The fuel rail consists of five parts:

The fuel rail is mounted on the intake manifold and distributes fuel to each cylinder through the individual injectors.

Fuel is delivered from the pump through the fuel feed pipe to the inlet port of the fuel rail pipe. From the fuel feed inlet, fuel is directed to the front rail pipe, then a crossover to the rear pipe, then through the rear rail pipe to the fuel pressure regulator. Fuel in excess of injector needs flows back through the pressure regulator assembly to the outlet port of the fuel rail. Fuel then flows through the fuel return pipe to the fuel tank to begin the cycle again.

The fuel injector is a solenoid device controlled by the PCM that meters pressurized fuel to a single cylinder. When the PCM energizes the injector coil (3), a normally closed ball valve (1) opens, allowing fuel to flow past a director plate (2) to the injector outlet. The director plate has holes that control the fuel flow, generating a dual conical spray pattern of finely atomized fuel at the injector outlet. Fuel from the outlet is directed at both intake valves, causing it to become further vaporized before entering the combustion chamber.

Fuel injectors will cause various driveability conditions if they will not open, are stuck open, leaking or have a low coil resistance.

The fuel pressure regulator (1) is a diaphragm-operated relief valve (6) with fuel pump pressure on one side, and regulator spring (2) pressure and intake manifold vacuum on the other side. The regulator's function is to maintain a constant pressure differential across the injectors at all times. The pressure regulator compensates for engine load by increasing fuel pressure as engine vacuum drops. The pressure regulator is mounted on the fuel rail.

The cartridge regulator is serviced as a separate component. When servicing the fuel pressure regulator, insure that the back-up O-ring (3), large O-ring (4), filter screen (5), and small O-ring (7) are properly placed on the pressure regulator.

With the ignition On, and engine Off (zero vacuum), system fuel pressure at the pressure test connection should be 317-407 kPa (46-59 psi). If the pressure regulator supplies fuel pressure which is too low or too high, a driveability condition will result. Refer to Fuel System Pressure Test for information on diagnosing fuel pressure conditions.

The throttle body contains a single throttle valve which controls the amount of air delivered to the engine. A coolant passage under the throttle valve heats the throttle body.

The Throttle Position (TP) sensor and Idle Air Control (IAC) valve are mounted on the throttle body. The TP sensor and minimum air stop are not adjustable.

The TP sensor is a potentiometer that is mounted on the throttle body and provides the PCM with information on throttle valve angle. The PCM provides a 5 volt reference signal and a ground to the TP sensor and the sensor returns a signal voltage that changes with throttle valve angle. At closed throttle (close to 0 degrees) the TP sensor output signal is low (below 1 volt) and at WOT (greater than 80 degrees) the TP sensor output signal is high (above 4 volts). Because the TP sensor is not adjustable, the PCM must account for build tolerances that could affect the TP sensor output at closed throttle. The PCM uses a learning algorithm so that it can correct for variations of up to 6 degrees of throttle angle.

The PCM uses TP information to modify fuel control based on throttle valve angle. For example, power enrichment occurs when the throttle angle approaches WOT. Acceleration enrichment occurs when the throttle angle increases rapidly (similar to an accelerator pump on a carburetor). A faulty TP sensor may cause various driveability conditions and should set a DTC.

Refer to Powertrain Control Module (General Information) .

The purpose of the Idle Air Control (IAC) Valve is to control engine idle speed, while preventing stalls due to changes in engine load.

The throttle blade (2) when closed allows a small amount of air into the intake manifold. However, most of the air for closed throttle engine operation passes through the IAC valve, bypassing the throttle blade. The IAC valve (1), mounted in the throttle body, controls air flow with a conical valve or pintle (3). By moving the pintle in towards the seat, air flow is decreased. By moving the pintle out away from the seat, air flow is increased.

The PCM moves the IAC pintle out if engine speed is too low or in if engine speed is too high. The PCM controls IAC valve movement in small steps called counts. The IAC valve position, measured in counts away from the valve seat, can be displayed on the scan tool.

The scan tool displays IAC position in counts. 0 counts indicates the PCM is commanding the IAC to be driven all the way into a fully seated position. This is usually caused by a vacuum leak.

The higher the number of counts, the more air is being commanded to bypass the IAC pintle. 320 counts indicates a fully open position.

Prior to starting the engine and at idle, the PCM calculates the desired position of the IAC valve based on coolant temperature, engine load, engine speed and battery voltage. While the throttle position is off idle, the IAC valve position follows or tracks throttle position to allow for smooth transitions from open to closed throttle.

If the IAC valve is disconnected or disabled with the engine running, the PCM may loose track of the IAC valve position causing erratic or incorrect idle speed. If this occurs, reset the IAC valve by doing the following:

1. The coolant temperature must be above 0°C (32°F)
2. The vehicle must be in Park (P) not Neutral (N)
3. Turn the ignition switch to Lock for 20 seconds

The Mass Air Flow (MAF) sensor is located in the MAF sensor housing between the air snorkel and the throttle body. It senses the amount of air entering the intake manifold. This information is required by the PCM to control fuel and emissions.

The MAF sensor used on this vehicle is a hot wire type and is used to measure air flow rate (mass/unit time). The MAF sensor output is a variable frequency square wave based on air mass traveling through the sensor. A low frequency output signal from the MAF sensor indicates low air flow and high frequency indicates high air flow. The MAF sensor has two wires crossing the air stream through it. One senses ambient temperature and the other, the hot wire, is kept at a fixed temperature above the ambient temperature. The MAF sensor senses air flow by measuring the power necessary to keep the hot wire hot, which is proportional to air flow, and converts it to a frequency modulate output signal.