Diagnosis and Adjustment

GENERAL INFORMATION

The electronic engine control system consists of the electronic control unit and the following sensors: – throttle position sensor – informs the electronic control unit of the throttle position and the throttle closing/opening speed; – engine coolant temperature sensor – informs the control unit of engine temperature; – air flow sensor – informs the control unit of the quantity and temperature of air passing through the air intake duct; – crankshaft angle sensor (installed in the ignition distributor) – informs the control unit of engine speed and crankshaft position; – vehicle speed sensor (installed in the speedometer) – informs the control unit of vehicle speed; – power steering switch and air conditioning switch (if equipped) – inform the control unit when these systems are switched on so that the control unit can adjust the engine idle speed to compensate for the additional engine load from these systems; – exhaust gas sensor (only on models with a catalytic converter) – sends oxygen content information in the exhaust gas to the control unit.

The electronic control unit (ECU) analyses the signals from all of these sensors. Based on this information, the ECU controls the fuel injectors by varying the pulse width, the length of time the injectors are open, to provide a richer or leaner air-fuel mixture as required. The air-fuel mixture and engine idle speed are continuously adjusted by the ECU to ensure optimum starting (cold or hot start), engine warm-up, idle, best running on the road and sharp acceleration.

The engine ECU also fully controls the engine idle speed through an auxiliary air valve. This valve, which is mounted on the throttle body (1.6 litre engine models) or the intake manifold (2.0 litre engine models), controls the opening of the air passage that bypasses the throttle plate. When the throttle is closed, the ECU controls the opening of the valve, which regulates the amount of air passing through the throttle body bypass passage, and so controls the engine idle speed.

On 1.6 litre engine models, the throttle body has a device that automatically increases the engine idle speed while the engine warms up. This function is controlled by a thermo valve. When the engine is cold, the wax capsule in the thermo valve (the valve is installed in the throttle body) is contracted and the fast idle cam holds the throttle plate slightly open. As the engine warms up, the wax capsule expands, raising the valve plunger, which in turn rotates the fast idle cam to the required position.

On 2.0 litre engine models, the increase in idle speed is controlled by an air control valve, which consists of a bimetallic spring and a flap. When the engine is cold, the bimetallic spring holds the flap open. Air passes through the valve, increasing the air supply to the intake manifold. When the engine is running, voltage is applied to the bimetallic spring of the valve, which gradually increases its temperature. As the temperature rises, the bimetallic spring deforms and closes the flap, stopping the additional air supply to the engine.

On all models, a solenoid valve for increasing engine idle speed, which is controlled by the ECU, is also installed on the throttle body. The solenoid valve controls the opening of the air passage that bypasses the throttle plate and is used to increase engine speed when the power steering system or the air conditioning is switched on. Note: On some models the valve may be fitted but will not function.

On models with a catalytic converter, the ECU also monitors the exhaust emission control system and the evaporative emission control system, which are described in the relevant chapter.

If the signals received by the ECU from the sensors are outside the permitted limits or do not agree with each other, the ECU switches to fail-safe (limp-home) mode. In this case, the ECU ignores the incorrect sensor signals and substitutes a predetermined value for the sensor signal, which allows the engine to run, although with reduced efficiency. When the ECU enters fail-safe mode, the warning lamp on the instrument panel illuminates and the corresponding fault code is stored in the ECU memory.

If the warning lamp illuminates, have the vehicle inspected by a qualified technician as soon as possible. A complete diagnosis of the engine management system can be carried out in a workshop using electronic diagnostic equipment, which is simply connected to the diagnostic socket. Note: The engine ECU also has a self-diagnosis mode that can be used by the vehicle owner. For further details, refer to sub-section 6.3.1.12.