Fuel Metering System

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

COMPUTER CONTROL
The main control sensor is the Oxygen Sensor (O2S), which is located in the exhaust manifold. The O2S provides the ECM with exhaust stream oxygen content information. The ECM modifies the air/fuel mixture by changing the injector pulse width signal sent to the injectors. By constantly measuring 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 Warm-Up Three-Way Catalytic Converter (WU TWC) operates most effectively and at which the vehicle's emissions are kept to a minimum. The constant tailoring of the air/fuel ratio based upon the O2S feedback is referred to as "Closed Loop" operation.

MODES OF OPERATION
There are several operating conditions that the ECM monitors, by sensing signals from various sensors, to determine how long to energize the injectors for each injection cycle. The fuel is delivered to the engine under one of several operating "modes." All of the modes are controlled by the ECM and are described below.

ENGINE STARTING ENRICHMENT
When the ignition switch is turned to the "START" position, the ignition start circuit energizes the circuit opening relay and powers the fuel pump. The ECM checks the Engine Coolant Temperature (ECT) sensor, Throttle Position (TP) sensor, and crank signal, and determines the proper air/fuel ratio for starting. The ECM controls the amount of fuel delivered in the starting mode by changing how long the injectors are energized for each injection cycle.

RUN MODE
The run mode has two conditions: "Open Loop" and "Closed Loop."

Open Loop
When the engine is first started and engine speed is above 400 RPM, the system goes into "Open Loop" operation. In "Open Loop," the ECM ignores the signal from the O2S and calculates injector pulse width based primarily on inputs from the ECT sensor and the Manifold Absolute Pressure (MAP) sensor.

The system will remain in "Open Loop" until the following conditions are met:
^ The O2S has varying voltage output - indicating that it is hot enough to operate properly.
^ The ECT sensor is registering something above 70°C (158°F).
^ A calibrated time delay has elapsed after starting the engine. This delay allows all sensors to stabilize.

Closed Loop
When these conditions have been satisfied, the system goes into "Closed Loop" operation. In "Closed Loop," the ECM calculates injector pulse width based primarily upon the signal from the O2S. This operation maintains an air/fuel mixture very close to the optimum 14.7:1 ratio.

ACCELERATION MODE
The ECM monitors changes in the TP sensor and MAP sensor signals to provide the necessary added fuel.

DECELERATION MODE
The ECM monitors changes in the TP sensor and MAP sensor signals and reduces injector pulse width. When deceleration is abrupt, the ECM will cut off fuel completely until a specified engine speed has been attained.

SYSTEM VOLTAGE COMPENSATION
A system voltage drop (due to weak battery or faulty charging system) interferes with the mechanical operation of the solenoid-operation fuel injectors. The actual injector pulse width becomes shorter which reduces the amount of fuel injected into the engine. If the ECM should sense such a condition, the injector pulse width is lengthened in an attempt to compensate for the inadequate electrical supply.

FUEL CUTOFF
Fuel injection stops (with operation of the fuel injector prevented) when decelerating (i.e., when the throttle lever is at idle position with engine speed 3,500 rpm or higher), so that unburned gas will not be exhausted. The fuel injectors will resume operating when the engine speed drops to 1,400 rpm.