The function of the fuel and air control system is to manage fuel and air delivery to each cylinder to optimize the performance and driveability of the engine under all driving conditions. The fuel supply is stored in a High Density Polyethylene (HDPE) fuel tank located in front of the rear wheels. The fuel sender allows retrieval of fuel from the tank and also provides information on fuel level. An electric fuel pump contained in the modular fuel sender pumps fuel through nylon pipes and an in-line fuel filter to the fuel rail. The pump is designed to provide fuel at a pressure above the regulated pressure needed by the injectors. Fuel is then distributed through the fuel rail to eight injectors inside the intake manifold. Fuel pressure is controlled by a pressure regulator mounted on the fuel rail. The fuel system in this vehicle is recirculating; this means that excess fuel that is not injected into the cylinders is sent back to the fuel tank by a separate nylon pipe. This removes air and vapors from the fuel as well as keeping the fuel cool during hot weather operation. Each fuel injector is located directly above each cylinder's two intake valves. An accelerator pedal in the passenger compartment is linked to a throttle valve in the throttle body by a cable. The throttle body regulates air flow from the air cleaner into the intake manifold, which then distributes this air to each cylinder's two intake valves. This allows the driver to control the air flow into the engine, which then controls the power output of the engine.

Unleaded fuel must be used with all gasoline engines for proper emission control system operation. Using unleaded fuel will also minimize spark plug fouling and extend engine oil life. Leaded fuel can damage the emission control system, and its use can result in loss of emission warranty coverage.

All vehicles with gasoline engines are equipped with an Evaporative Emission Control (EVAP) System that minimizes the escape of fuel vapors to the atmosphere. Information on this system is found in Driveability and Emissions (Powertrain and Emissions System).

The Northstar engine is fueled by eight individual injectors, one for each cylinder, that are controlled by the Powertrain Control Module. The PCM controls each injector by energizing the injector coil for a brief period generally once every other engine revolution. The length of this brief period, or pulse, is carefully calculated by the PCM to deliver the correct amount of fuel for proper driveability and emissions control. The length of time the injector is energized is called the pulse width and is measured in milliseconds (thousandths of a second).

While the engine is running, the PCM is constantly monitoring its inputs and recalculating the appropriate pulse width for each injector. The pulse width calculation is based on the injector flow rate (mass of fuel the energized injector will pass per unit of time), the desired air/fuel ratio, and actual air mass in each cylinder and it is adjusted for battery voltage, short term and long term fuel trim. The calculated pulse is timed to occur as each cylinders intake valves are closing to attain largest duration and most vaporization.

Fueling during crank is slightly different than during engine run. As the engine begins to turn, a prime pulse may be injected to speed starting. As soon as the PCM can determine where in the firing order the engine is, it begins pulsing injectors. The pulse width during crank is based on coolant temperature and barometric pressure.

The Northstar fueling system has several automatic adjustments to compensate for differences in fuel system hardware, driving conditions, fuel used, and vehicle aging. The basis for fuel control is the pulse width calculation described above. Included in this calculation are an adjustment for battery voltage, short term fuel trim, and long term fuel trim. The battery voltage adjustment is necessary since changes in voltage across the injector affect injector flow rate. Short term and long term fuel trims are fine and gross adjustments to pulse width designed to maximize driveability and emissions control. These fuel trims are based on feedback from oxygen sensors in the exhaust stream and are only used when the fuel control system is in closed loop.

Under certain stringent conditions, the fueling system will not energize injectors, individually or in groups, for a period of time. This is referred to as Fuel shut-off. Fuel shut-off is used to improve traction, save fuel, improve starting, and protect the vehicle under certain extreme or abusive conditions.

In case of a major internal problem, the PCM is equipped with a back-up fueling system that will run the engine until service can be performed.

Notice: Do not operate the engine in the back-up fuel for extended periods. Back-up fueling will negatively impact driveability and fuel economy, and may cause damage to the emissions system.

As part of the fueling system, the PCM records and sends data about fueling to the Instrument Panel Cluster from which the IPC calculates fuel economy, range, and fuel used displays. The PCM calculates the amount of fuel delivered to the engine through the injectors based on injector pulse width and flow rate. This data is accumulated and sent to the IPC periodically via the data link.

The PCM controls the fuel injectors based on information it receives from several information sensors. Each injector is fired individually in engine firing order, which is called Sequential Multiport Fuel Injection. This allows precise fuel metering to each cylinder and improves driveability under all driving conditions.

Due to increasing awareness towards vehicle emissions (Evaporative and Exhaust) and their impact 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 its effects on the vehicles fuel system must be gained.

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/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 spark knock. This vehicle requires 91 octane ([R+M]/2 method) in order to ensure proper performance of the fuel control system. Using fuel with an octane rating lower than 91 can create spark knock, which would cause the PCM to retard ignition timing to eliminate the knock. In a case such as this, poor engine performance and reduced fuel economy could 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, it 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 three 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 temperature and the percentage of fuel evaporated. The fuel components that boil at relatively low temperatures (below about 90°F) are known as light ends and those 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. It is the proper mixture of these components that provide 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 (psi) vaporized fuel exerts within a sealed container when heated to 100°F. Volatility increases proportional to RVP. While RVP can easily be measured in the field, it 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, improper volatility can create several driveability problems. Low volatility can cause poor cold starts, slow warm ups, and poor overall cold weather performance. It may also cause deposits in the crankcase, combustion chambers and spark plugs. Volatility that is too high could cause high evaporative emissions and 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 fuel injectors affect 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 and 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 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. It could also produce excessive exhaust emissions or a rotten egg smell from the exhaust system.

Notice: Do not use fuels containing methanol in order to prevent damage and corrosion to the fuel system.

Oxygenated fuels contain oxygen in their chemical structure. The advantages that oxygenated fuels offer are improved octane quality, 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 it does not exceed 10% by volume. MTBE is permitted up to 15% by volume.