Engine Control Module: Description and Operation

System Diagram:

This vehicle is equipped with a 3.0L, DOHC 60° V6 cylinder gasoline engine, and a computer controlled sequential electronic fuel injection system. The system maintains an air/fuel mixture as close as possible to the ideal stoichiometric ratio, for all operating conditions. During normal operation, the Powertrain Control Module-Engine (PCME) monitors various engine sensors, including an exhaust gas oxygen sensor, and operates injectors sequentially according to firing order, for varying lengths of time depending on amount of fuel required.

FUEL INJECTION
Fuel is supplied to the fuel rail and injectors at high pressure by the fuel pump in the tank. A fuel pressure regulator varies fuel pressure depending on manifold vacuum. When fuel demand is high as indicated by low manifold vacuum, pressure is increased slightly. A pulsation damper helps to reduce fuel pump pressure oscillations. Injectors are electric solenoid valves that get power when the engine is cranking or running and turn "ON" when the PCME completes their circuits to ground. Injectors spray fuel into the intake port when they are turned "ON". The amount of fuel sprayed depends on the injector "pulse width" (length of time the injectors are turned "ON" during each injection cycle). Injectors are operated sequentially, which means injectors turn "ON" in the same sequence as the engine firing order, so that injection occurs at each cylinder while the intake valves for that cylinder are open.

DECHOKE CONTROL SYSTEM
Cuts fuel injection under certain conditions to aid engine starting if the spark plugs become fouled (such as when the engine is flooded). This allows plugs to dry and purges excess fuel from cylinders.

PCME will cut fuel injection while cranking under these conditions:
^ PCME receives ignition switch START signal
^ Engine speed below 500 rpm
^ Throttle valve fully open

STARTING AND WARM-UP
During cranking when the engine is cold, a cold start injector sprays additional fuel into the intake manifold dynamic chamber to provide a richer mixture for easy starting. When the engine is running cold, the fuel injection system operates in what is known as "OPEN LOOP" mode. In this mode, the PCME determines the amount of fuel needed depending on the engine coolant temperature and the amount of air flow as measured by the air flow meter. A specific pulse width is assigned for certain air flow rates at various temperatures. These are the "base" pulse width values stored in the computer memory which provide a slightly rich air/fuel ratio for good driveability while the engine is cold and during warm-up.

NORMAL WARM ENGINE OPERATION
When the engine warms up, the fuel injection system switches to "CLOSED LOOP" operation. In this mode, the computer monitors several sensors in addition to the air flow meter and temperature sensor, including an exhaust gas oxygen sensor. Signals from the oxygen sensor tell the PCME if the air/fuel ratio is rich or lean. The PCME then modifies the base pulse width accordingly to increase or decrease the amount of fuel being injected, depending on the need at that moment. Injector pulse width is adjusted several times each second to maintain the air/fuel ratio as close as possible to the ideal ratio.

IDLE SPEED CONTROL
Idle speed is controlled by the Bypass Air Control (BAC) valve. The BAC valve regulates how much measured air is allowed to bypass the throttle valves, depending on engine temperature and signals from the PCME. The BAC valve has two parts, the "Air Valve" which is open when cold to provide cold start fast idle, and the Idle Speed Control (ISC) valve which is electrically operated by the PCME. In the air valve, a temperature sensitive wax pellet is heated by engine coolant. As the engine warms up, the pellet expands against the air valve, closing it. When the engine is warm, idle speed is controlled by the PCME through duty cycle signals to the ISC valve (the ratio of "ON" time vs."OFF" time).

DECELERATION FUEL CUT-OFF
The PCME recognizes when the engine is decelerating by monitoring the engine speed and an idle switch that tells the PCME when the throttle is closed. When the throttle is closed and the engine rpm is above a certain speed, the PCME will leave injectors turned "OFF" and no fuel is injected. This reduces emissions, prolongs catalytic converter life, and improves fuel economy.

INTAKE AIR SYSTEM - VARIABLE INERTIA CHARGING SYSTEM (VICS)

Variable Inertia Charging System (VICS):

The intake manifold has two paths that air can take on the way to the cylinders, a long and a short path. The two paths are separated by a shutter valve, like a throttle valve in each intake runner. When the shutter valves are closed, air must take the long path. When the valves are open, air can take a more direct short path to the engine. The PCME opens and closes the shutter valves at different engine speeds to take advantage of harmonic oscillations in the intake manifold runners. This helps "pack" more fuel and air into the cylinders during each cylinder cycle (improved volumetric efficiency), increasing torque and horsepower over a broader rpm range.

IGNITION TIMING CONTROL
Ignition timing is also controlled by the computer. The PCME monitors signals from sensors in the distributor. From these signals, the PCME can calculate crankshaft position and engine speed. The PCME uses this information, compared with information from other engine sensors, to determine the correct spark advance for the engine speed and load at any given time. The ignition coil "fires" when the PCME interrupts the low voltage signal to a power switching transistor (igniter). The igniter turns the primary coil circuit "OFF" causing the ignition coil to discharge. For more information on the ignition coil, igniter, and other secondary ignition components, refer to Ignition System

SELF-DIAGNOSTICS
The EGI system has self-diagnostic capability. When a fault is detected in a monitored component and/or circuit, the PCME will store a numbered code in its memory (malfunction code) and the Malfunction Indicator Lamp (MIL) will light and stay on while the engine is running, alerting the driver to seek service. The technician can then access PCME fault memory and display trouble codes identifying the faulty component(s) or circuit(s), to aid in troubleshooting and repair.

FAIL-SAFE MODE
To provide a margin of safety and maintain driveability in the event of certain system failures, the PCME has a fail-safe mode. In this mode, it will substitute a fixed "in range" signal for that of the failed sensor. This allows the vehicle to be operated until repaired, although driveability will probably be affected. The PCME automatically switches to fail-safe mode if a fault is detected in any of these inputs:

^ Airflow meter
^ Atmospheric pressure sensor
^ EGR position sensor
^ Feedback system
^ Intake air thermosensors
^ Oxygen sensor
^ Throttle sensor
^ Water thermosensor