Fuel quality may cause driveability problems such as hesitation, lack of power, stall, and no start. Fuel testing is limited. Before you diagnose the fuel quality, perform all other system checks, such as the Powertrain System Check, the Fuel System Check, the Ignition System Check, and the Emissions System Check. If these checks fail to solve the problem, ask the customer 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 Fuel System Diagnosis Alcohol in Fuel test. If this test is inconclusive, drain the tank and refill the tank with a known good-quality fuel.

Due to increasing awareness about vehicle emissions and their impact on the environment, federal regulations limit certain fuel characteristics. These limitations can cause driveability problems that are extremely difficult to diagnose. In order to make a diagnosis, you need a basic understanding of fuel and of the effect of fuel on the fuel system.

Octane is a measure of a fuel's ability to resist spark knock. Spark knock occurs in the combustion chamber just after the spark plug fires, when the air-to-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 or a ping. Fuel with a high octane number has a greater resistance to spark knock. This vehicle requires 87 octane (VIN K) in order to ensure proper performance of the fuel control system. Using fuel with an octane rating lower can create spark knock, which would cause the powertrain control module (PCM) to retard the ignition timing in order to eliminate the knock. This situation can result in poor engine performance and in reduced fuel economy. During severe knock cases, engine damage can occur.

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

You can determine volatility with 3 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 of the relationship between the temperature and the percentage of evaporated fuel. The fuel components that boil at relatively low temperatures, below approximately 90°F, are known as light ends. Fuel components that boil at approximately 300°F are known as heavy ends. The light ends are important for cold starting and for cold weather driveability. Heavy ends provide engine power and are important for hot weather driveability. The proper mixture of these components provides proper operation across a wide range of temperatures. However, an accurate distillation curve of a gasoline usually requires laboratory testing. The RVP test measures the pressure that vaporized fuel exerts within a sealed container when the fuel is heated to 38°C (100°F). Volatility increases in proportion to the RVP. Although you can easily measure RVP, the results may be misleading. Two fuels with the same RVP may have different distillation curves and different driveability characteristics.

Low volatility can cause the following conditions:

Volatility that is too high can cause the following conditions:

Because volatility depends on the 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 fuel injectors can their 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. The effectiveness of deposit inhibitors varies by manufacturer. If intake valve deposits occur, fuel may be suspected. These deposits can cause symptoms such as excessive exhaust emissions, power loss, and poor fuel economy.

The sulfur content in fuel is 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 cause the following conditions: