I live in a small town, and the Gas station there has a bad reputation for gas quality. With rumors like peoples gas tanks freezing solid and such.

Is there a quick, safe, cheap way to test for water content in gasoline so we can put the final word on these rumors? Our regular gasoline is E10.

2 Answers 2


Water in fuel is easy to figure out. Put a sample (say 5 to 10 ounces) of the fuel into a clear glass container. Let it sit. If it separates into two layers, you have water in the fuel.

demonstration of water and gasoline separating

Shown: an obviously contrived example of gasoline (10% ethanol) with approximately 2 ounces of water added - water has visibly sunk to the bottom of the jar.

  • Any guidance on how long you need to wait?
    – Rory Alsop
    Oct 14, 2011 at 11:49
  • 1
    I'd assume it's similar to Avgas, which separates at the rate of around an inch a minute (Foot deep fuel tank will end up with all the water on the bottom in 12 minutes). Oct 14, 2011 at 12:17
  • Will the water still separate out with the 10% ethanol? I know that has some effect on the water solubility.
    – Kratz
    Oct 14, 2011 at 12:42
  • I would guess so, but can't say for certain. I only fly certified airplanes, and any ethanol at all is prohibited in them. However, the experimental guys in the hangar next door have a Rotax that's allowed to run 5% ethanol. They're still technically required to sump the fuel for water, so I'm guessing it still separates in a similar manner? Either way, it's an empirical question. Stick some of your gas in a cup and drop in some water to see what happens. :-) Oct 14, 2011 at 13:21
  • 5
    Ethanol is hygroscopic (it absorbs water). As such, gasoline with ethanol in it will not separate as completely as gasoline without ethanol. With enough water content, it will still separate, but the "gasoline" portion will still contain some unseen water. See here for some more info. This site suggests you might be able to cool your sample (in the fridge or freezer?) to render more of the water (but also some ethanol) insoluble.
    – Flimzy
    Oct 15, 2011 at 19:14

most diesel vehicles have a fuel/water separator some with an audible alarm if not they are available aftermarket.Be worth enquiring if these are compatible with gasoline engines

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