plastic bottles of gas-line antifreeze sold at the gas station, which are often the same stuff. Honda approves gas with up to 5% methanol. So as long as you add it to a full tank, a cup or so won't hurt. Honda warns that more than 5% can damage your fuel system though!

Isopropanol is the third choice, and the best one since it is five times as effective as methanol, but you may have to read the fine print on that little plastic bottle of gas-line antifreeze to see whether it's methanol or isopropanol. Rubbing alcohol also contains isopropanol but it's mixed with water already so it's of no use as a gas-line de-icer. Warning! Buying large amounts of either methanol or isopropanol at one time will likely provoke a visit from the local drug enforcement agents - both are used as solvents for certain illegal substances.

More on alcoholized/oxygenated fuels here:

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