Due to an increasing awareness towards vehicle emissions (Evaporative and Exhaust) and the impact that the emissions have on the environment, federal regulations are limiting certain characteristics of fuel. These limitations are causing driveability problems that are extremely difficult to diagnose. In order to make a diagnosis, a basic understanding of fuel and the effects on the vehicle's fuel system must be gained.

Octane is the measure of a fuel's ability to resist a spark knock. A spark knock occurs in the combustion chamber just after the spark plug fires, when the air/fuel mixture in the cylinder does not completely burn. The remaining mixture spontaneously combusts due to temperature and pressure. This secondary explosion causes a vibration that is heard as a knock (ping). Fuel with a high octane number has a greater resistance to a spark knock. This vehicle requires 91 octane ([R+M]/2 method) in order to ensure the proper performance of the fuel control system. Using fuel with an octane rating lower than 91 can create a spark knock. This can cause poor engine performance and a reduction in fuel economy. Also, in severe knock cases, engine damage may occur.

Volatility is a fuel's ability to change from a liquid state to a vaporized state. Liquid gasoline must vaporize before entering the combustion chamber, since liquid gasoline will not burn. The rate at which the gasoline vaporizes determines the amount of evaporative emissions that are released from the fuel system.

Volatility can be determined through three different tests:

The Vapor-Liquid Ratio test determines what temperatures must exist in order to create a vapor-liquid ratio of 20.

The distillation curve is a graph showing the relationship between the temperature and the percentage of evaporated fuel. The fuel components that boil at low temperatures (below about 90°F) are known as the light ends and the fuel components that boil at about 300°F are known as the heavy ends. The light ends are important for cold starting and cold weather driveability. The heavy ends provide engine power and are important for hot weather driveability. The proper mixture of these components provide a proper operation across a wide range of temperatures. However, the distillation curve of a gasoline usually requires laboratory testing.

The Reid Vapor Pressure (RVP) test measures the pressure (kPa/psi) that the vaporized fuel exerts within a sealed container when the fuel is heated to 100°F. The volatility increases proportional to the RVP. While the RVP can easily be measured in the field, this may be misleading because it is possible for two fuels with the same RVP to have different distillation curves, and therefore, different driveability characteristics.

As stated, an improper volatility can create several driveability problems. A low volatility can cause poor cold starts, slow warm ups, and poor overall cold weather performance. A low volatility may also cause deposits in the crankcase, the combustion chambers and the spark plugs. A volatility that is too high could cause high evaporative emissions and purge canister overload, vapor lock, and hot weather driveability conditions. Since the volatility is dependent on temperature, different fuels are used during certain seasons of the year, thus creating problems during sudden temperature changes.

Fuel system deposits can cause various driveability problems. Deposits usually occur during hot soaks after the key is in the off position. Poor fuel quality or driving patterns such as short trips followed by long cool down periods can cause injector deposits. This occurs when the fuel remaining in the injector tip evaporates and leaves deposits. Leaking injectors can increase injector deposits. Deposits on the fuel injectors affect the spray pattern, which in turn can cause reduced power, an unstable idle, hard starts, and poor fuel economy.

The intake valve deposits can also be related to the fuel quality. While most fuels contain deposit inhibitors, some do not and the effectiveness of deposit inhibitors varies by the manufacturer. If the intake valve deposits occur, the fuel may be suspected. These deposits can cause symptoms such as excessive exhaust emissions, power loss and a poor fuel economy.

The sulfur content in the fuel is also regulated to a certain standard. The premium grades of fuel generally have a lower sulfur content than the less expensive blends. A high sulfur content can promote the formation of acidic compounds that could deteriorate the engine oil and increase the engine wear. A high sulfur content could also produce excessive exhaust emissions or a rotten egg smell from the exhaust system.

Notice: Do Not use fuel that contains more than 5% methanol. Use of a fuel (gasohol) that contains more than 5% of methanol can corrode metal fuel system components and damage plastic and rubber parts.

The oxygenated fuels contain oxygen in the chemical structure. The advantages that the oxygenated fuels offer are an improved octane quality, a better combustion, and reduced carbon monoxide emissions. The most commonly used oxygenated fuels are ethanol (grain alcohol) and MTBE (Methyl Tertiary Butyl Ether). Ethanol is acceptable if the ethanol does not exceed 10 percent by volume. The MTBE is permitted up to 15 percent by volume. Another type of fuel oxygenator is Methanol, which is permitted up to 5 percent by volume.

The fuel must meet the ASTM Standard: D4814 (US), CGSB 3.5-M87 (Canada).

Minimum Octane requirement: 91 [(R+M)/2] (pump) octane where R=research octane number and M=motor octane number.

Notice: Do not spill fuel containing any alcohol on the vehicle. Alcohol can cause damage to the paint finish and trim.

You may use fuel containing Methyl Tertiary-butyl Ether (MTBE), providing that there is no more than 15 percent MTBE by volume.

You may use fuel containing ethanol (ethyl) or grain alcohol, providing that there is no more than 10 percent ethanol by volume.

Notice: Do Not use fuel that contains more than 5% methanol. Use of a fuel (gasohol) that contains more than 5% of methanol can corrode metal fuel system components and damage plastic and rubber parts.

You may use fuel containing methanol (methyl) or wood alcohol, providing that there is no more than 5 percent methanol by volume.