For 1990-2009 cars

The fuel metering system consists of the following parts:

• | The fuel supply components (fuel tank, pump, pipes).

• | The fuel pump electrical circuit.

• | The fuel rail.

• | The fuel injectors.

• | The fuel pressure regulator.

• | The throttle body.

• | The Idle Air Control (IAC) valve.

• | The Throttle Position (TP) sensor.

### System Overview

The fuel tank stores the fuel supply. An electric fuel pump, located in the fuel tank with the fuel sender assembly, pumps fuel through an in-line fuel filter to the fuel rail assembly. The pump provides fuel at a pressure greater than is needed by the injectors. The fuel pressure regulator, part of the fuel sender assembly, keeps fuel available to the injectors at a regulated pressure. A separate pipe returns unused fuel to the fuel tank.

### Fuel Pump Electrical Circuit

When the ignition switch is in the ON position (before engaging the starter), the PCM energizes the fuel pump relay for two seconds, causing the fuel pump to pressurize the fuel system. If the PCM does not receive ignition reference pulses (engine cranking or running) within two seconds, it shuts OFF the fuel pump relay, causing the fuel pump to stop.

### Fuel Rail Assembly

The fuel rail assembly attaches to the engine intake manifold. The fuel rail assembly performs the following functions:

• | It positions the injectors (1) in the intake manifold.

• | It distributes fuel evenly to the injectors.

• | It integrates the fuel pulse dampener (2) into the fuel metering system.

### Fuel Injectors

The top-feed fuel injector assembly is a solenoid operated device, controlled by the PCM, that meters pressurized fuel to a single engine cylinder. The PCM energizes the injector solenoid, which opens a ball valve, allowing fuel to flow past the ball valve, and through a recessed flow director plate. The director plate has multiple machined holes that control the fuel flow, generating a conical spray pattern of finely atomized fuel at the injector tip. Fuel is directed at the intake valve, causing it to become further atomized and vaporized before entering the combustion chamber. An injector stuck partly open can cause a loss of pressure after engine shutdown. Consequently, long cranking times would be noticed on some engines.

### Fuel Pulse Dampener

The fuel pulse dampener attaches inside a housing on the fuel rail assembly. The fuel pulse dampener is diaphragm operated, with fuel pump pressure on one side and spring pressure on the other side. The function of the dampener is to dampen fuel pulsation.

### Accelerator Controls

The accelerator control system is cable operated. There are no linkage adjustments, therefore use the specific cable for each application.

### Throttle Body Assembly

The throttle body assembly attaches to the intake manifold. The throttle body controls air flow into the engine, thereby controlling engine output. The vehicle operator opens the throttle valve within the throttle body through the accelerator controls. During engine idle, the throttle valves are almost closed. A fixed air bypass orifice and the Idle Air Control (IAC) valve (2) handle the air flow control. Engine coolant flows through the coolant cavity on the bottom of the throttle body in order to prevent throttle valve icing during cool weather operation. The throttle body also provides the location for mounting the Throttle Position (TP) sensor (1).

### Idle Air Control (IAC) Valve

The purpose of the IAC valve is to control engine idle speed, while preventing stalls due to changes in engine load. The IAC valve (1), mounted in the throttle body, controls the bypass air around the throttle valve (2). By moving a conical valve known as a pintle (3), in, towards the seat (to decrease air flow); or out, away from the seat (to increase air flow), a controlled amount of air can be bypassed. If engine speed is too low, more air is bypassed to increase RPM. If engine speed is too high, less air is bypassed to decrease RPM. The PCM moves the IAC valve in small steps, called counts. These can be measured and displayed by a scan tool, which plugs into the Data Link Connector (DLC). The PCM calculates the proper position of the IAC valve during idle based on battery voltage, coolant temperature, engine load, and engine RPM. If the RPM drops below specification and the throttle valve is closed, the PCM senses a near stall condition and calculates a new valve position in order to prevent stalling.

• | Engine idle speed is a function of total air flow into the engine. Idle speed is based on IAC valve pintle position + crankcase ventilation valve flow + throttle valve opening + bypass orifice air flow + calibrated vacuum loss through accessories.

• | Controlled idle speed is programmed into the PCM, which determines the correct IAC valve pintle position to maintain the desired idle speed for all engine operating conditions and loads.

• | The minimum idle air rate is set at the factory with a stop screw. This setting allows enough air flow by the throttle valves to cause the IAC valve pintle to be positioned a calibrated number of steps (counts), from the seat, during controlled idle operation.

• | If the IAC valve is disconnected and reconnected with the engine running, the idle speed may be wrong. If this occurs, reset the IAC valve by depressing the accelerator pedal slightly, start and run the engine for five seconds, then turn the ignition OFF for ten seconds.

### Throttle Position (TP) Sensor

The TP sensor attaches to the side of the throttle body opposite the throttle lever. It senses the throttle valve angle and relays that information to the PCM. The PCM requires knowledge of throttle angle to generate the required injector control signals (pulses).