Alcohol concentrations of more than 10 percent in fuel can be detrimental to the fuel system components and may cause driveability problems such as hesitation, lack of power, stall, no start, etc.

The problems may be due to fuel system corrosion and subsequent fuel filter plugging, deterioration of rubber components, and/or air-fuel mixture leaning. Various types and concentrations of alcohol are used in commercial fuel. Some alcohol is more detrimental to fuel system components than others.

The fuel sample should be drawn from the bottom of the tank so that any water present in the tank will be detected. The sample should be bright and clear. If alcohol contamination is suspected, use the following procedure to test the fuel quality:

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

If alcohol is present in the fuel, the volume of the lower layer, which would now contain both alcohol and water, will be more than 10 ml. For example, if the volume of the lower layer increased to 15 ml, at least 5 percent alcohol in the fuel is indicated . The actual amount of alcohol may be somewhat more because this procedure does not extract all of the alcohol from the fuel.

The fuel sample should be drawn from the bottom of the tank so that any water present in the tank will be detected. The sample should be bright and clear. If the sample appears cloudy, or contaminated with water, as indicated by a water layer at the bottom of the sample, use the following procedure to diagnose the fuel.

1. Using an approved fuel container, draw approximately 0.5 liter of fuel.
2. Place the cylinder on a level surface for approximately 5 minutes in order to allow settling of the particulate contamination.
3. Observe the fuel sample. If any physical contaminants or water are present, clean the fuel system. Refer to Fuel System Cleaning .

Particulate contamination will show up in various shapes and colors. Sand will typically be identified by white or light brown crystals, rubber particles typically as black and irregular. Any particles that show up, regardless of color or shape, are not acceptable, and the entire fuel system should be thoroughly cleaned.