Details on Diesel Fuel

Water in diesel fuel is a contaminant that can cause damage to the fuel system. Water in fuel can take two forms: coarse water droplets that drop out of suspension and can be filtered out or emulsified water particles suspended in the fuel, which can pass through some filters.

 

Proper fuel filter servicing and the use of clean diesel fuel that is free of water or contaminants are critical for the longevity of fuel system components of a modern direct injected diesel engine. Adherence to the fuel filter change interval will help to ensure that the diesel fuel system will be protected from contaminants and that the engine will continue to operate as designed. Purchasing fuel from a high volume fuel retailer increases the chances that the fuel is fresh and of good quality.

 

Water in Fuel Sensor

 

The Water in Fuel Sensor on the GM Duramax diesel engine can be tested after removing it from the fuel filter assembly. (Fig. 5) A good sensor will have continuity when the float is manually raised. Use a DVOM to check for continuity. The float can be tested by placing it in water and in clean diesel fuel. A good float will float in water and sink in diesel fuel.

 

Contaminants

 

If water is present in diesel fuel, fungi and other microorganisms can survive and multiply, especially in warmer climates. The fungi can be present in any part of the fuel handling system. These fungi grow into long strings and will form large globules. The growths appear slimy and are usually black, green or brown. Fungi use the fuel as their main energy supply and need only trace amounts of water and minerals.

 

The most common symptom of fuel contamination is fuel filter plugging, however, various metal fuel system components also can corrode. If fungi have caused fuel system contamination, flush the fuel system, replace the fuel filter and refill the tank with clean diesel fuel.

 

If a customer desires to use a biocide after flushing the fuel system, it should not contain alcohol or other water emulsifiers.

 

Diesel Fuel Additives

 

Customer use of diesel fuel additives are neither required nor recommended for the 6.5L diesel and 6.6L Duramax diesel engines. (Fig. 6) Alcohol-based additives permit water to pass the fuel filter and water separator, causing damage to the fuel system. GM Diesel Fuel Conditioner, part number 88861009, is alcohol-free and utilizes water demulsifiers to cope with water in the fuel.

 

Fuel distributors blend no. 1 and no. 2 diesel fuels for seasonal requirements in a particular region. No other blending of fuels is recommended. If a customer desires to use a winter fuel additive to prevent fuel waxing or icing during extreme cold snaps, the winter fuel additive must not contain alcohol or other water emulsifiers that may compromise the water removal effectiveness of the fuel filtering system.

 

The use of additives such as a cetane improver to enhance engine performance or a lubricity additive to aid in the longevity of fuel system components are not fixes for poor quality or contaminated fuel. If such additives are used, they must not contain alcohol or other water emulsifiers.

 

Biodiesel

 

Diesel fuel containing greater than five percent biodiesel (B5) or the use of unmodified bio-oils blended into diesel fuel at any concentration may damage the diesel fuel system and engine.

 

Waxing

 

Diesel fuel gelling or waxing (also called cloud point) at low temperatures may result if the fuel is not blended properly for the temperatures being experienced. Further, biodiesel begins to wax at temperatures of 10 to 20 degrees higher than no. 2 winter blend fuel.

 

A below freezing hard start/no start condition may be caused by fuel waxing or fuel contamination. Fuel waxing may also restrict the amount of fuel flow through the fuel filter. A high fuel system vacuum reading will be found if the system is checked when the fuel is cold and waxing.

 

The use of engine block heaters in extreme cold temperatures may alleviate some fuel waxing concerns.

 

Refer to the appropriate Service Information for additional information about diagnosing the effects of contaminants in diesel fuel and testing for the presence of contaminants, including fungi, water, gasoline, and biodiesel in amounts larger than five percent.

 

- Thanks to Bill Carnevale