The concentration of alcohol-in-fuel can be detrimental to the fuel system. Higher than recommended alcohol-in-fuel concentrations may cause driveability problems such as hesitation, lack of power, stall, or no start. High concentrations may cause corrosion of the fuel system components or fuel filter plugging, and deteriorate the rubber and the plastic components.

Commercial automotive fuel can contain alcohol of various types and levels of concentration. Some forms of alcohol are very detrimental to the fuel system components. The presence of an excessive amount of alcohol-in-fuel may be the cause of a driveability problem. The following procedure can be used in order to detect an excessive amount of alcohol-in-fuel. The following procedure uses water in order to separate the alcohol from the fuel. You cannot determine the specific type of alcohol contamination with this test.

Draw the fuel sample from the bottom of the fuel tank, because water in the tank will concentrate there. The fuel sample should be bright and clear. Do not use a fuel sample that appears cloudy or contaminated for this procedure. Clean the fuel system if the fuel sample is cloudy or contaminated.

1. Fill a 100-ml cylinder with 1-ml graduation marks to the 90-ml mark with the fuel sample.
2. Add 10-ml of water in order to bring the volume to 100-ml, and install a stopper.
3. Shake the cylinder for 10 to 15 seconds.
4. Carefully loosen the stopper in order to release the pressure.
5. Close the stopper, and shake the cylinder again for 10 to 15 seconds.
6. Put the graduated cylinder on a level surface for approximately 5 minutes in order to allow adequate time for the liquid to separate.

Alcohol is present in the fuel if the volume of the lower layer, which now contains alcohol and water, is more than 10-ml.

For example, if the volume of the lower layer increases to 15-ml, there is at least 5-percent alcohol in the fuel. The actual amount of the alcohol may be somewhat more because this procedure does not extract all of the alcohol from the fuel.