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
Computerized engine control system monitors and controls engine operation. Input sensors supply information to an Powertrain Control Module (PCM). PCM processes information from input devices and sends output voltage signals to various control devices. See INPUT DEVICES and OUTPUT SIGNALS . Information received from input devices is processed by PCM to calculate proper fuel delivery, ignition timing, fuel pressure, fuel evaporative purge, exhaust gas recirculation and air injection.
CONTROL UNIT
Note. Powertrain Control Module (PCM) may also be referred to as Electronic Control Module (ECM) or Vehicle Control Module (VCM). This is the same component.
PCM is located below center console.
PCM analyzes all electrical data signals from input devices to control fuel injection, ignition and emissions. PCM includes a back-up fail-safe control system. If a malfunction develops in PCM microcomputer, back-up control system will maintain necessary engine functions to permit operation of vehicle.
PROM is factory-programmed engine calibration data, which modifies PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. EEPROM (Electronically Erasable Programmable Read Only Memory) provides fuel delivery back-up so engine runs in case of PROM or PCM failure. Anytime PCM is replaced, install original PROM and/or EEPROM into replacement PCM. If battery voltage is removed, PROM and EEPROM information is retained.
INPUT DEVICES
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the ECM. The second category covers OUTPUT SIGNALS, which are components controlled by the PCM.
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input usage on specific model, see appropriate wiring diagram in WIRING DIAGRAMS article. The available input signals include the following
A/C Request (A/C On)
This provides PCM with a simple "on" (A/C Request) signal. PCM make calculations for extra load, fuel enrichment and idle.
Mass Air Flow Sensor (MAF)
AFS is located in air cleaner housing. Airflow sensor measures rate (volume) of air intake.
Battery Voltage
Battery voltage is monitored by PCM. PCM must have 12 volts to operate. PCM can adjust idle speed, timing or enrich air/fuel ratio to maintain a constant 12 volts.
Engine Coolant Temperature (ECT) Sensor
ECT sensor is located in engine block. ECT sensor informs PCM of engine coolant temperature.
Crankshaft Position (CKP) Sensor
CKP sensor is located in distributor housing or engine block. CKP sensor detects engine speed and relative position of each piston. With this information, PCM calculates ignition timing and dwell angle.
Camshaft Position (CMP) Sensor
CMP sensor is located in timing cover or engine block. CMP sensor continuously sends PCM a signal to help with ignition timing and starting.
EGR Pintle Position Sensor
This sensor is mounted inside linear EGR valve, and informs PCM of EGR pintle movement. PCM uses this information to control EGR flow.
Heated Oxygen Sensor (HO2S)
Sensor is located in exhaust manifold, and informs PCM of amount of oxygen in exhaust gases. By heating up HO2S, it can send accurate information before a standard O2S would.
Intake Air Temperature (IAT) Sensor
IAT sensor is located in fresh air duct to monitor air temperature entering throttle body and notifies PCM.
Knock Sensor (KS)
Knock sensor is located in engine block. If a detonation occurs, detonation sensor sends a signal to PCM to retard ignition timing.
Manifold Absolute Pressure (MAP) Sensor
MAP sensor monitors intake manifold vacuum and informs PCM of engine load.
Oxygen Sensor (O2S)
Sensor is located in exhaust manifold, and informs PCM of amount of oxygen in exhaust gases.
Park/Neutral Switch (A/T) & Inhibitor Switch (M/T)
Switch is used to inform PCM of gear selection. Information is used by PCM to allow starter operation and control engine idle speed.
Power Steering Pressure Switch (PSPS)
When power steering pressure is high, PSPS sends a signal to PCM to increase idle speed. PCM will also turn off A/C clutch when high P/S pressure is detected.
RPM Reference Signal
PCM uses signal to determine control of timing, fuel delivery, injector pulse width and idle speed.
Throttle Position (TP) Sensor
TP sensor is mounted on throttle body throttle shaft, and informs PCM of incremental changes in throttle position. TP sensor is sometimes combined with a throttle valve switch.
Vehicle Speed Sensor (VSS)
VSS is located in speedometer or transmission, and informs PCM of vehicle road 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 on each output component, refer to the system indicated after component.
CHECK ENGINE Light or Malfunction Indicator Light (MIL)
See SELF-DIAGNOSTIC SYSTEM .
Direct Ignition System (DIS)
See IGNITION SYSTEM .
EGR Valve
See EXHAUST GAS RECIRCULATION (EGR) SYSTEM under EMISSION SYSTEMS.
Electronic Spark Advance
See IGNITION TIMING CONTROL SYSTEMS under IGNITION SYSTEM.
Electronic Spark Timing (EST)
See IGNITION TIMING CONTROL SYSTEMS under IGNITION SYSTEM.
Fuel Disable Mode (Engine Speed)
See FUEL CONTROL under FUEL SYSTEM.
Deceleration Mode
See FUEL CONTROL under FUEL SYSTEM.
Clear Flood Mode
See FUEL CONTROL under FUEL SYSTEM.
Acceleration Mode
See FUEL CONTROL under FUEL SYSTEM.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pressure Regulator
See FUEL DELIVERY under FUEL SYSTEM.
Fuel Pump
See FUEL DELIVERY under FUEL SYSTEM.
Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEM.
See FUEL CONTROL under FUEL SYSTEM.
Idle Air Control (IAC) Valve
See IDLE SPEED under FUEL SYSTEM.
See FUEL CONTROL under FUEL SYSTEM.
Fuel pump is located inside fuel tank. Pump and sending unit are integral in design and are replaced as an assembly.
Located in fuse/relay box under hood. When ignition switch is turned on, PCM will activate fuel pump relay to run fuel pump. Fuel pump will operate as long as engine is cranking or running and PCM is receiving ignition reference pulses. If there are no reference pulses, PCM will shut off fuel pump within 2 seconds after ignition is turned on.
Fuel pressure regulator governs flow of fuel to injectors. Pressure varies depending on different vehicle speed and load conditions.
Fuel pressure regulator consists of a fuel chamber and a vacuum chamber separated by a diaphragm. (Scheme 1) Fuel chamber has a fuel inlet pipe and a fuel outlet pipe. Fuel inlet pipe delivers fuel from fuel distributor pipe. Appropriate amount of fuel is then delivered to fuel injector. Excess fuel is returned to fuel tank by fuel outlet pipe.
Vacuum chamber is connected to intake manifold by a hose. Any change in fuel pump delivery pressure or intake manifold pressure will cause diaphragm to move. This movement will maintain pressure balance between intake manifold and fuel chamber to ensure a steady supply of fuel to fuel injectors.
Scheme 1
FUEL CONTROL
The purpose of the fuel control system is to deliver the correct amount of fuel to the engine under all operating conditions. Fuel is delivered by the fuel injectors which are controlled by the PCM. The PCM checks Engine Coolant Temperature (ECT) and Throttle Position Sensor (TPS) to determine proper air/fuel ratio for starting. PCM changes the air/fuel ratio to the engine by modifying fuel injector pulse width. Fuel system operates in one of 2 modes: open loop or closed loop.
Open Loop
When engine is cold, PCM ignores signal from heated oxygen sensor (HO2S). Air/fuel ratio is calculated based on inputs from TPS, ECT and MAF sensors. System will remain in open loop until HO2S reaches operating temperature, coolant temperature reaches preset temperature, a specific period of time has elapsed after engine starts and engine has been greater than a specified RPM since start up.
Closed Loop
When HO2S reaches operating temperature, coolant temperature reaches a preset temperature, and a specific period of time has passed since engine start-up, PCM operates in closed loop to control air/fuel ratio based on HO2S signals (in addition to other input parameters). PCM maintains air/fuel ratio as close as possible to 14.7:1. If HO2S cools off (due to excessive idling) or a fault occurs in the HO2S circuit, vehicle once again enters open loop mode.
Fuel is metered into cylinders by electrically controlled solenoid valves in injectors. PCM controls on/off time (duty cycle) of fuel injectors to regulate air/fuel ratio.
PCM monitors engine speed, this system shuts off fuel flow when engine speed increases above 6400 RPM. Injectors are turned back on when RPM decreases below 6150.
Flooded engine can be cleared by depressing accelerator pedal fully. PCM will de-energize fuel injectors. Fuel injectors will remain de-energized as long as throttle remains open 80 per cent and engine speed is below 800 RPM.
PCM will reduce amount of fuel injected when it detects a decrease in throttle position and air flow. PCM may cut off fuel completely for short periods, if deceleration is very fast.
PCM provides extra fuel when a rapid increase in throttle position and air flow is detected.
Sensor is located in exhaust manifold, and informs PCM of amount of oxygen in exhaust gases. Electrically pre-heating this sensor provides PCM information sooner than a standard O2S.
Sensor is located in exhaust manifold, and informs PCM of amount of oxygen in exhaust gases.
Battery Voltage Correction
PCM compensates for low battery voltage by increasing injector pulse width, ignition dwell time and increasing idle RPM. PCM is able to perform these commands because of a built-in memory/learning function.
IAC is located in throttle body, and controls engine idle speed, while preventing stalls due to changes in engine load. IAC controls bypass air around throttle plate by moving pintle in (to decrease air flow) or out (to increase air flow).
Proper position of IAC pintle is calculated by PCM based on battery voltage, coolant temperature, engine load and engine RPM. It does this by sending voltage pulses to proper motor winding in IAC motor. This will cause motor shaft and valve to move in and out of motor a given distance for each pulse received. PCM pulses are referred to as counts.
IAC should be disconnected only while ignition is OFF. Each time ignition is turned off, PCM will reset IAC valve. This is done by sending enough counts to seat valve. Fully seated valve is PCM reference point. A given number of counts are then issued to open valve and normal PCM control of IAC valve will begin from this point. To increase idle speed, PCM will increase counts to retract IAC valve to allow more airflow through idle air passage and by-pass throttle plate until idle speed reaches proper RPM. To decrease idle speed, PCM will reduce counts to extend IAC valve to reduce airflow through idle passage around throttle plate. This will reduce PCM counts. IAC operates as an air by-pass during cold fast idle conditions. When engine is cold, air regulator allows additional flow of air to enter intake manifold.
Direct ignition system consists of 2 coil packs, ignition module, crankshaft position sensor and PCM. Coil packs consists of 6 independently mounted ignition coils (at plug). Ignition control module (ICM) receives ignition control signals from PCM which in turn triggers corresponding ignition coils. PCM controls spark timing and ignition control during crank and run, there is no by-pass mode. ICM is not repairable.
Crankshaft position sensor magnetic pick-up provides a signal to PCM to identify correct firing sequence and crank signals to trigger each coil at proper time. PCM controls timing and fuel injector pulse width using crankshaft position, engine RPM, engine temperature, MAP sensor and VSS input signals.
VCM monitors and controls ignition timing.
EMISSION SYSTEMS
Note. For emission systems usage, see appropriate EMISSION APPLICATIONS article.
EVAPORATIVE EMISSION SYSTEM
Fuel Evaporation Emission Control (EVAP) system prevents escape of gasoline vapors (hydrocarbons) from fuel tank into atmosphere. To reduce hydrocarbon (HC) emissions, evaporated fuel from fuel tank is absorbed into a charcoal canister to hold vapors and is purged by PCM control when engine coolant temperature is over 140 degrees (60 C) into intake manifold for combustion in cylinders. (Scheme 2) Purging of canister is controlled by an PCM-operated EVAP purge solenoid (normally closed). Attached to some charcoal canisters is a EVAP canister vent solenoid allows fresh air to enter charcoal canister to purge fuel vapors. EVAP vent solenoid closes to seal off evaporative emission system for leak testing.
Scheme 2
EXHAUST GAS RECIRCULATION (EGR) SYSTEM
EGR system reduces oxides of nitrogen (NOx) by recycling (through EGR valve) some exhaust gas back into intake manifold in order to lower combustion chamber temperature.
Linear EGR valve system is used. ECM continuously monitors EGR pintle position, and continuously adjusts it in order to obtain correct flow. PCM uses ECT,TPS and MAF information to compute pintle position. This makes linear EGR valve systems a "closed loop" system. (Scheme 3)
Scheme 3
PCV SYSTEM
Positive Crankcase Ventilation (PCV) system prevents hydrocarbon (HC) build-up in crankcase from escaping into atmosphere. PCV system is a closed type. It consists of a valve cover baffle plate, PCV valve and oil separator (except some models). Oil separator removes oil particles from blow-by gases. Crankcase vapors are routed from crankcase, through oil separator, to PCV valve, and into intake manifold to be burned in combustion chamber.
DESCRIPTION
PCM constantly looks at information from various input devices to control systems that affect vehicle performance. PCM self-diagnostic capabilities allow for trouble shooting system through a Malfunction Indicator Light (MIL). MIL will illuminate to alert driver of a system malfunction. The type of failure (hard or intermittent) will affect how the MIL illuminates. The specific malfunction can be determined via a 2-digit code displayed by MIL. To retrieve codes, see appropriate TESTS W/CODES article.
MALFUNCTION INDICATOR LIGHT (MIL)
MIL is on instrument panel and has the following functions
- It informs driver that a problem has occurred and that vehicle should be taken for service as soon as possible.
- It displays trouble codes stored by Electronic Control Module (ECM), which help technician diagnose system problems.
HARD FAILURES
Hard failures (PCM detects a DTC that impacts vehicle emissions) cause MIL to illuminate and remain ON until system or component passes the same test, for 3 consecutive drive cycles, without emission related faults. If vehicle is experiencing a misfire malfunction which may cause damage to Three-Way Catalytic Converter (TWC), MIL will flash once per second. This will continue until vehicle is outside of speed and load conditions which could cause possible catalyst damage, and MIL will stop flashing and remain on steady. If light comes on and remains on (light may flash) during vehicle operation, determine cause of malfunction using diagnostic (code) charts. If a sensor fails, Powertrain Control Module (PCM) will use a substitute value in its calculations to continue engine operation. In this condition (commonly known as limp-in mode), the vehicle runs but driveability will not be optimum.
INTERMITTENT FAILURES
Intermittent failures may cause MIL to flicker or illuminate and go out after intermittent fault goes away. However, corresponding trouble code will be retained in Electronic Control Module (ECM) memory. If related fault does not reoccur within a certain time frame, related trouble code will be erased from ECM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in TESTS W/O CODES article.