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.
AIR INTAKE SYSTEM
Air intake system, used on fuel injected vehicles, supplies air for engine. System consists of an air cleaner, air intake pipe, throttle body, Electronic Air Control Valve (EACV), by-pass control system (Prelude 2.0L), tandem control valve system (Civic and CRX TBI), fast idle mechanism (except TBI) and intake manifold. A resonator chamber in the air intake pipe reduces noise as air is drawn into system.
BY-PASS CONTROL SYSTEM (PRELUDE 2.0L)
By-pass control system consists of 2 separate intake paths in intake manifold, a by-pass valve, a by-pass control diaphragm and a by-pass control solenoid valve. ECU selects air intake path most favorable for engine performance by operating by-pass valves to direct airflow through a long or short intake path. A long intake path is used for high torque at low RPM. A short intake path is used for high power at high RPM.
TANDEM CONTROL VALVE SYSTEM (CIVIC & CRX TBI)
Tandem control valve system is used to improve fuel atomization from main fuel injector by regulating venturi vacuum to tandem control valve diaphragm through an ECU-controlled solenoid valve. Solenoid is energized when engine coolant temperature is less than 160°F (70°C) and engine speed is above 1500 RPM on A/T models or 2000 RPM on M/T models. When coolant temperature reaches 160°F (70°C), solenoid is energized full time.
When solenoid is de-energized, spring-loaded control valve closes tandem valve located above injectors, decreasing throttle bore diameter, increasing air velocity and promoting improved atomization of injected fuel in response to airflow rate. When engine solenoid is energized, venturi vacuum is allowed to act against tandem control diaphragm to open tandem control valve.
THROTTLE CONTROL SYSTEM
Dashpot diaphragm functions as a cranking opener. When engine is at idle, intake manifold vacuum is applied on dashpot diaphragm and pulls up diaphragm rod so throttle valve is in idle position. During cranking with starter, spring in dashpot diaphragm pushes throttle valve open a small amount for assisting engine starting.
THROTTLE BODY (PFI)
Throttle body is a single-barrel sidedraft type. Lower portion of throttle valve is heated by engine coolant from cylinder head. Idle adjusting screw (to increase and decrease by-pass air) and canister/purge port are both located on top of throttle body. A dashpot is used to slow throttle valve as it approaches closed position, preventing rich mixtures on deceleration.
THROTTLE BODY (TBI)
Throttle body is a single-barrel downdraft type. Idle adjusting screw (opens throttle valve) and canister purge port are located on bottom of throttle body. A dashpot is used to slow throttle valve as it approaches closed position, preventing rich mixtures on deceleration.
ELECTRONIC CONTROL UNIT (ECU)
Computerized engine controls are used to control fuel, ignition and emission control systems. Electronic Control Unit (ECU) receives input signals from various sensors and components. ECU then compares each signal with a preprogrammed parameter in its memory. Based on this comparison, output signals are then adjusted to allow vehicle to perform optimally under all operating conditions. ECU is located under passenger-side carpet on all models.
Note. Components are grouped into 2 categories. First category covers INPUT DEVICES, which control or produce voltage signals monitored by ECU. Second category is OUTPUT SIGNALS, which are components controlled by ECU.
CEC INPUT DEVICES
Vehicles are equipped with various input devices. Not all devices are used on all models. To determine component usage of a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section. Available input signals include these signals
A/C SWITCH SIGNAL
This switch signals ECU of demand for air conditioning. ECU then increases engine RPM to compensate for additional engine load.
ALTERNATOR (FR) SIGNAL
This signals ECU when alternator field circuit is energized. ECU compensates for changes in idle speed and low battery voltage which can cause erratic injector pulse width.
ATMOSPHERIC PRESSURE (PA) SENSOR
PA sensor converts atmospheric pressure into electrical signals and relays information to ECU.
AUTOMATIC TRANSMISSION SHIFT POSITION SIGNAL
This signals ECU when transmission selector lever is in Park, Neutral or D4 position.
BATTERY VOLTAGE (IGN.1)
This provides ECU with a battery voltage signal from ignition circuit when ignition is on.
BRAKE SWITCH SIGNAL (CIVIC & CRX)
This signals ECU when brake pedal is depressed.
COOLANT TEMPERATURE (TW) SENSOR
Coolant temperature sensor is a temperature dependent variable resistor (thermistor). Resistance of thermistor decreases as coolant temperature increases.
CYL SENSOR (PRELUDE)
CYL sensor is mounted on end of camshaft. Sensor detects position of No. 1 cylinder for sequential firing of each fuel injector.
ELECTRIC LOAD DETECTOR (CIVIC & CRX)
This signals ECU when an electrical load (headlights, radio, etc.) exists so ECU can compensate for additional engine load.
EGR VALVE LIFT SENSOR (EXCEPT CIVIC & CRX 1.6L)
EGR valve lift sensor detects amount of EGR valve lift and sends information to ECU. ECU uses this information, along with other sensor inputs, to determine regulation of EGR control solenoid valve.
INTAKE AIR TEMPERATURE (TA) SENSOR
Intake air temperature sensor is a temperature dependent variable resistor (thermistor). Thermistor resistance decreases as intake air temperature increases.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
MAP sensor converts manifold absolute pressure into electrical signals and sends signals to ECU. MAP sensor signals are a measurement of engine load.
OXYGEN (O2) SENSOR
Oxygen sensor detects oxygen content of exhaust gases and sends signal to ECU, which varies air/fuel ratio to maintain a 14.7:1 ratio under most conditions. This ratio is most efficient for combustion and catalytic converter operation.
Accord and Prelude 2.1L models use heated oxygen sensors. Prelude 2.0L models use 2 non-heated oxygen sensors. All other models use a single, non-heated oxygen sensor.
POWER STEERING PRESSURE SWITCH (ACCORD & PRELUDE)
This switch signals ECU when power steering load is high. ECU then compensates for load by increasing engine RPM.
STARTER SIGNAL
Signals ECU when engine is cranking.
TDC/CRANK SENSOR (CIVIC, CRX 1.5L TBI & PRELUDE)
TDC/CRANK sensor is a combination sensor, located inside distributor housing. Each sensor generates a separate signal.
TDC sensor determines ignition timing at start-up (cranking) and when crank angle signal is abnormal. CRANK sensor detects engine RPM, fuel injection timing and ignition of each cylinder.
TDC/CRANK/CYL SENSOR (ACCORD, CIVIC 1.6L & CRX PFI)
TDC/CRANK/CYL sensor is a combination sensor located inside distributor. Each sensor generates a separate signal. CRANK sensor detects engine RPM, fuel injection timing and ignition of each cylinder. TDC sensor detects ignition timing at start-up (cranking) and when crank angle signal is abnormal. CYL sensor detects position of No. 1 cylinder for sequential fuel injection to each cylinder.
THROTTLE ANGLE (POSITION) SENSOR
Throttle angle sensor is a 3-wire potentiometer connected to throttle shaft. As throttle angle changes, throttle angle sensor varies voltage signal monitored by ECU. Sensor voltage ranges from about one volt at closed throttle to about 5 volts at wide open throttle.
VEHICLE SPEED SENSOR/PULSER
Vehicle speed signal is generated by speed sensor (speed pulser on some models), which produces 4 pulses (switch closures to ground) per speedometer cable revolution.
CEC 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 indicated system.
A/C CLUTCH ENGAGEMENT DELAY
See IDLE SPEED under FUEL SYSTEM.
CHECK ENGINE LIGHT & LED INDICATOR
See SELF DIAGNOSTIC SYSTEM .
ELECTRONIC AIR CONTROL VALVE
See IDLE SPEED under FUEL SYSTEM.
EXHAUST GAS RECIRCULATION CONTROL SOLENOID VALVE
See EXHAUST GAS RECIRCULATION (EGR) SYSTEM under EMISSION SYSTEMS.
FAST IDLE CONTROL SOLENOID VALVE
See IDLE SPEED under FUEL SYSTEM.
IGNITOR UNIT
See IGNITION SYSTEM .
FUEL INJECTOR
See FUEL CONTROL under FUEL SYSTEM.
LOCK-UP CONTROL SOLENOID VALVE
See TRANSMISSION under MISCELLANEOUS CONTROLS.
MAIN RELAY
See FUEL DELIVERY under FUEL SYSTEM.
PURGE CUT-OFF CONTROL SOLENOID VALVE
See EMISSION SYSTEMS .
Fuel Injection
Fuel system consists of an in-tank high pressure electric fuel pump, main relay, fuel filter, pressure regulator, injectors (1.5L TBI engines use 2 injectors, main and auxiliary) and injector resistor(s) (except Civic & CRX 1.5L PFI). This system delivers pressure regulated fuel to injectors and cuts fuel delivery when engine is not running.
Fuel Pump
Fuel pump consists of a DC motor, a circumference flow pump assembly, an internal relief valve for protecting fuel line system, an internal check valve for retaining residual pressure, an inlet port and discharge port. Pump assembly consists of impeller (driven by motor), pump casing (which forms pumping chamber) and pump cover.
Injector Resistor(s) (PFI)
Injector resistor(s) lowers current supplied to injectors to prevent damage to injector coils, allowing injectors a faster response time.
Main relay contains 2 individual relays. One relay is energized whenever ignition is on. It supplies battery voltage to ECU, power to injectors (except Civic and CRX 1.5L TBI), and power for second relay. Second relay supplies power to fuel pump (and injectors on Civic and CRX 1.5L TBI). Second relay is energized for 2 seconds when ignition switch is initially turned on and when engine is running.
Fuel Pressure Regulator
Fuel pressure regulator maintains proper fuel pressure to injectors. Regulator uses manifold vacuum to sense engine load and modifies fuel pressure to maintain driveability.
When manifold vacuum is high, vacuum diaphragm is drawn back, overcoming spring pressure. Excess fuel passes through pressure regulator and is returned to tank via fuel return line. When manifold vacuum decreases (engine load increases), spring pressure closes off return passage, thereby maintaining fuel pressure and volume.
Programmed Fuel Injection (PGM-FI) ECU
PGM-FI ECU is main controlling device of electronic fuel control system. The basic fuel injector duration is built into ECU memory. The ECU modifies basic injector duration according to input signals from various sensors to obtain final injector duration for fuel delivery.
Fuel injector consists of a solenoid, plunger needle valve and housing. When current is applied to solenoid coil, valve lifts and pressurized fuel is injected close to intake valve. Since needle valve lift and fuel pressure are constant, air/fuel ratio is determined by time valve is open (duration of current supplied to solenoid coil).
On TBI models, injector is sealed by 2 "O" rings and a rubber mount. On PFI models, injector is sealed by an "O" ring and seal ring at top and bottom. All seals, "O" rings and rubber mounts reduce injector operating noise and heat transfer.
A/C Clutch Engagement Delay (Civic, CRX & Prelude)
When ECU receives a demand for cooling from air conditioning system (A/C switch), it delays A/C clutch relay activation for a short time. This prevents compressor clutch from being energized until idle speed is increased by ECU, ensuring smooth transition into A/C mode without overloading engine.
Electronic Air Control Valve (EACV)
Engine idle speed is controlled by EACV (and by fast idle valve on Accord and Prelude). EACV varies amount of air by-passing throttle plate (into intake manifold) in response to signals from ECU. After engine starts, EACV opens for a short time to increase idle speed. Activation time is dependent upon engine coolant temperature. When coolant temperature is low, EACV is held open to obtain proper fast idle speed. After engine reaches normal operating temperature, EACV is activated only to maintain minimum idle speed.
Fast Idle Valve (Accord & Prelude)
Fast idle valve allows additional air to by-pass throttle plate into intake manifold. Increased idle speed prevents engine from running erratically during warm-up. Valve is controlled by a thermowax plunger, which contracts when cold and expands when hot.
Fast Idle Control Solenoid Valve (Civic & CRX 1.6L PFI)
When intake and coolant temperature are extremely low, ECU energizes solenoid valve. Fast idle control solenoid valve opens, increasing airflow and resulting in higher idle speed.
IGNITION TIMING CONTROL
ECU has complete control of ignition timing. Timing is controlled in response to signals from various sensors. Input signals from TDC, CRANK and/or CYL (if equipped), throttle angle, coolant temperature, and MAP sensors are all used by ECU to determine optimum ignition timing control.
Accord
Battery voltage is supplied through ignition switch to ignition coil and ignitor unit. Ignitor, located inside distributor, triggers ignition coil based upon signal from distributor pick-up coil. When triggered, high voltage from ignition coil is distributed to each spark plug by distributor.
Civic, CRX & Prelude
Battery voltage is supplied through ignition switch to ignition coil and ignitor unit. ECU triggers ignitor unit based upon signals from TDC, CRANK and/or CYL (if equipped) and other sensors. High voltage from ignition coil is distributed to each spark plug by distributor.
Except Civic & CRX 1.6L
EGR system reduces oxides of nitrogen (NOx) emissions by recirculating exhaust gases through EGR valve into intake manifold and back to combustion chambers.
System is composed of EGR valve, Constant Vacuum Control (CVC) valve and EGR control solenoid valve. System is operated by ECU, which analyzes input signals from EGR valve lift sensor and various sensors to provide optimum EGR flow.
EGR flow is cut when combustion gas temperatures are low (when engine emits a relatively low NOx emission) to assure good cold driveability. Purge cut-off solenoid valve is operated by ECU to cut off EGR operation.
EGR Valve
When opened, EGR valve circulates exhaust gas through intake manifold and back into combustion chamber to be reburned, resulting in reduced combustion chamber temperature. Lower temperature reduces oxides of nitrogen (NOx) and helps to control spark knock.
EGR Control Solenoid Valve
When ECU determines it is necessary to recirculate exhaust gases, it grounds EGR control solenoid valve, regulating vacuum controlling EGR valve. By regulating vacuum to EGR valve, EGR flow is adjusted for optimum control of NOx emission.
Exhaust Gas Recirculation Valve Lift Sensor
The EGR valve lift sensor detects EGR valve lift and sends information to ECU. The ECU uses this information, along with other sensor inputs, to determine regulation of EGR control solenoid valve.
EVAPORATIVE EMISSION SYSTEM (EVAP)
Evaporative emission system minimizes fuel vapor escaping into atmosphere. For emission control applications and components used for each model and engine, see appropriate EMISSION APPLICATION and VACUUM DIAGRAMS article.
Charcoal Canister
Charcoal canister temporarily stores fuel vapor until it can be purged, drawn into engine and burned in combustion chamber.
Fuel Tank Vapor Control System
Fuel tank vapor control system consists of a fuel cut-off valve, liquid/vapor separator, a 2-way valve and fuel filler cap. All fuel vapor inside fuel tank is directed to charcoal canister through fuel cut-off valve and liquid/vapor separator.
Fuel cut-off valve and liquid/vapor separator prohibit liquid fuel from entering 2-way valve. When fuel vapor pressure in fuel tank is greater than set value of 2-way valve, valve opens and regulates flow of fuel vapor into canister. The 2-way valve regulates both pressure and vacuum in tank. The filler cap contains a relief valve to prevent excessive pressure or vacuum build-up.
Vapor Purge Control System
The vapor purge control system controls when charcoal canister is to be purged. On some models, canister purging is accomplished when ECU activates purge cut-off solenoid valve, allowing fresh air to be drawn through bottom of charcoal canister and into a port on throttle body. On other models, canister purging is temperature-controlled by a thermovalve directing manifold vacuum to purge control valve, allowing venturi vacuum to purge charcoal canister.
POSITIVE CRANKCASE VENTILATION (PCV) SYSTEM
The Positive Crankcase Ventilation (PCV) system is designed to prevent blow-by gas (in engine crankcase) from escaping into atmosphere. The PCV valve contains a spring-loaded plunger. When engine starts, plunger in PCV valve is lifted in proportion to intake manifold vacuum and blow-by gas is drawn directly into intake manifold.
When ignition is initially turned on, ECU supplies ground to illuminate CHECK ENGINE warning light. The light remains on until vehicle starts. When an abnormal sensor signal occurs, ECU lights CHECK ENGINE light, stores failure code in erasable memory and indicates code with an LED (located on ECU) anytime ignition is on. For additional information, see appropriate G - TESTS W/ CODES article in the ENGINE PERFORMANCE Section.
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
A/C CLUTCH
When a demand for air conditioning exists in A/C switch circuit, ECU supplies ground to A/C clutch relay to operate the A/C compressor. ECU can also change engine idle RPM to compensate for additional engine load.
Lock-Up Control Solenoid Valve (A/T)
The ECU analyzes input signals from speed sensor and throttle angle sensor to determine when to apply an on/off voltage signal to lock-up control solenoid valve. This provides precise timing of torque converter lock-up system.
When energized, lock-up control solenoid valve allows hydraulic pressure in torque converter to lock transmission main shaft (input shaft) to torque converter cover (engine crankshaft).
See also:
• WIRING DIAGRAMS
• IDLE SPEED