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
The air intake system used on carbureted vehicles supplies fresh, filtered, temperature-controlled air to carburetor. The system consists of an air cleaner, air intake pipe, carburetor, intake manifold and intake air control system. The intake air control system maintains a uniform air temperature inside air cleaner. The carburetor receives fresh air, controlled within a narrow temperature range, regardless of outside temperature.
CARBURETOR
Prelude models use two 1-barrel sidedraft carburetors, each of which has a variable venturi. The variable venturi carburetors allow a smooth increase of engine speed and engine output due to change in venturi area. The changes made are in proportion to carburetor intake airflow rate.
The 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 2.1L), tandem control 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)
The by-pass control system consists of 2 separate intake paths in the intake manifold, a by-pass valve, a by-pass control diaphragm and a by-pass control solenoid valve. The ECU selects the 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)
The tandem control valve system is used to improve fuel atomization from main fuel injector. This is accomplished by regulating venturi vacuum to tandem control valve diaphragm through an ECU-controlled solenoid valve.
When engine RPM is low, solenoid is de-energized, and spring-loaded control valve closes tandem valve located above injectors. This decreases throttle bore diameter, increasing air velocity, promoting improved atomization of injected fuel in response to airflow rate. When engine RPM increases, solenoid is energized, venturi vacuum is allowed to act against tandem control diaphragm to open tandem control valve.
THROTTLE CONTROL SYSTEM
The 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 (EXCEPT CIVIC & CRX TBI)
The throttle body is a single-barrel sidedraft type. Lower portion of throttle valve is heated by engine coolant from cylinder head. The 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 (CIVIC & CRX TBI)
The throttle body is a single-barrel downdraft type. The idle adjusting screw (that 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.
COMPUTERIZED ENGINE CONTROLS
The computerized engine controls are used to control fuel, ignition and emission control systems. The Electronic Control Unit (ECU) used for each system accepts input signals from various input components. The ECU compares each of input signals it receives with a preprogrammed parameter preset in its memory. It instantly analyzes each of the input values and adjusts output signals, accordingly. This allows vehicle to perform optimally under a wide variety of operating conditions.
ELECTRONIC CONTROL UNIT (ECU)
The Electronic Control Unit (ECU) receives input signals (data) from various sensors. The ECU processes data and sends output signals to various controlling devices. This allows for precise control of fuel system, ignition system and emission system. See ECU LOCATION table.
| Application | Location |
|---|---|
| Accord | Under Driver's Seat |
| All Other Models | Under Passenger's Side Carpet |
ECU LOCATION
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the ECU. The second category covers OUTPUT SIGNALS, which are components controlled by ECU.
INPUT DEVICES
Vehicles are equipped with various input devices. Not all devices are used on all models. To determine the input usage of a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article. Available input signals include the following
A/C Switch Signal
This switch signals ECU when there is a demand for air conditioning (in A/C switch circuit), so ECU can increase engine RPM to compensate for additional engine load.
Alternator (FR) Signal (EFI)
This signals ECU when alternator is charging. ECU compensates for changes in idle speed and low battery voltage that can cause injector lag (duration/injector opening).
Atmospheric Pressure (PA) Sensor (EFI)
The 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 (Carb.)
This provides ECU with a constant battery voltage signal from battery.
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 PGM-FI control unit when brake pedal is depressed.
Clutch Switch Signal (M/T With Carb.)
This signals ECU when clutch is engaged.
Coolant Temperature (TW) Sensor
The coolant temperature sensor is a temperature dependent resistor (thermistor). The resistance of thermistor decreases as coolant temperature increases.
CYL Sensor (Prelude SFI)
The CYL sensor is mounted on end of camshaft. The CYL sensor detects position of No. 1 cylinder for sequential firing of each fuel injector.
Electric Load Detector (Civic & CRX)
This signals ECU when there is an electrical load (headlights, radio, etc.), so ECU can compensate for engine RPM change.
Exhaust Gas Recirculation Valve Lift Sensor (Calif. Except Carb. & Civic 1.6L)
The EGR valve lift sensor detects amount of 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.
Ignition Coil Output Signal (Carb.)
This signal allows ECU to monitor engine RPM through ignition circuit. This signal also controls fuel pump cut-off relay. If signal is lost, relay will stop electric fuel pump.
Intake Air Temperature (TA) Sensor
The intake air temperature sensor is a temperature dependent resistor (thermistor). The thermistor resistance decreases as intake air temperature increases.
Manifold Absolute Pressure (MAP) Sensor
The 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
The 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 SFI models use 2 non-heated oxygen sensors. All other models use a single non-heated oxygen sensor
Power Steering Pressure Switch (Accord & Prelude SFI)
This switch signals ECU when power steering load is high, so ECU can compensate for load by increasing engine RPM.
Starter Signal
Signals ECU when engine is cranking.
TDC/CRANK Sensor (Civic, CRX 1.5L TBI & Prelude)
The TDC/CRANK sensor is a combination sensor, located inside distributor housing. Each sensor generates a separate signal.
The TDC sensor determines ignition timing at start-up (cranking) and when crank angle signal is abnormal. The CRANK sensor detects engine RPM, fuel injection timing and ignition of each cylinder.
TDC/CRANK/CYL Sensor (Accord, Civic 1.6L & CRX HF & Si)
The TDC/CRANK/CYL sensor is a combination sensor located inside distributor. Each sensor generates a separate signal. The CRANK sensor detects engine RPM, fuel injection timing and ignition of each cylinder. The TDC sensor detects ignition timing at start-up (cranking) and when crank angle signal is abnormal. The CYL sensor detects position of No. 1 cylinder for sequential fuel injection to each cylinder.
Throttle Angle (Position) Sensor
The 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 (or speed pulser on some models), which produces 4 pulses (switch closures to ground) for each speedometer cable revolution.
OUTPUT DEVICES
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 system indicated in brackets after component.
- A/C Compressor Clutch Relay Delay (Fuel System)
- A/C Idle Boost Solenoid Valve (Fuel System)
- Air Leak Solenoid Valve - Prelude (Fuel System)
- Air Suction Control Solenoid Valve (Emission Systems)
- Air Vent Cut-Off Solenoid Valve (Emission Systems)
- CHECK ENGINE Light (Self-Diagnostics System)
- Cranking Leak Solenoid Valve (Fuel System)
- Early Fuel Evaporative Heater Relay (Emission Systems)
- Electronic Air Control Valve (Fuel System)
- Exhaust Gas Recirculation Control Solenoid Valve (Emission Systems)
- Fast Idle Control Solenoid Valve (Fuel System)
- Feedback Control Solenoid Valve (Fuel System)
- Frequency Solenoid Valve "A" (Fuel System)
- Fuel Pump Cut-Off Relay (Fuel System)
- Ignition Control Solenoid Valve (Ignition System)
- Ignitor Unit (Ignition System)
- Fuel Injector (Fuel System)
- Inner Vent Cut-Off Valve (Emission Systems)
- Lock-Up Control Solenoid Valve (Miscellaneous)
- Main Relay (Fuel System)
- Primary Slow Mixture Cut-Off Solenoid (Fuel Systems)
- Purge Cut-Off Control Solenoid Valve (Emission Systems)
- Vacuum Piston Control Solenoid Valve (Fuel System)
Carbureted
The fuel system consists of an low-pressure in-tank electric fuel pump, 2 fuel filters and a fuel pump cut-off relay. This system delivers pressurized fuel to carburetor when engine is running.
Fuel Injection
The fuel system consists of an in-tank high pressure electric fuel pump, main relay, fuel filter, pressure regulator, injectors (on 1.5L TBI engines, there are 2 injectors; main and auxiliary) and injector resistor(s) (except Civic 1.5L and CRX Std.). This system delivers pressure regulated fuel to injectors and cuts fuel delivery when engine is not running.
Fuel Pump
Fuel pump is comprised 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. The pump assembly consists of impeller (driven by motor), pump casing (which forms pumping chamber) and pump cover.
Fuel Pump Cut-Off Relay (Prelude Carb.)
The fuel pump cut-off relay applies battery voltage (IGN. 1 circuit) to fuel pump whenever ignition pulses from ignition coil are applied to fuel cut-off relay. During deceleration with throttle valve closed, current is cut off at speeds greater than 1200 RPM to improve fuel economy and reduce emissions. Fuel cut-off also takes place when engine speed exceeds 7000 RPM, regardless of position of throttle valve, which protects engine from over-speeding.
Injector Resistor(s) (Except Civic 1.5L & CRX Std.)
The injector resistor(s) lower(s) current supplied to injectors to prevent damage to injector coils. This allows injectors a faster response time.
Main Relay (Fuel Injection)
The main relay actually contains 2 individual relays. One relay is energized whenever ignition is on. It supplies battery voltage to ECU, power to injectors (except Civic 1.5L and CRX Std.), and power for second relay. The second relay supplies power to fuel pump (and injectors on Civic 1.5L and CRX Std.). The second relay is energized for 2 seconds when ignition switch is initially turned on and when engine is running.
Pressure Regulator (Fuel Injection)
The fuel pressure regulator maintains a constant fuel pressure to injectors. Fuel pressure is modified by a vacuum diaphragm connected to manifold vacuum.
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 Carburetor (PGM-CARB) (Prelude)
The main controlling device is PGM-CARB control unit, which contains logic circuits sensing inputs from various sensors and signals. It applies voltage to output devices as required to control fuel and maintain emissions for optimum engine performance.
Air Jet Controller
The air jet controller is an atmospheric pressure sensing device controlling airflow into slow and main air jets of carburetor throat. As atmospheric pressure is reduced by increasing altitude, bellows expand to open valve in air jet controller, increasing airflow to jets to maintain an optimum air/fuel ratio.
Air Leak Solenoid Valve
The air leak solenoid valve is energized when car is being started or when vehicle is running in power mode. The solenoid valve functions to close off extra air passage in carburetor, richening air/fuel mixture.
Feedback Control Solenoid Valve
The feedback control system is designed to provide proper air/fuel ratio (14.7:1) whenever possible. This allows Three-Way Catalyst (TWC) to reduce hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) emissions. The feedback control solenoid valve is provided to stop feedback operation at low engine speed (engine idle).
Power Valve
A power valve circuit is provided in the carburetor, to enrich air/fuel ratio when engine is operated in power (performance) mode. Power valve circuit is controlled by manifold vacuum, for enrichment at low manifold vacuum (wide open throttle) and no enrichment at high manifold vacuum (deceleration).
Primary Slow Mixture Cut-Off Solenoid
The primary slow mixture cut-off solenoid valve is provided to cut off idle mixture passage between air/fuel passage and by-pass port to prevent run-on when ignition is turned off. The solenoid valve also functions to cut-off mixture flow under deceleration.
Slow Air Jet Control System
To maintain an optimum air/fuel ratio, slow air jet control system controls airflow into primary jets of carburetor throats. When vehicle is started or running in power mode, ECU energizes air leak solenoid valve in air cleaner to close extra air passage, richening air/fuel mixture.
Vacuum Piston Control Solenoid Valve
When ECU receives proper signals of intake manifold vacuum (from MAP sensor) and engine speed (from ignition coil), it sends an electric current to activate vacuum piston control solenoid valve. When this solenoid is activated, manifold vacuum is directed to diaphragm of vacuum piston control valve through check valve "C" to open passage from vacuum piston upper chamber to atmosphere. When solenoid valve is deactivated, vacuum piston lowers by its own weight and force of return spring.
Programmed Fuel Injection (PGM-FI) ECU
The 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
The fuel injector consists of a solenoid, plunger needle valve and housing. When current is applied to solenoid coil, valve lifts up 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 mount rubber. On SFI models, injector is sealed by an "O" ring and seal ring at each end (top and bottom). All seals, "O" rings and mount rubbers reduce injector operating noise and heat transfer.
A/C Idle Boost Solenoid Valve
Valve is energized when A/C compressor is energized, applying vacuum to diaphragm of idle boost throttle controller.
Cranking Leak Solenoid Valve
The cranking leak solenoid valve is activated when cranking (starting) engine to bleed vacuum from choke opener.
Electronic Air Control Valve (EACV)
Depending upon signals received from oxygen sensor, ECU operates EACV to control air/fuel mixture ratio feedback. The EACV opens air passage from air cleaner case to intake manifold in proportion to intensity of electric current received from ECU.
Idle Control System
System prevents idle speed from dropping when A/C compressor is turned on. When A/C compressor is on, manifold vacuum is introduced into diaphragm chamber of idle boost controller through A/C idle boost solenoid valve which is activated by compressor switch. The idle controller's diaphragm rod is retracted to open throttle valve a small amount. Throttle valve opening is adjusted with idle control screw on idle controller to maintain original idle speed.
When compressor is off, A/C idle boost solenoid valve is deactivated to close vacuum passage. Vacuum stored in controller is then released through filter on solenoid valve.
IDLE SPEED (FUEL INJECTION)
The idle speed of engine is controlled by Electronic Air Control Valve (EACV) and fast idle valve (Accord and Prelude).
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, preventing compressor from being energized until idle speed can be increased by ECU. This ensures a smooth transition into A/C mode without loading down engine.
Engine idle speed is controlled by EACV (and by fast idle valve on Accord and Prelude). The EACV changes amount of air by-passing throttle plate (into intake manifold) in response to signals sent 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)
When coolant temperature is less than preset temperature, fast idle valve opens, allowing additional air to by-pass into intake manifold and increase idle speed. This prevents engine from running erratically during warm-up. The valve is controlled by a thermowax plunger, which contracts when cold and expands when hot.
Fast Idle Control Solenoid Valve (Civic 1.6L & CRX Si)
The ECU controls ground circuit for fast idle control solenoid valve. When energized (ECU senses intake and coolant temperatures are extremely low), fast idle control solenoid valve opens, increasing airflow for fast idling.
Accord
Battery voltage is supplied through ignition switch to ignition coil and ignitor unit. The ignitor, located inside distributor, triggers ignition coil based upon its pick-up signals. When triggered, induced 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. The ignitor unit is triggered by ECU based upon signals from TDC, CRANK and/or CYL (if equipped) and other sensors. Induced voltage from ignition coil is distributed to each spark plug by distributor.
Ignition timing is mainly controlled by distributor centrifugal and vacuum advance units. Vacuum to vacuum advance diaphragm is controlled by engine load (available vacuum) and coolant temperature. Distributor has 2 separate vacuum advance diaphragms ("A" and "B") which operate on manifold vacuum.
Civic, CRX & Prelude SFI
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.
Prelude Carb.
Ignition timing is mainly controlled by distributor centrifugal and vacuum advance components. Ignition control by vacuum advance corresponds to manifold vacuum and coolant temperature. Distributor has 2 separate vacuum advance diaphragms ("A" and "B") which operate on manifold vacuum. Manifold vacuum to diaphragm "B" is controlled by a thermovalve.
This system improves driveability at cold engine temperatures by advancing ignition timing. When coolant temperature is higher than set value, vacuum in thermovalve is released and vacuum advance is provided only by manifold vacuum to diaphragm "A".
Ignition Control Solenoid Valve (Accord SFI)
The Ignition Control Solenoid Valve (ICSV) opens and closes off vacuum to diaphragm "B" (vacuum advance diaphragm), that advances ignition timing in distributor. The ICSV is operated by ECU which receives signals from TDC sensor (engine speed), coolant temperature and manifold vacuum inputs to provide maximum performance and fuel economy, while protecting engine by preventing detonation from occurring.
Ignitor Unit
The ignitor receives power when ignition switch is in ON position. Ignitor unit controls ground circuit for ignition coil, which also receives power from ignition switch. Ignitor triggers ignition coil based upon pick-up coil (carbureted), or ECU (fuel injection) signals.
AIR INJECTION SYSTEM (CARBURETED)
The Air Injection System (AIS) improves emission performance by supplying fresh air from air cleaner into exhaust manifold via air suction control solenoid valve.
Air Suction Control Solenoid Valve
When air suction control solenoid valve is activated, manifold vacuum raises diaphragm valve of air suction valve and fresh air from air cleaner is induced to exhaust manifold. The fresh air is induced into exhaust manifold through reed valve of air suction valve by pulsation of exhaust gases.
EXHAUST GAS RECIRCULATION (EGR) SYSTEM
The EGR system reduces oxides of nitrogen (NOx) emissions by recirculating exhaust gases through EGR valve into intake manifold and back to combustion chambers.
Fuel-injected system is composed of EGR valve, Constant Vacuum Control (CVC) valve and EGR control solenoid valve. The system is operated by ECU, which analyzes input signals from EGR valve lift sensor and various sensors to provide optimum EGR flow.
Carbureted EGR system is composed of EGR valve, EGR control valves "A" and "B" and purge cut-off solenoid valve. The EGR valve is operated by ported vacuum and provides an EGR function is proportional to engine load by a balanced operation of both EGR control valves "A" and "B".
An EGR cut-off function on carbureted models responds to low engine coolant temperatures or vehicle speed. 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. Manifold vacuum pulls exhaust gases into combustion chambers to cool and dilute air/fuel mixture.
EGR Control Solenoid Valve
When ECU determines it is necessary to recirculate exhaust gases, it grounds EGR control solenoid valve, regulating vacuum that controls EGR valve. By regulating vacuum to EGR valve, EGR flow is adjusted for optimum control of NOx emission.
EGR Control Valves "A" & "B" (Prelude Carb.)
EGR control valve "A" opens when ported vacuum is directed to its diaphragm and manifold vacuum is bled into air filter through an orifice in air passage. This generates vacuum across an orifice in air passage which applies vacuum to upper diaphragm of EGR control valve "B", causing it to open. (Scheme 1)
As EGR control valve "B" opens, ported vacuum is bled off to carburetor venturi. This makes composite vacuum which is applied to diaphragm of EGR control valve "A". As this composite vacuum decreases, EGR control valve "A" begins to close and vacuum in air passage decreases, causing EGR control valve "B" to begin to close. (Scheme 1)
As EGR control valve "B" closes, composite vacuum increases, causing EGR control valve "A" to begin to open, completing cycle which repeats very rapidly.
Scheme 1
Exhaust Gas Recirculation Valve Lift Sensor (Calif. Except Carbureted & Civic 1.6L)
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.
FUEL EVAPORATION (EVAP)
The fuel evaporation 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 articles.
Air Vent Cut-Off Solenoid Valve (Prelude Carbureted)
The air vent cut-off solenoid valve regulates vapor flow from carburetor float bowls. When engine is not running, outer air vent passage opens, so fuel vapor in float bowls can be vented into charcoal canister. When engine speed is greater than 400 RPM, air vent cut-off solenoid valve opens inner air vent passage, so fuel vapor in float bowls can vent into air cleaner.
Carburetor Vapor Control System (Prelude Carb.)
The carburetor vapor control system consists of an air vent cut-off solenoid, inner vent solenoid valve, inner and outer carburetor air vents and a sub inner air vent. The air vent cut-off solenoid valve regulates airflow to carburetor float bowls.
When engine is not running, air vent cut-off solenoid valve allows outer air vent passage of carburetor to be open, allowing fuel vapor in float bowls to be vented into charcoal canister. When engine speed is greater than 400 RPM, air vent cut-off solenoid valve opens inner air vent passage allowing fuel vapor in float bowls to be vented into air cleaner. When starting engine with coolant temperature greater than 176°F (80°C), opening of inner vent is momentarily delayed to ease engine starting.
The inner vent solenoid valve also controls air into carburetor float bowls. When engine speed is greater than 400 RPM, inner vent solenoid valve normally opens sub inner air vent passage, but allows only fuel vapor to vent to air cleaner.
Charcoal Canister
The canister temporarily stores fuel vapor until it can be purged, drawn into engine and burned in combustion chamber.
Fuel Tank Vapor Control System
The 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.
The 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.
Inner Vent Solenoid Valve (Prelude Carb.)
The inner vent solenoid valve remains closed during engine cranking for easier starts. When inner vent solenoid valve is closed, it prevents fuel vapor in float chamber from being drawn into carburetor. When engine starts (exceeds 400 RPM), inner vent solenoid valve is activated by ECU to open passage from air cleaner case.
Purge Cut-Off Control Solenoid Valve (Prelude Carb.)
When coolant temperature is less than a preset temperature (specifications differ according to application), ECU supplies voltage to purge cut-off solenoid valve, cutting vacuum to purge control valve.
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.
Thermostatic Air Cleaner (TAC)
When air temperature in air cleaner is less than a predetermined temperature, air bleed valve will remain closed, directing manifold vacuum to air control diaphragm. When vacuum is applied to air control diaphragm, it closes off incoming ambient air and directs heated air from hot air intake duct (from exhaust manifold) into air cleaner. As under-hood temperatures increase, air bleed valve will gradually open, bleeding vacuum from air control diaphragm until air delivery door closes off hot air delivery.
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.
CHECK ENGINE LIGHT & LED INDICATOR
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. On all models except Accord, ECU is located under passenger's side floorboard carpet. On Accord models, ECU is located under driver's seat. For additional information, see G - TESTS W/ CODES article.
MISCELLANEOUS
Note. Although not considered true Engine Performance related systems, some controlled devices may affect driveability if they malfunction.
A/C Clutch
When there is a demand for air conditioning 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 (Civic & CRX 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
• ECU LOCATION