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.
Scheme 1
911 Carrera & 911 Carrera 4
Intake system is divided by 2 resonance flaps. This allows natural occurrence of intake air oscillations to draw more air into engine. Resonance flaps are activated by ECM through a resonance flap solenoid, which opens resonance flaps with help of a diaphragm valve when engine speed exceeds 5500 RPM. Solenoid is connected with a vacuum supply tank.
Solenoid and diaphragm valve will close when engine speed drops to less than 5400 RPM. To avoid seizure of resonance flaps (from deposits in intake), solenoid is activated briefly each time ignition is turned on. This causes resonance flaps to move from closed to open and back to closed position.
THROTTLE VALVE ASSEMBLY
To improve engine pick-up and transition behavior during slow acceleration, movement of throttle valve assembly has been modified so that smaller diameter throttle valve plate opens first. Large diameter throttle valve plate is also opened by a drag arm after small diameter throttle valve is opened about 5 degrees. In addition, idle speed switch must open before small throttle valve plate moves.
* PLEASE READ FIRST *
All models are equipped with Digital Motor Electronics (DME) engine management system. (Scheme 1) DME system uses various sensors to monitor intake air volume, engine speed, crankshaft position, intake air temperature and throttle position. Signals from these sensors, along with start and oxygen sensor signals, are sent to DME Electronic Control Module (ECM). ECM uses input from these sensors to determine correct fuel amount and ignition timing.
ELECTRONIC CONTROL MODULE (ECM)
An 88-pin ECM monitors and controls all engine management functions. (Scheme 2) ECM is located under driver's seat (911 Carrera and 911 Carrera 4). ECM has the ability to store fault codes related to fuel injection and ignition system. Detected faults remain stored for at least 50 engine starts.
ECM coding distinctions are made between manual and automatic transmissions. ECM will activate Malfunction Indicator Light (MIL).
Scheme 2
Note. Components are grouped into 2 categories. The first category is INPUT DEVICES, which are components that control or produce voltage signals monitored by ECM. The 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 device usage on a specific model, see SYSTEM WIRING DIAGRAMS (1997) or SYSTEM WIRING DIAGRAMS (1998). Available input signals include
Mass Airflow (MAF) Sensor
MAF sensor is located in intake air stream and supplies air volume information to ECM. ECM uses this and other information to regulate fuel injection rate.
Knock Sensors
Engine is fitted with 2 knock sensors. If engine knock is detected, ignition timing for pertinent cylinder is retarded 3 degrees. If engine knock continues, ignition will continue to be retarded (up to 9 degrees maximum). When engine knock stops, ignition timing is returned in small steps to its optimal value.
Engine Speed/Crankshaft Position (CKP) Sensor
Engine speed/CKP sensor measures engine speed by monitoring teeth on flywheel. Sensor sends 2 voltage pulses to ECM as each tooth passes. Sensor detects crankshaft position in relation to TDC and sends signal to ECM.
Engine (Coolant) Temperature Sensor
Sensor is located in cylinder head. Sensor supplies information on engine temperature to control air/fuel ratio (as engine temperature varies during cold start) and spark timing.
Full Throttle Switch
Full throttle switch is mounted on throttle housing. Full throttle switch signals ECM of optimum power demands.
Heated Oxygen Sensor (HO2S)
Four Heated Oxygen Sensors (HO2S) are used: 2 upstream of converters and 2 downstream of converters. HO2S supplies a low voltage signal (less than 0.5 volt) when fuel mixture is lean (high oxygen), and a higher voltage (up to one volt) when fuel mixture is rich (low oxygen). This rich-to-lean fluctuating signal is used by ECM in calculations of air/fuel mixture (fuel injector) control.
Idle Switch
Switch is used to sense closed throttle position and is mounted on throttle housing, opposite full throttle switch. Idle switch signals ECM when to control idle stabilization and coasting fuel cut-off.
Intake Air Temperature (IAT) Sensor
Sensor is located in air stream of airflow meter and supplies incoming air temperature information to control unit. ECM uses IAT sensor signal, along with other information, to regulate fuel injection rate.
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 Compressor Control
See A/C COMPRESSOR CONTROLS under MISCELLANEOUS CONTROLS.
DME (Fuel Pump) Relay
See FUEL DELIVERY under FUEL SYSTEM.
Double-Ignition
See IGNITION TIMING CONTROL SYSTEM under IGNITION SYSTEM.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Tank Vent Solenoid
See EVAPORATIVE EMISSIONS SYSTEM under EMISSION SYSTEMS.
Idle Speed Control
See IDLE SPEED CONTROL under FUEL SYSTEM.
Malfunction Indicator Light (MIL)
See MALFUNCTION INDICATOR LIGHT under SELF-DIAGNOSTIC SYSTEM.
Resonance Flap Solenoid
See RESONANCE FLAPS under AIR INDUCTION SYSTEM.
Fuel Pressure Regulator
Pressure regulator is located at end of injection collection line. Pressure regulator maintains constant fuel pressure to fuel injectors.
Fuel Pump & DME (Fuel Pump) Relay
Electric fuel pump is located in fuel tank. It maintains a constant fuel supply to injection system. DME relay supplies power for fuel pump. ECM energizes fuel pump portion of DME relay.
Scheme 3
Fuel System
ECM meters amount of fuel distributed to each cylinder by sequential control of fuel injectors. Amount of fuel reaching each cylinder is controlled separately, once for each working (compression) stroke, according to firing order. ECM changes injection timing depending on engine temperature, engine speed and load, and other engine operating conditions.
A fuel rail links fuel pressure regulator with fuel injectors. Each cylinder is provided with a solenoid-operated injector, which sprays fuel toward backside of each intake valve. ECM determines length of time each injector is open. Injector ON time determines amount of fuel delivered.
All models use an Idle Air Control (IAC) valve to provide auxiliary air valve operation and idle speed stabilization. Valve is located near throttle body and is controlled by ECM.
Engine speed/CKP sensor sends a signal to ECM. ECM uses this signal to fire coils for secondary ignition. Based on information received from sensors, spark control allows ECM to determine exact instant that ignition is required.
Two spark plugs per cylinder ignite air/fuel mixture. With this system, burn time of air/fuel mixture is shorter because of shorter spark travel. Detonation is also less likely, making it possible to increase compression ratio and improve thermodynamic efficiency in engine. Ignition must be triggered about 6 degrees later because of shorter spark travel and burn time. Ignition triggering signal goes from ECM to 2 ignition control units installed on left side in engine compartment.
In each ignition final stage, a transistor is activated, which interrupts secondary circuit of both ignition coils (also installed on left side of engine compartment). In this manner, an ignition spark is created by both ignition coils for each cylinder in sequence. A Hall Effect sensor, installed in double-ignition distributor, identifies cylinder No. 1 (ignition TDC). (Scheme 4) Hall signal is required by ECM for assignment of both knock sensor signals, as well as for sequential fuel injection control.
Each distributor rotor is controlled by centrifugal weights. This ensures alignment between distributor rotor and corresponding ignition contact in distributor cover.
Scheme 4
EVAPORATIVE EMISSIONS SYSTEM
Fuel evaporation control system is designed to prevent fuel vapors from escaping into atmosphere. (Scheme 5) System consists of a non-vented fuel tank filler cap, expansion tank, evaporation tank, charcoal canister, fuel tank vent solenoid, purge diaphragm valve, restrictor, and connecting lines and hoses. Expanded fuel, caused by high ambient temperatures, is collected in expansion tank. Liquid fuel is returned to main tank by venting action as fuel is used from main tank.
Fuel vapor passes through a vent line to charcoal canister where it is stored. A second vent line connects canister to a purge control diaphragm valve. Purge control diaphragm valve is closed when engine is not running. When engine has been started, manifold vacuum is applied to top fitting of purge diaphragm valve. This opens an internal passage in diaphragm valve, allowing vapors stored in charcoal canister to be drawn into intake manifold for burning during combustion process. Purge line also contains a fuel tank vent solenoid which is controlled by ECM.
Scheme 5
ECM can store trouble codes related to fuel injection and ignition systems. Detected codes remain stored for at least 50 engine starts. If battery cable or ECM connector is disconnected, trouble code memory and system adaptation will be cleared. For additional information, see appropriate TESTS W/CODES article in this Section.
MALFUNCTION INDICATOR LIGHT
All models are equipped with a Malfunction Indicator Light (MIL), which comes on if a component related to fuel injection and ignition system fails. See ELECTRONIC CONTROL MODULE (ECM) under COMPUTERIZED ENGINE CONTROLS. MIL is located in gauge cluster. For additional information, see appropriate TESTS W/CODES article in this Section.
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if theymalfunction.
A/C COMPRESSOR CONTROLS
ECM receives input and sends output signals for A/C compressor control. See appropriate TESTS W/CODES article in this Section article.
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