The basic function of the fuel metering system is to control fuel delivery to the engine. The fuel is delivered to the engine by individual fuel injectors mounted in the intake manifold near each cylinder.

The heated oxygen sensor (HO2S 1) provides the powertrain control module (PCM) with exhaust stream oxygen content information. The PCM modifies the air/fuel mixture by changing the injector pulse width signal sent to the injectors. By constantly measuring the air intake and the exhaust oxygen content and adjusting the injector pulse width, the air fuel ratio is kept very close to the optimal 14.7:1. This is the range at which the three-way catalytic converter (TWC) operates most effectively and at which the emissions of this vehicle are kept to a minimum. The constant tailoring of the air/fuel ratio is based upon feedback from the HO2S 1 and is referred to as Closed Loop operation.

The fuel in the fuel tank is pumped by the fuel pump, filtered by the fuel filter and fed under pressure to each injector through the fuel rail. The fuel pressure applied to the fuel injector, or the fuel pressure in the fuel feed pipe, is always kept higher than the pressure in the intake manifold by the fuel pressure regulator, located in the fuel tank. The fuel is injected into the intake port of the cylinder head when the injector opens according to the injection pulse signal from the PCM.

The PCM controls the amount of fuel the fuel injector supplies to each cylinder by controlling the ON time of each individual injector. The delivery timing of the fuel into the cylinder head intake port by the fuel injector is controlled by the PCM. The timing and pulse of the fuel injectors is carefully calculated with inputs from the various sensors, so that a suitable air/fuel mixture is supplied to the engine for every driving condition.

There are 2 types of injection timing. One is synchronous injection, in which the injection is synchronous with the ignition signal, or the signal from the camshaft position (CMP) sensor. The other is asynchronous injection, in which injection takes place independently of the ignition signal, or the signal from the CMP sensor.

The powertrain control module (PCM) first calculates the correct timing of the fuel injectors by factoring the engine speed and the air volume together. Then the PCM applies certain compensations that are based on the information provided by various sensors which detect the state of the engine and the current driving conditions.

When starting the engine, the fuel injectors inject the fuel simultaneously and synchronously at every BTDC 75 degrees signal of the CMP sensor. When the engine is starting at a cold state, the amount of fuel is determined by the engine coolant temperature (ECT) sensor and is divided and injected.

Once the engine is running, the fuel injection occurs in a cylinder only when it is in the exhaust stroke. The PCM detects the compression stroke of the number 1 cylinder through the CMP sensor signal.

Whenever a change in the throttle valve opening exceeds a specified value, as determined by the PCM, additional fuel is injected simultaneously into the cylinders which are in the intake and exhaust strokes. This is in addition to the above synchronous injection and is not based on the ignition signal.

In order to improve starting performance, fuel enrichment during start up is carried out. For a certain time after the engine is started, the air/fuel mixture is enriched slightly in order to stabilize the engine speed. The amount of compensation varies, depending on the engine coolant temperature, as measured by the ECT sensor.

When the engine is cold, additional fuel is added in order to ensure good driveability. The level of enrichment of the air/fuel mixture is gradually decreased until the ECT sensor reaches a specified value.

During acceleration, the pulse of the fuel injectors is lengthened in order to deliver more fuel. The additional fuel required is relative to the engine coolant temperature. Acceleration Enrichment ensures smooth and reliable engine acceleration.

In order to provide maximum power during high engine load driving conditions, the air/fuel mixture is enriched when the throttle valve opening is more than a specified value determined by the PCM.

A power supply system voltage drop will delay the mechanical operation of the fuel injector. The actual injector ON time becomes shorter when the system voltage decreases. In order to compensate for this, the fuel injector pulse width signal is lengthened.

The base air/fuel ratio may vary due to differences in individual engines and mileage. In order to compensate for such variations, feedback information is used to adjust the base air/fuel mixture to maintain the optimum air/fuel ratio.

Fuel injection stops when decelerating, when the throttle valve is at idle position and the engine speed is high), so that unburned gas will not be exhausted. Fuel injection starts again when the above conditions are no longer present.

Fuel delivery also stops when the engine speed exceeds 6,800 RPM. This will prevent engine overrun which adversely affects the engine. Fuel delivery starts again when the engine speed decreases to less than 6,500 RPM.

In order to obtain efficient performance of the three-way catalytic converter (TWC) and a high clarification rate of CO, HC and NOx in the exhaust gas stream, the air/fuel mixture must be kept as close to the theoretical air/fuel ratio (14.7:1) as possible. In order to accomplish this the PCM first compares the input voltage from the heated oxygen sensor 1 (HO2S 1) with a specified reference voltage. If the HO2S 1 input voltage is higher than the specified reference voltage, the PCM determines that the air/fuel ratio is richer than the theoretical air/fuel ratio and reduces the fuel (lean command). If the input voltage from the HO2S 1 is lower than the specified reference voltage, the PCM determines that the air/fuel ratio is leaner and increases the fuel (rich command). By repeating these operations, the PCM can adjust the air/fuel ratio in order to be closer to the theoretical air/fuel ratio. Control of the fuel delivery system as just described is known as Closed Loop operation.

The Closed Loop fuel control operation will not take place under any of the following conditions: