Certain driveability problems can be related to the fuel quality. Fuel testing is limited. Therefore, all other system checks must be performed before the fuel quality is diagnosed. If these checks fail to solve the problem, gain information from the customer about when the condition occurs, when and where the fuel was purchased, and which octane level was used. Ask the customer to change fuel brands. If the symptom persists, perform the Alcohol/Contaminants-in-Fuel Diagnosis . If this is not conclusive, drain the fuel tank and refill the tank with a known good quality fuel.

Due to increasing awareness of vehicle emissions (evaporative and exhaust) and the emissions' impact on the environment, federal regulations limit 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 of fuel on the vehicle's fuel system must be gained.

Octane is a measure of the ability of a fuel to resist spark knock. 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 the temperature and pressure. This secondary explosion causes a vibration that is heard as a spark knock. A fuel with a high octane number has increased resistance to spark knock. This vehicle requires 91 octane ([R+M]/2 method) in order to ensure correct performance of the fuel control system. Using fuel with a lower octane rating can create spark knock, which causes the PCM to retard the ignition timing to eliminate the knock. Poor engine performance and reduced fuel economy can result. Also, in severe knock cases, engine damage may occur.

Volatility is a fuel's ability to change from a liquid state to a vapor state. Since liquid gasoline will not burn, the fuel must vaporize before entering the combustion chamber. The rate at which gasoline vaporizes determines the amount of evaporative emissions released from the fuel system, and therefore has made volatility an environmental concern. The federal government has lowered the maximum allowable volatility, but certain driveability conditions have resulted.

Volatility can be determined through 3 different tests: the Vapor-Liquid Ratio, the Distillation Curve, and the Reid Vapor Pressure Test (RVP). The Vapor-Liquid Ratio test determines what temperatures must exist to create a vapor-liquid ratio of 20. The distillation curve is a graph showing the relationship between the temperature and the percentage of fuel evaporated. The fuel components that boil at relatively low temperatures of less than about 90°F are known as light ends. Fuel components that boil at about 300°F are known as heavy ends. The light ends are important for cold starting and cold weather driveability. Heavy ends provide engine power and are important for hot weather driveability. The correct mixture of these components provides correct 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 (psi) that vaporized fuel exerts within a sealed container when heated to 100°F. The fuel's volatility increases proportional to the RVP. While the RVP can easily be measured in the field, the RVP may be misleading as it is possible for 2 fuels with the same RVP to have different distillation curves, and therefore, different driveability characteristics.

As stated, improper volatility can create several driveability problems. Low volatility can cause poor cold starts, slow warm ups, and poor overall cold weather performance. Improper volatility may also cause deposits in the crankcase, the combustion chambers, and on the spark plugs. A volatility that is too high could cause high evaporative emissions and a purge canister overload, vapor lock, and hot weather driveability conditions. Since 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 key Off. 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 injectors' spray pattern, which in turn could cause reduced power, unstable idle, hard starts, and poor fuel economy.

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

The sulfur content in fuel is also regulated to a certain standard. 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 engine oil and increase engine wear. A high sulphur content could also produce excessive exhaust emissions or a rotten egg smell from the exhaust system.