Water contamination in the fuel system may cause driveability conditions such as hesitation, stalling, no start, sporadic misfires as well as causing a misfire at a single cylinder caused by water pooling near a single injector, usually at the lowest point in the fuel rail in some fuel systems. Once enough water has collected in this area it is possible for the injector to receive a shot of water causing an intermittent misfire in that cylinder.

Alcohol concentrations more than 10 percent in the fuel may be detrimental to 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. If an excessive amount of alcohol in the fuel is suspected as the cause of a driveability condition, the following procedure may be used to detect the presence of alcohol in 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, this procedure should not be used, and the Fuel System should be cleaned.

1. Using a 100 ml specified 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 fluid 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 is increased to 15 ml, this indicates at least 5 percent alcohol in the fuel. 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, then clean the fuel system. Refer to .

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