Determining if Fuel has had Oil Added

by Had Robinson

Running a 2-cycle engine on unmixed fuel is an expensive mistake.  The engine will seize after a few minutes and is pretty much ruined and will require a complete rebuild including a new cylinder, piston, connecting rod, crankshaft, and bearings – and maybe more.

If you need to know whether or not your fuel has oil in it do this:

  1. Pour a couple of tablespoons of the suspect fuel in a clean white bowl.
  2. Do the same with fuel you KNOW has not been mixed with oil in another bowl.
  3. Let them sit awhile, occasionally swirling the fuel near the edges of the bowl.
  4. The fuel with the oil will leave a distinct ring of oil (usually red or blue) in the bowl because it does not evaporate.

This test will not tell you the ratio of the mix.  Hopefully, you already know this....

In the photo below, the bowl on the right contained the fuel mixed with oil. The oil is clearly visible as a ring around the bowl. The bowl on the left contained unmixed fuel. The blue color of the residues is due to the color of the oil. You want to have the bowl with the unmixed fuel as a standard of comparison.  Caution: AVGAS has a blue color which can be confused with the oil.

oil in fuel test

Note: It is always a good idea to put a label on your gas can that it has been mixed with oil. A piece of duct tape works well as you can put the date on it when the oil was added. A label on a piece of glider line also works well.

NEVER refill a fuel container without first removing any labels!

Turkey Vulture