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, no start, etc. High concentrations may also cause corrosion of the fuel system components and subsequent fuel filter plugging as well as deterioration of the rubber and the plastic components.

Commercial automotive fuel can contain alcohol in various types and levels of concentration. Some forms of alcohol are more detrimental to the fuel system components than others. 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 extract the alcohol from the fuel. The specific type of alcohol contamination cannot be determined from this test.

The fuel sample should be drawn from the bottom of the fuel tank, because any water present in the tank will be concentrated there. The fuel sample should be bright and clear. A fuel sample that appears to be cloudy or contaminated with the water (as indicated by a water layer at the bottom of the sample), should not be used with this procedure. Clean the fuel system if a cloudy or contaminated fuel sample is found.

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 the water to bring the total fluid volume to 100 ml and install a stopper.
3. Shake vigorously for 10 to 15 seconds.
4. Carefully loosen the stopper to release the pressure.
5. Close the stopper and shake the cylinder vigorously 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 will now contain both alcohol and water) is greater than 10 ml.

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