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
Fuel is metered to intake system through individual injectors, mounted in intake manifold next to intake valves, one for each cylinder. Electronic Control Module (ECM) monitors data from various sensors and controls such functions as fuel injector pulse width ("on" time), ignition timing, and emission control devices.
ELECTRONIC CONTROL MODULE (ECM)
If a system malfunction occurs, a back-up system within ECM controls fuel and ignition system functions according to preprogrammed values (limp-in mode). This allows vehicle to be driven, but performance may not be optimal.
A self-diagnostic function allows ECM to store Diagnostic Trouble Codes (DTCs) in its memory. If a system fault occurs, Malfunction Indicator Light (MIL) will illuminate to inform driver of system problems and the appropriate DTC will be stored in memory. For further self-diagnostic system information, see TESTS W/CODES article in this section.
Note. Components are grouped into 2 categories. First category is INPUT DEVICES, which are components that control or produce signals monitored by ECM. Second category is OUTPUT SIGNALS, which are components controlled by ECM.
INPUT DEVICES
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine input usage on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article in this section. Available input signals include the following
A/C Switch
Signals ECM of A/C operation.
Atmospheric Pressure Sensor
Detects atmospheric pressure and intake manifold pressure. ECM uses this signal to compensate for variations in altitude, which affect air/fuel mixture ratios.
Camshaft Position (CMP) Sensor
Sensor is located on camshaft support on left cylinder bank. Based on signals received from CMP sensor, ECM identifies cylinder No. 1. ECM then uses these signals to trigger ignition system and fuel injectors. This device may also be referred to as cam angle sensor.
Crankshaft Position (CKP) Sensor
Sensor is located on oil pump, at front center of cylinder block. Based on signals received from CKP sensor, ECM determines crankshaft angle and triggers ignition system and fuel injectors. This device may also be referred to as a crank angle sensor.
Engine Coolant Temperature (ECT) Sensor
ECM supplies reference voltage signal to thermistor. Resistance of ECT sensor changes with variations in coolant temperature, causing signal voltage to increase or decrease.
Front & Rear Oxygen Sensors (O2S)
Sensors generate voltage according to exhaust gas oxygen content. Signal voltage increases when oxygen content is low (rich), and decreases when oxygen content is high (lean). ECM controls air/fuel ratio based on voltage generated.
Fuel Lever Sensor
Monitors fuel level in the fuel tank.
Fuel Tank Pressure Sensor
Detects evaporation gas pressure in fuel tank.
Fuel Temperature Sensor
Monitors temperature of fuel in fuel tank.
Ignition Switch
Signals ECM of ignition switch operation.
Intake Air Temperature (IAT) Sensor
ECM applies reference voltage to a thermistor installed on air cleaner housing. Signal voltage increases and decreases with variations in air temperature.
Knock Sensor (KS)
Installed on cylinder block, this sensor responds to cylinder block vibrations resulting from detonation. If detonation occurs, KS generates a signal to ECM. Using this signal, ECM retards spark timing until detonation stops. This device may also be referred to as a detonation sensor.
Mass Airflow (MAF) Sensor
Hot-wire type airflow sensor uses heat transfer between incoming air and a heating resistor located in air intake to convert quantity of airflow into engine to an electrical signal.
Park/Neutral Position (PNP) Switch
Switch sends transmission gear position signal to ECM.
Starter Switch
Signals ECM condition of engine cranking.
Throttle Position (TP) Sensor
TP sensor consists of a potentiometer and an idle switch. TP sensor sends ECM a signal corresponding to throttle valve position. Idle switch signal occurs when throttle is near idle position. ECM uses these signals to control air/fuel ratio during acceleration, deceleration, and idle.
Vehicle Speed Sensor (VSS)
ECM supplies voltage to one side of a reed switch located in speedometer. Speedometer cable revolution opens and closes reed switch, providing ECM with continuity/no continuity input. ECM interprets this signal as vehicle speed.
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 of each output component, refer to indicated system.
Canister Purge Control Valve (CPCV)
See FUEL EVAPORATIVE SYSTEM under EMISSION SYSTEMS.
Data Link Connector (DLC)
See SELF-DIAGNOSTIC SYSTEM .
EGR Solenoid Valve
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
Fuel Injector(s)
See FUEL CONTROL (SFI) under FUEL SYSTEM.
Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEM.
Idle Air Control (IAC) Solenoid Valve
See IDLE SPEED under FUEL SYSTEM.
Malfunction Indicator Light (MIL)
See SELF-DIAGNOSTIC SYSTEM .
Power Transistor (Distributor Ignitor)
See IGNITION SYSTEMS .
Self-Diagnostics
See SELF-DIAGNOSTIC SYSTEM .
Tachometer
See TACHOMETER under MISCELLANEOUS CONTROLS.
Vent Control Solenoid Valve
See FUEL EVAPORATIVE SYSTEM under EMISSION SYSTEMS.
Fuel Pressure Regulator
Regulator maintains constant fuel system pressure by bleeding excess fuel at injector rail back to fuel tank. Intake manifold vacuum acts on regulator diaphragm to control position of bleed-off valve in regulator.
Fuel Pump
Impeller-type pump is used. On all models, fuel pump is located in fuel tank. On all models, fuel pump pressurizes fuel through in-line filter, to fuel rail. Fuel pump receives battery power through fuel pump relay.
ECM energizes fuel pump relay based on inputs from ignition switch and ignition coil. During cranking, cranking circuit supplies current to energize fuel pump relay. After engine starts and key is released to RUN position, ECM provides fuel pump relay ground. This activates fuel pump.
Fuel Injectors
Fuel is supplied to engine through injector valves located near intake valve ports. ECM controls injector pulse width, which controls quantity of fuel metered through injectors.
Injectors are pulsed sequentially in spark plug firing order. Constant fuel pressure is maintained to the injectors. Air/fuel mixture is regulated by amount of time injector stays open (pulse width). Various sensors provide information to the ECM to control pulse width.
Fast Idle Control Device (FICD) Solenoid
FICD solenoid is mounted in-line with fresh air intake duct and intake manifold. When A/C request is present, FICD solenoid opens and allows fresh air to by-pass throttle valve, increasing idle speed for the extra engine load.
The IAC solenoid valve is incorporated in injection body and regulates amount of air which by-passes throttle valve built into injection body. It is activated by signals sent by ECM to mainly maintain engine idle speed to specified engine speed.
Idle air control solenoid valve is a "current-proportion" solenoid type which consist of a coil, valve shaft, spring and housing. Housing is integral part of injection body and is provided with an air passage opened or closed by valve.
Idle Speed Control (ISC) Valve
Valve is mounted on air filter housing, in-line between air filter housing and intake manifold. At idle, ECM opens and closes solenoid valve. This allows fresh air to by-pass throttle valve through solenoid valve, into intake manifold.
DISTRIBUTORLESS IGNITION
Distributorless ignition system is controlled by ECM. ECM receives signals from mass airflow sensor, engine coolant temperature sensor, crankshaft position sensor, camshaft position sensor, knock sensor, throttle position sensor and park/neutral switch to operate engine at optimum efficiency. Ignition timing is also determined by ECM which immediately transmits a primary current off signal to the power transistor (distributor ignitor) to control ignition timing. Ignition timing is fixed at 10 degrees during a starting condition.
Ignition Timing Advance Control
On all models, ignition timing advance is controlled by ECM. Based on sensor input signals, ECM adjusts ignition timing to preprogrammed advance and retard specifications.
When engine knock occurs, the KS generates a signal to ECM. ECM retards spark timing until engine knocking stops, then gradually advances spark timing.
EXHAUST GAS RECIRCULATION (EGR)
EGR lowers oxides of nitrogen (NOx) exhaust emissions by admitting exhaust gases back into intake system. Exhaust gases lower peak combustion temperatures, which lowers NOx emissions.
This system consists of EGR valve, EGR solenoid valve and backpressure transducer (BPT). EGR valve, located between exhaust manifold and collector, is controlled by vacuum through BPT from throttle body and controls exhaust gas flow from exhaust manifold to intake manifold.
EGR solenoid valve, located between throttle body and EGR valve, is controlled by ECM according to engine driving conditions, and opens and closes the vacuum line between BPT and EGR valve diaphragm.
BPT valve, located between throttle body and EGR solenoid valve, varies throttle port vacuum operating EGR valve according to throttle opening and exhaust gas pressure, and controls rate of exhaust flow to EGR valve.
Purge Control Solenoid (PCS) Valve
Purge Control Solenoid (PCS) valve, located in evaporation line between canister and ported vacuum source, is built into collector chamber. See CANISTER PURGE CONTROL VALVE (CPCV). Under appropriate conditions, ECM energizes PCS valve. This allows ported vacuum signal to CPCV.
Located on top of canister, this vacuum-controlled valve opens and closes purge line between intake manifold and canister. CPCV valve remains closed at idle because ported vacuum activates its control diaphragm. When vacuum activates control diaphragm, purge line opens, and stored vapors are drawn into intake manifold.
Vent Control Solenoid Valve (VCSV)
Vent control solenoid valve, located at intake port of canister, is normally open to atmosphere. ECM sends a signal to VCSV to close the port to air when evaporation gas is detected at evaporation line.
Fuel Cut Valve (FCV)
On AWD models, fuel cut valve is built onto evaporation pipe of fuel tank. Rising level of fuel in fuel tank causes a float to move up and close the cap hole to prevent fuel from entering evaporation line.
Two-Way Valve
Two-way valve is located in evaporation line between fuel tank and canister. When evaporation gas pressure exceeds specific value, two-way valve is pressurized to push a spring which lifts the valve seat so that evaporative gas is discharged to canister. When evaporation gas pressure drops to specified value, valve moves to close valve seat.
Rollover Valve
Rollover valve is incorporated into the fuel system for prevention of fuel leakage in the event of vehicle rolling over. Valve will cut fuel flow if vehicle is rolled 90 degrees or more.
POSITIVE CRANKCASE VENTILATION (PCV)
PCV system is designed to prevent blow-by gases in engine crankcase from escaping into atmosphere. System consists of a sealed oil filler cap, rocker cover with fresh air inlet, connecting hoses, air inlet duct and PCV valve.
PCV valve contains a spring-loaded plunger. By opening and closing in direct relation to engine load, PCV valve meters blow-by gas flow to combustion chamber. During periods of high manifold vacuum, such as at idle or deceleration, valve is almost completely closed, limiting flow of blow-by gases. During cruise speeds, valve permits a greater flow of blow-by gases.
Under conditions in which high amounts of crankcase pressure are produced (such as heavy loads or at wide open throttle), system allows excess blow-by gas back through air intake duct.
Self-diagnostic system monitors output signals through DLC. Diagnostic Trouble Codes (DTCs) can only be read using a scan tool connected to DLC. For additional information, see TESTS W/CODES article in this section.
All vehicles are equipped with MIL (CHECK ENGINE light) on instrument panel. Light comes on when ignition switch is turned on (bulb check), and when system malfunctions occur. For additional information, see TESTS W/CODES article in this section.
ECM provides signal to drive tachometer.