[jdmccright]

When it gets cold out, water can condense from the air inside the tank and settle at the bottom (it has a higher density and is immiscible). This can prevent starting, cause misfire, or stalling if there is enough accumulation. The main ingredient in Heet, isoproyl alcohol, is used to make any water in the tank miscible in the gasoline so that it can be sucked out by the fuel pump and expelled in the exhaust as combustion takes place...a little at a time so that it doesn't affect engine performance too much to set off warning lights. It is not designed as a fuel stabilizer or cleaner, only to remove built-up water.

As mentioned, the dilution ratio is low enough (when added to a full tank as directed) that the alcohol won't dry out seals. To be safe, don't add to a completely empty tank then fill up to provide good mixing. Hope this helps!