Beware of Gasoline Phase Separation!

Written by Dr. Klaus L.E. Kaiser

Phase separation of gasoline — avoid it if you can! I’ll tell you why and have a simple experiment that lets you measure the ethanol content in your gasoline too.

Phase separation (PS) is what you can get from the ethanol content in your gasoline. It can damage your engine and possibly get you stranded at an out of way place. It is more likely to occur with lubricant-added gasoline used for two-cycle engines commonly found in ATVs, outboard motors, motorcycles, snowmobiles and other equipment with small engines, but can also happen with regular gasoline.


New” Gasoline

Gasoline without any ethanol used to be what you got when filling up your gas tank a decade ago or so, but the wannabe-savers-of-the-world had a “better” idea. Why not dilute the gasoline with ethanol. At first your “new” fuel was limited to an ethanol content of 5% (E5). Then the US Environmental Agency (EPA) mandated up to 10% ethanol content (E10) and as of late EPA is talking about a 15% ethanol (E15) mandate.

If you have experienced problems with E5 or E10 gasoline, E15 could just about be disastrous. The main reason is that the likelihood of PS occurring increases exponentially with the ethanol content in the gasoline.

Properties of Gasoline and Ethanol

Pure gasoline is a mixture of hydrocarbons which does not dissolve in or mix with water. In contrast, ethanol can be mixed — is miscible — with water at any ratio. The reason is ethanol’s hydroxyl group which provides a great affinity for water. For the same reason, ethanol is not completely miscible with common gasoline under all temperatures. Moreover, small amounts of water in your fuel can dramatically change that solubility and foster PS.

Let me demonstrate that with a simple experiment. Anyone can do that experiment and it will show you the ethanol content of your gasoline at the same time. To begin, let’s build the device to undertake the experiment. It is small, lightweight, portable, inexpensive, and can be re-used as often as you like.

Ethanol Measuring Device

For this experiment, we are using the following materials:

  1. a turkey baster, made from clear plastic, available for a dollar or two,

  2. a wine bottle cork,

  3. a quarter inch size drill bit,

  4. a dab of a clay-based putty, such as plumber’s pipe joint,

  5. a small twig of any kind, long enough to reach the bottom inside the baster,

  6. approximately 2 oz. (75 ml) of gasoline and

  7. three drops of water.

The setup is equally simple:

  1. using the drill bit, drill a hole approximately 3/8 inch deep into the cork (can easily be done by hand),

  2. place a small amount of pipe joint compound (or other gasoline and water compatible sealant) into the hole and use the back side of the drill bit to push it to the bottom to seal the hole of the baster,

  3. remove the rubber bulb from the baster and press the baster into the hole in the cork,

  4. secure the cork with baster in an upright position,

  5. fill the baster to the top of any graduation (1.5 oz. in my model, or approximately 75 mL, or about 80% of total volume) with the gasoline to be tested,

  6. finally, add three drops (approximately 1/8 teaspoon or 0.5 mL) of water to the baster.


At this point in the setup I used, there are now two phases visible: a clear bottom layer of water (barely above the top of the cork) and the slightly colored gasoline above it (I used a 75:1 gasoline to oil 2-cycle engine oil mix with dye).

Now, watch what happens when you use the little twig to stir up the system. Move the twig up and down a few times to get the water and gasoline to have more contact. Initially, the gasoline will turn milky but it will soon separate again into two clean phases, except now, the phase boundary is at a significantly higher level than before. In my setup, the difference was approximately ¾ inch (18 mm) in the conical part of the baster, representing an ethanol content of 5% in the original gasoline.

You can calculate the amount of ethanol separated out from the gasoline if the graduation on the baster is detailed enough, or by determining its weight or volume in other ways.


The observed increase in the volume of the aqueous (water) phase at the bottom is solely due to the ethanol content of the gasoline. With the help of a few drops of water you have separated the ethanol from the gasoline and collected it in the separate phase at the bottom of the baster.

This simple experiment proves several things at once:

  1. You demonstrated the process of phase separation.

  2. Ethanol is more soluble in water than in gasoline.

  3. There is ethanol in your gasoline.

  4. A small amount of water can cause phase separation between the ethanol and (now nearly pure) gasoline on top.

  5. The bottom layer consists of nearly pure ethanol with a small content of water.

When is PS likely to occur?

As shown by the experiment, very little water is required to cause PS. For example, that small amount of water can be introduced to your gas tank from humid air. When that air in your tank is cooled below the dew point the water vapor will condense to liquid water and collect at the bottom of your tank. The normal shaking action of any vehicle in motion will provide the stirring effect used in the experiment and voila – there is your two-phase system.

Think of snowmobiles and all other equipment with two-cycle engines. They are even more likely to experience PS because the small amount of lubricant additive to the gasoline reduces the ethanol solubility in gasoline even further. Think of a boat which has its tank below the water level and therefore more at the water rather than air temperature.

Temperature also has an effect on the solubility of ethanol in gasoline. The lower the temperature the lower is its solubility in gasoline and the higher the chance of PS occurring.

Consequences of PS

It ought to be quite obvious that PS in a tank of gasoline can have severe consequences. Depending on your tank and fuel withdrawal design your engine could receive nearly straight ethanol (with some water) as fuel. That will certainly result in much less power and its corrosive effects are likely to damage the engine and fuel system.

Alternatively, the (now ethanol-depleted) gasoline will have a lower octane rating than the original one due to the (slight) octane-boosting effect of small amounts (less than 1%) of ethanol in gasoline. Its removal via PS will also lead to less power of the remaining gasoline, possibly causing engine knocking and also irreparable damage your engine.

As you can see, PS is something to be avoided. No wonder most car manufacturers have said that all warranties are null and void with the use of E15 gasoline.

What You Can Do

  1. Express your concerns to the politician(s) representing you.

  2. Where possible at all, get gasoline without ethanol at all or as little ethanol (5% max) content as possible.

  3. Use “phase separation inhibitors.” There are companies producing such types of gasoline additives. However, I have no personal experience with such yet. Perhaps readers who have used any of these products may wish to provide some feedback here.

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