INFO EFFECT FUEL VOLATILITY (DRIVEABILITY CONDITIONS)

Subject: EFFECT OF FUEL VOLATILITY ON DRIVEABILITY CONDITIONS

Model and Year: ALL MODEL YEARS, ALL PASSENGER CARS AND TRUCKS

THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 92-281-6C, DATED SEPT. 1992. THE 1993 MODEL YEAR HAS BEEN ADDED. ALL COPIES OF 92-281-6C SHOULD BE DISCARDED.

Recent changes in EPA regulations have effectively lowered the maximum allowable fuel volatility. Volatility, which can be defined as a gasoline's ability to change from a liquid to a vapor, directly affects the amount of evaporative emissions produced by the fuel. Higher volatility means that more unburned hydrocarbons will be released into the atmosphere. Unfortunately, reducing fuel volatility can cause problems during cold engine operation when low temperatures impede the fuel's ability to vaporize and burn.

Two terms are often used to describe volatility characteristics. These are:

- Distillation Curve

A graph showing the relationship between temperature and the percentage of fuel evaporated. The fuel components that boil at relatively low temperatures (below about 90 degrees F) are known as the "light ends", and are essential for good cold engine performance. The "heavy ends", which begin to boil at about 300 degrees F, contain the most energy but are more difficult to burn. Laboratory analysis is usually required to determine the distillation curve of a gasoline sample.

- Reid Vapor Pressure (RVP)

RVP is the pressure (psi) that vaporized fuel exerts within a sealed container as it is heated to 100 degrees F. The higher the RVP, the higher the fuel volatility. While RVP is readily tested in the field, fuels of the same RVP can have different distillation curves and cold driveability characteristics.

Fuel volatility will vary depending on geographic location and time of year (fuel intended to be used in higher ambient conditions is formulated with less volatility). This can make cold driveability as big a problem during summer months as during the winter. There may be additional variation in the volatility characteristics of pump gasoline, caused by the differences in fuel manufacturers, blends and storage times. As EPA fuel volatility standards are lowered, variations between fuels (which may further reduce volatility) becomes a critical factor influencing cold engine performance. No matter how thoroughly the relationship between fuel quality and cold driveability is understood, eliminating fuel quality as an issue can be difficult, because:

TOOLS WHICH TEST FUEL VOLATILITY MEASURE ONLY RVP, NOT THE DISTILLATION CURVE.

- The distillation curve has a greater effect on cold driveability than RVP.

THE CUSTOMER MAY PERCEIVE RAISING THE FUEL QUALITY ISSUE AS AN ATTEMPT TO EVADE HIS PROBLEM.

- This may cause difficulty in getting accurate fuel usage information.

WATER, AND OTHER TYPES OF FUEL CONTAMINATION, MAY CAUSE CONTINUED DRIVEABILITY COMPLAINTS.

- The problem symptoms may remain even though the vehicle has been refueled several times with a quality gasoline.

CUSTOMER CONCERNS

CONDITION: Poor Cold Engine Operation, symptoms may include; hard start/extended crank, stalling, backfiring, hesitation and/or lack of power.

POSSIBLE CAUSE: Low volatility fuel will not vaporize sufficiently to allow normal combustion.

CORRECTION: Replace Fuel.

DIAGNOSTIC PROCEDURE:

1. Perform basic system checks in section 6E of the service manual.

2. Check for service bulletins which relate to cold driveability issues specific to the problem vehicle.

- An updated engine control calibration, or service procedure, may be available to make the engine less susceptible to low volatility fuels.

3. If basic checks do not reveal a vehicle fault, then advise the customer that fuel quality may be an issue.

Recommend the following actions:

- Change brands of fuel

- Use 87 octane gasoline, unless the vehicle is designed for premium gasoline.

- Try to empty the fuel tank as much as practical before refilling.

- Run a minimum of three tanks of new fuel before returning for service.

4. If above steps are ineffective, do not proceed with additional vehicle diagnosis and/or parts replacement until the fuel tank has been drained and refilled with a known good quality gasoline AT THE DEALERSHIP.

5. If the problem remains refer to service manual, service bulletins and/or technical assistance.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.