Information on Diesel Fuel Additives

This bulletin is being revised to add RPO LBZ. Please discard Corporate Bulletin Number 03-06-04-017C (Section 06 -- Engine/Propulsion System).

Important: The use of diesel fuel additives is not required or recommended for the 6.5L diesel or the 6.6L Duramax® Diesel engine under normal conditions. The filtering system is designed to block water and contaminants without the use of additives. However, some customers may desire to use fuel additives to improve the characteristics of available diesel fuels.

If the customer desires to use a fuel additive, care must be taken in its selection. There are two common methods that fuel additives use to cope with water in the fuel.

One method is through demulsification of water in the fuel. This method causes water particles to combine together to form larger particles, which drop out of suspension. This allows the fuel filter/water separator to separate the water from the fuel as it is designed to.

The other method of coping with water in the fuel is through emulsification. This method, often using alcohol as the emulsifier, keeps water particles suspended in the fuel. Emulsification of water in the fuel can allow water to get past the fuel filter/water separator, in most cases causing damage to the fuel system. Only alcohol free water demulsifiers should be used in General Motors diesel engines. GM Diesel Fuel Conditioner®, P/N 88861009 (in Canada, 88861038) or both *Racor Power Shot Plus® and *Stanadyne® diesel fuel additives are alcohol free and utilize water demulsifiers to cope with water in the fuel. Other brands may be available in different areas; be sure that they clearly state that they are alcohol free demulsifiers before use.

*We believe these sources and their products to be reliable. General Motors does not endorse, indicate any preference for or assume any responsibility for the products from these firms or for any such items which may be available from other sources.

Fuel Waxing/Icing

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

Bacteria and Fungi Growth

Bacteria and fungi growth can occur in diesel fuel when there is water present, especially during warmer weather. The best prevention against bacteria and fungi growth is to use clean fuel that is free of water. There are diesel fuel biocides available that are designed to kill bacterial growth in the fuel system. However, the dead bacteria can still cause blockages throughout the fuel system. If bacterial growth is found in the fuel system, the proper method of removal is to flush the fuel system using Service Manual procedures, replace the fuel filter element 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.

Low Cetane Number

The cetane number is one indicator of a diesel fuel's ability to ignite. There are many indicators of overall fuel quality such as cleanliness, specific gravity, volatility, viscosity, detergency, corrosion inhibiting abilities, and lubricity. Increasing the cetane number alone is not a fix for poor quality fuel. Additionally, increasing the cetane number beyond the engine's requirements will not increase performance. However, the cetane number of diesel fuel is not always consistent and some customers may desire to use a cetane improver to ensure full performance of their engine. If such an additive is to be used, it must not contain alcohol or other water emulsifiers.

Poor Lubricity

The 6.5L diesel and the 6.6L Duramax® Diesel engines are designed to operate on today's low sulfur fuel without the use of additives. A fuel additive designed to increase lubricity is not a fix for poor quality or contaminated fuel, but some customers may desire to use a lubricity additive to aid in the longevity of their fuel system components. If such an additive is to be used, it must not contain alcohol or other water emulsifiers.

If a vehicle is properly maintained but has fuel contamination issues, consider obtaining fuel from a different source. Purchasing fuel from a high volume fuel retailer increases the chance that the fuel is fresh and of good quality.