Engine Controls - Theory & Operation Geo Prizm pre-I

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.

COMPUTERIZED ENGINE CONTROLS

Port Fuel Injection (PFI) is used on Prizm. The electronic fuel injection engine control system monitors vehicle operating conditions through input signals and regulates air/fuel mixture and other engine control operations by output signals. This lowers exhaust emissions, while maintaining fuel economy and driveability.

The control systems have a fail-safe mechanism. If a fault occurs while driving, the system will substitute pre-programmed values. Driving performance will be affected, but vehicle may still be driven. These systems have a self-diagnostic feature capable of recognizing a system fault and storing a related trouble code in memory for future retrieval and diagnosis.

ELECTRONIC CONTROL MODULE (ECM)

Note. For electronic control module locations, see ECM LOCATION table.

Power for ECM is received from stoplight fuse. The ECM distributes power or controls ground of various sensors, switches and solenoids for engine control.

ECM LOCATION (1)

INPUT DEVICES

The ECM controls various output devices based on signals from input devices. These devices include sensors, switches, or simple monitored circuits, such as a RPM reference signal from the ignition coil. Available input signals include the following

A/C Signal

On models with A/C, a signal from the A/C control module is sent to ECM when air conditioner is operating. The ECM uses this signal with other inputs to determine idle speed.

Battery Voltage Signal

The ECM monitors battery voltage. A drop in battery voltage directly effects pulse width of the fuel injector. As battery voltage drops, pulse width decreases, causing a leaner air/fuel mixture. The ECM compensates by increasing pulse width to provide richer mixture.

Coolant Temperature Sensor

Coolant temperature sensor monitors coolant temperature. A reference voltage (supplied and monitored by ECM) is modified by sensor resistance, which changes according to temperature. High temperature causes low resistance. Low temperature causes high resistance. ECM uses this information to determine control (output) signals to injectors and emission components.

Crank Angle Sensor

Engine speed signal is generated by pick-up coil/crank angle sensor and trigger wheel assembly in the distributor. This pulsing signal is sent to ECM where it is used to calculate engine speed. It is also used as input to determine control system functions.

Engine Start Signal

Engine start signal, sent from starter circuit, is used by ECM to determine whether engine is cranking or not. Using this signal, ECM calculates fuel injection timing and idle speed control.

Knock Sensor (Prizm GSi 4A-GE VIN 5)

Knock sensor, located in rear of cylinder block, sends a variable AC voltage signal to ECM depending on engine detonation. The ECM uses this signal to determine whether to retard ignition timing.

Manifold Absolute Pressure (MAP) Sensor (Prizm 4A-FE)

MAP sensor connects to ECM by a wire harness and to engine by a manifold vacuum hose. ECM supplies monitors a 5-volt reference signal to MAP sensor. MAP sensor voltage varies according to changes in engine load (manifold vacuum).

ECM interprets voltage change as changes in engine load and uses signal to help determine control of fuel injector, shift-indicator light and EGR solenoid.

Mass Airflow (MAF) Sensor (Prizm GSi 4A-GE)

Airflow sensor measures air entering the engine and produces an appropriate output signal proportional. The ECM uses this information for fuel injector control.

Manifold Air Temperature (MAT) Sensor

Air temperature sensor resistance changes with respect to temperature. High temperature causes low resistance. Low temperature causes high resistance.

A reference voltage, supplied and monitored by ECM, is modified by sensor resistance. The ECM uses this information to determine control output signals to injectors. See MANIFOLD AIR TEMPERATURE SENSOR (MAT) LOCATION table.

MANIFOLD AIR TEMPERATURE SENSOR (MAT) LOCATION

Oxygen (O2) Sensor

Oxygen sensor is mounted in exhaust manifold or exhaust pipe, where it contacts exhaust gases. The O2 sensor generates voltage according to oxygen content of exhaust gases. Voltage will vary from .1 volt (lean condition) to as high as one volt (rich condition).

Sensor will not generate a voltage signal until it reaches operating temperature. Sensor sends voltage signal to ECM, which uses it to obtain correct emissions by adjusting air/fuel ratio. Until sensor is warmed up, ECM adjusts air/fuel mixture based upon preprogrammed tables in ECM memory.

Note. Prizm GSi (4A-GE VIN 5) models use a heated oxygen sensor.

Park/Neutral Switch On A/T Models

On A/T applications, the ECM monitors switch position and receives a voltage signal when transaxle/transmission is in any forward or reverse gear. The ECM uses this input signal to help control of fuel injectors and idle speed control.

Sub-Oxygen (O2) Sensor (Prizm GSi 4A-GE VIN 5 Calif. Only)

Sub-oxygen sensor is mounted in exhaust system, where it contacts exhaust gases after the catalytic converter. The sensor rechecks emission level after the catalytic converter and sends reference signal to the ECM.

Throttle Position Sensor (Except Prizm 4A-FE)

The ECM supplies throttle position sensor with a 5-volt reference signal. Sensor contains variable resistor and idle switch. Throttle position sensor sends ECM an output signal corresponding to opening of throttle valve and an idle switch signal (only when throttle is in idle position). ECM uses these signals to control air/fuel ratio during acceleration, deceleration and idling. These signals are also used to determine idle speed.

Throttle Switch (Prizm 4A-FE)

Signals ECM of closed throttle condition. The idle switch and Wide Open Throttle (WOT) switch are a part of the throttle switch. Switch is open except when throttle is closed or wide open.

At closed throttle, idle switch closes. At wide open throttle, WOT switch closes. These on/off signals are monitored by ECM and used to control fuel injection, idle speed control, shift indicator light (if equipped) and EGR vacuum switching valve (if equipped).

Vehicle Speed Sensor

Vehicle speed sensor consists of a lead switch and magnet built into speedometer head. As magnet rotates with speedometer cable, its magnetic force causes lead switch to turn on and off. This on/off pulse is sent to ECM and interpreted as vehicle speed. ECM uses this signal to help determine control of idle speed control solenoid.

OUTPUT SIGNALS

Note. Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. For theory and operation on each output component, refer to the system indicated in brackets after component.

The ECM processes information from the input signals and sends appropriate voltage control signals to the following engine controls

1. A/C Vacuum Switching Valve (Fuel System)
2. Automatic Transmission Lock-Up Relay (Miscellaneous Controls)
3. CHECK ENGINE Light (Self-Diagnostic System)
4. Circuit Opening Relay (Fuel System)
5. Control Relay (Fuel System)
6. Distributor Ignitor (Ignition System)
7. EFI Main Relay (Fuel System)
8. EGR Vacuum Switching Valve (Emission Systems)
9. Fuel-Cut System (Fuel System)
10. Fuel Injector (Fuel System)
11. Fuel Pressure Regulator Vacuum Switching Valve Fuel System)
12. Fuel Pump Relay (Fuel System)
13. Idle Air Control (IAC) Valve (Fuel System)
14. Idle Speed Control (ISC) Solenoid (Fuel System)
15. Idle Speed Control (ISC) Valve (Fuel System)
16. Idle-Up Vacuum Switching Valve (IVSV) (Fuel System)
17. Ignition Coil (Ignition System)
18. Positive Temperature Coefficient (PTC) Heater Relay (Emission Systems)
19. Secondary Air Vacuum Switching Valve (SEC-VSV) Air Induction System)
20. Shift Indicator Light (Miscellaneous Controls)
21. Throttle Opener Vacuum Switching Valve (Fuel System)

Circuit Opening Relay

Circuit opening relay is located behind center console of instrument panel. Relay is mounted on bracket attached to the ECM. During cranking, a signal from the ignition switch closes the contacts in circuit opening relay to energize fuel pump. Once engine is started, ECM controls the circuit opening relay and fuel pump remains on.

EFI Main Relay

EFI main relay supplies voltage to circuit opening relay when ignition is turned on. The ECM controls the fuel pump relay for fuel pump operation.

Fuel Pressure Regulator

Fuel pressure regulator is a spring/vacuum-operated, diaphragm-type relief valve which maintains a regulated fuel pressure under all conditions. When manifold vacuum is high (low fuel requirements), diaphragm is drawn in, counteracting spring pressure. In this condition, excess fuel is routed back to fuel tank. When manifold vacuum drops (engine load), spring pressure overcomes vacuum, closing off fuel tank return line. This maintains pressure and volume to fuel injectors.

Fuel Pressure Regulator Vacuum Switching Valve (Prizm GSi)

Fuel pressure regulator vacuum switching valve controls vacuum supply to the fuel pressure regulator. The ECM controls vacuum switching valve ground by monitoring engine operation.

Fuel Pump

Electric fuel pump, located in fuel tank, receives power when ECM provides ground for circuit opening relay. Fuel pump delivers fuel to the injectors where system pressure is controlled by fuel pressure regulator. Fuel pump internal check valve maintains pressure in fuel lines after fuel pump is turned off.

The ECM monitors battery voltage. A drop in battery voltage directly affects fuel injector pulse width. As battery voltage drops, pulse width decreases, causing a leaner air/fuel mixture. The ECM compensates by increasing pulse width to provide richer mixture.

Fuel-Cut System

The fuel-cut system will stop fuel injection during deceleration to prevent unburned gases from being exhausted.

Fuel Injector

When injector solenoid coil is energized by the ECM (on time), coil becomes an electromagnet. This lifts injector plunger upward, allowing fuel to be injected into the intake manifold or cylinder. The air/fuel mixtures are controlled by the injector pulse width (on time). ECM determines proper pulse width based upon input signals from various sensors and switches.

Fuel injection timing is determined by ECM based upon RPM signals received from either the distributor or ignition coil. Prizm uses Port Fuel Injection (PFI), which incorporates fuel injectors mounted in a fuel rail assembly.

Air Valve

Air valve is used to increase idle speed when engine coolant is less 176°F (80°F) on Prizm GSi models or 140°F (60°F) on all others. The coolant temperature alters the thermowax pellets to increase or decrease air into the intake.

When coolant temperature is less than specified, valve opens by spring pressure, allowing air into intake. This increased air flow increases the idle speed. As coolant temperature increases, valve closes and shuts off the airflow, causing idle speed to decrease. When engine coolant temperature is greater than 176°F (80°F) on Prizm GSi models or 140°F (60°F) on all others, valve fully closes and normal idle speed is obtained.

Idle Speed Control (ISC) Valve (Prizm)

The ISC valve, located on end of intake plenum, by-passes air around throttle valve directly into the intake manifold. Air is allowed to pass around IAC valve when it is activated by the ECM. Valve is energized whenever idle speed drops to less than desired RPM due to engine load (i.e. electrical, A/C, P/S, A/T in Drive, etc.).

Idle-Up Vacuum Switching Valve (IVSV) (Prizm GSi)

The IVSV, located on mass airflow sensor, by-passes air around the throttle valve of mass airflow sensor when it is activated by the ECM. The VSV is energized whenever idle speed drops to less than desired RPM due to engine load (i.e. electrical, A/C, P/S, A/T in Drive, etc.).

Prizm

Ignition system consists of a distributor which uses a signal rotor and pick-up coil/crank angle sensor to produce reference signals to the ECM. Ignition coil is mounted on the side of the distributor. External ignitor is mounted on left strut tower below diagnostic connector.

Power for ignition coil is provided through the ignition switch. As rotating signal rotor passes pole piece of pick-up coil/crank angle sensor, a reference signal is sent to the ECM. The ECM uses this signal to determine when to signal the ignitor to open ground circuit for primary ignition.

When ignitor opens ground circuit for primary ignition, magnetic field around ignition coil windings collapses, producing an induced high voltage surge which is used to fire the spark plugs. Ignitor sends a signal back to ECM to confirm primary ignition circuit operation was completed.

Prizm GSi

Ignition system consists of a distributor which uses a signal rotor and pick-up coil/crank angle sensor to produce reference signals to the ECM. External ignitor is mounted on right side of firewall in engine compartment.

Power for ignition coil is provided through the ignition switch. As rotating signal rotor passes pole piece of pick-up coil/crank angle sensor, a reference signal is sent to the ECM and ignitor. The ignitor sends a RPM reference signal to the ECM. The ECM uses reference signals to determine when to signal the ignitor to open ground circuit for primary ignition.

When ignitor opens ground circuit for primary ignition, magnetic field around ignition coil windings collapses, producing an induced high voltage surge which is used to fire the spark plugs. Ignitor sends a signal back to ECM to confirm primary ignition circuit operation was completed.

Ignition Timing Advance Control

Ignition timing is controlled by ECM based upon sensor input signals. Input signals may be supplied by coolant temperature sensor, pick-up coil/crank angle sensor, MAP sensor (Prizm), MAF sensor (Prizm GSi), knock sensor (Prizm GSi), throttle position switch or sensor and vehicle speed sensor.

EXHAUST GAS RECIRCULATION (EGR)

To lower oxides of nitrogen (NOx) exhaust gas emissions, an Exhaust Gas Recirculation (EGR) system is used. The EGR system introduces exhaust gases into intake system. Exhaust gases are noncombustible gases which, when combined with the incoming air/fuel mixture, lower peak combustion chamber temperatures.

EGR valve receives ported operating vacuum from an ECM regulated EGR Vacuum Switching Valve (VSV). Various inputs to ECM are used to determine EGR operation. Vacuum signal to EGR valve is further controlled by a vacuum modulator located in the vacuum line between the EGR VSV and ported vacuum supply.

Under low driving speeds and light load conditions, vacuum modulator diaphragm is pushed downward and opens vacuum modulator. This allows air to enter modulator from the outside. This reduces vacuum supply to EGR valve and EGR valve closes slightly, reducing amount of recirculated exhaust gases.

Under high driving speeds and heavy load conditions, vacuum modulator diaphragm is pushed upward and closes vacuum modulator. No air can enter modulator from the outside. This increases vacuum supply to EGR valve and valve opens slightly, increasing amount of recirculated exhaust gases. Under following conditions, ECM does not allow EGR operation

1. Coolant temperature is low
2. Engine operating under heavy load
3. Low intake manifold pressure
4. Mass airflow sensor reading is low (Prizm GSi only)
5. Throttle valve is at idle position

EVAPORATIVE FUEL EMISSIONS SYSTEM

Fuel tank vapors are stored in charcoal canister. A check valve in fuel tank cap maintains constant pressure in the fuel tank. When fuel tank pressure exceeds specified pressure, fuel tank vapors flow to charcoal canister. Charcoal canister retains vapors until canister purge valve opens in accordance with engine conditions, allowing fuel vapors to enter intake manifold.

Bimetallic Vacuum Switching Valve (BVSV) is mounted in engine coolant passage. When engine coolant reaches specified temperature, BVSV opens, allowing air to flow through the valve and into intake manifold. This airflow opens canister purge valve and allows fuel vapors to flow from carbon canister into the intake manifold. Canister purge valve will only be opened with engine running, engine at normal operating temperature and throttle valve is above idle position.

POSITIVE CRANKCASE VENTILATION (PCV)

The PCV system circulates crankcase blow-by gases (hydrocarbons) into the air induction system rather than allowing them to escape to the atmosphere. Crankcase gases are mixed with air/fuel mixture and are burned in combustion chamber.

Crankcase ventilation system uses a PCV valve which prevents hydrocarbon fumes from collecting in intake manifold when engine is not running. When engine is above idle, manifold vacuum is low, spring pressure opens PCV, allowing crankcase fumes to be drawn into intake manifold. When engine is at idle, manifold vacuum is high, which reduces PCV opening and amount of crankcase fumes drawn into intake manifold. The PCV valve is a metered orifice type on Prizm GSi and Storm GSi models or check valve type on all others.

CHECK ENGINE LIGHT

All vehicles are equipped with a CHECK ENGINE light, located on the instrument panel. Light will illuminate when ignition is turned on and engine is not running. Light should go out when engine is started. When CHECK ENGINE engine light remains on or flashes, with engine running, the self-diagnostic system has detected a problem. If problem goes away, light will go out after 10 seconds, but a trouble code will remain stored in the ECM memory.