Engine Control System & Fuel System - 2.4L (Lax) - Introduction: Overview Pontiac Vibe II

Powertrain Control Module Description

The powertrain control module (PCM) is a precision microprocessor that is an essential part of the electronic control system. The PCM is responsible for engine and automatic transmission operation. The PCM is located in the engine compartment. The PCM communicates with other vehicle control modules using the controlled area network (CAN) communication protocol. Outside communication with the PCM is through the data link connector (DLC) located below the left side of the instrument panel (IP).

The PCM performs the OBD II diagnostic tests of the emission related systems. The PCM supplies a buffered voltage, called reference voltage, to the various information sensors and switches. The PCM controls most components with an electronic switch that completes a ground circuit when turned ON. The electronic switch is commonly referred to as an output driver. The PCM is also responsible for a self-diagnosis function and a fail-safe function.

Self-Diagnosis Function

The powertrain control module (PCM) diagnoses any troubles which may occur in the engine control system when the ignition switch is in the ON position with the engine running. The PCM indicates a malfunction by illuminating the malfunction indicator lamp (MIL) when a fault occurs in any of the following systems

1. The heated oxygen sensor 1 (HO2S 1)
2. The heated oxygen sensor 2 (HO2S 2)
3. The engine coolant temperature (ECT) sensor
4. The accelerator pedal position (APP) sensors
5. The throttle position (TP) sensors
6. The vehicle speed sensor (VSS)
7. The intake air temperature (IAT) sensor
8. The mass air flow (MAF) sensor
9. The camshaft position (CMP) sensor
10. The crankshaft position (CKP) sensor
11. The knock sensor (KS) system
12. The evaporative emission (EVAP) control system
13. The electronic throttle control (ETC) or throttle actuator control (TAC) system
14. The CMP actuator solenoid system
15. The misfire detection
16. The fuel-trim
17. The catalyst monitor
18. The central processing unit (CPU) of the PCM

When the PCM detects a malfunction in one of the above areas, the PCM will illuminate or flash the MIL in order to notify the driver of the occurrence of a fault.

Malfunction Indicator Lamp (MIL) Operation

The malfunction indicator lamp (MIL) is located in the instrument panel cluster. The MIL will display as either SERVICE ENGINE SOON or one of the following symbols when commanded ON

Scheme 74

Scheme 75

The MIL indicates that an emissions related fault has occurred and vehicle service is required.

The following is a list of the modes of operation for the MIL

1. The MIL illuminates when the ignition is turned ON, with the engine OFF. This is a bulb test to ensure the MIL is able to illuminate.
2. The MIL turns OFF after the engine is started if a diagnostic fault is not present.
3. The MIL remains illuminated after the engine is started if the control module detects a fault. A diagnostic trouble code (DTC) is stored any time the control module illuminates the MIL due to an emissions related fault. The MIL turns OFF after three consecutive ignition cycles in which a Test Passed has been reported for the diagnostic test that originally caused the MIL to illuminate.
4. The MIL flashes if the control module detects a misfire condition which could damage the catalytic converter.
5. When the MIL is illuminated and the engine stalls, the MIL will remain illuminated as long as the ignition is ON.
6. When the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition is cycled OFF and then ON.

Fail-Safe Function

When a malfunction occurs within the engine control system, the PCM maintains control over the fuel injection system, the idle speed control system, etc. The PCM controls these systems by using calculated values and/or backup programs stored within the PCM.

This function is called the fail-safe function. With the fail-safe function, a certain level of engine performance is available even when a malfunction occurs. The fail-safe function prevents a complete loss of engine performance.

The systems covered by the fail-safe function are as follows

1. The MAF sensor
2. The IAT sensor
3. The APP sensor
4. The TP sensor
5. The HO2S heater circuits
6. The KS system
7. The TAC system
8. The CPU in the PCM
9. The fuel cut-off for ignition system failures

Catalyst Monitor Diagnostic Operation

The powertrain control module (PCM) uses certain diagnostic strategies known as primary system based diagnostics that evaluate the various primary system operations. The primary system based diagnostics also evaluate the various primary system operations affect on vehicle emissions. Some of the primary system based diagnostics are listed here with a brief functional description of the diagnostics involved.

The OBD 2 catalyst monitor diagnostic measures the oxygen storage capacity of the 3-way catalytic converter (TWC). Heated oxygen sensors (HO2S) are installed before (pre-catalyst) and after (post-catalyst) the TWC. Voltage variations between the sensors allow the PCM to determine the performance of the TWC catalyst. When the TWC catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity to store and release oxygen is generally degraded. The OBD 2 catalyst monitor diagnostic is based on a correlation between the conversion efficiency of the TWC catalyst and the oxygen storage capacity of the catalyst. A good catalyst, e.g., 95 percent hydrocarbon conversion efficiency, will show a relatively flat output voltage on the post-catalyst sensor, HO2S 2. A degraded catalyst, 65 percent hydrocarbon conversion, will show greatly increased activity in the output voltage from the post catalyst HO2S.

The post-catalyst HO2S is used to measure the oxygen storage and release capacity of the catalyst in the TWC. A high oxygen storage capacity indicates a good catalyst. A low oxygen storage capacity indicates a failing catalyst. The TWC and the HO2S 2 must be at operating temperature in order to achieve the correct oxygen sensor voltages, like those shown in the post-catalyst HO2S Outputs graphic.

The catalyst monitor diagnostic is sensitive to the following conditions

1. Exhaust leaks
2. HO2S contamination
3. Alternative fuels

Exhaust system leaks may cause any of the following results

1. A false failure for a normally functioning, good catalyst.
2. Prevent a degraded catalyst from failing the catalyst monitor diagnostic.
3. Prevent the catalyst monitor diagnostic from running.

The presence of HO2S contaminants may prevent the catalyst monitor diagnostic from functioning properly.

Three-Way Catalyst Oxygen Storage Capacity

The TWC catalyst must be monitored for efficiency. In order to accomplish this, the control module monitors the pre-catalyst (HO2S 1) and post-catalyst (HO2S 2) oxygen sensors. When the TWC is operating properly, the post-catalyst oxygen sensor will have significantly less activity than the pre-catalyst oxygen sensor. The TWC stores and releases oxygen as needed during the normal reduction and oxidation process. The control module will calculate the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor voltage levels. If the activity of the post-catalyst oxygen sensor approaches that of the pre-catalyst oxygen sensor, the catalyst's efficiency is degraded.

Stepped or staged testing levels allow the PCM to statistically filter test information. This prevents falsely passing or falsely failing the catalyst monitor oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. Post-catalyst oxygen sensor activity should not be used to determine the oxygen storage capacity unless directed by the service manual.

A 2-stage test is used to monitor the catalyst efficiency. Failure of the first stage of the test will indicate that the catalyst requires further testing in order to determine the catalyst efficiency. The second stage test looks at the inputs from the pre-catalyst and post-catalyst HO2S sensors more closely in order to determine if the catalyst is actually degraded. This two stage test further increases the accuracy of the oxygen storage capacity monitor. Failing the first stage test DOES NOT indicate a failed catalyst. The catalyst may be marginal or the fuels sulfur content could be very high.

Aftermarket HO2S characteristics may be significantly different from the original equipment manufacturer HO2S. An inferior HO2S may lead to a false pass or a false fail of the catalyst monitor diagnostic. An aftermarket catalytic converter that does not contain the same amount of cerium as the original catalytic converter can cause a false DTC to set. An incorrect amount of cerium in the catalyst can alter the correlation between the oxygen storage and the conversion efficiency of the TWC.

Scheme 76

A good TWC catalyst will show a very active output voltage on the pre-catalyst heated oxygen sensor (1). A good catalyst, 95 percent hydrocarbon conversion, will show a relatively flat output voltage on the post-catalyst heated oxygen sensor (2).

Scheme 77

A degraded TWC catalyst, 65 percent hydrocarbon conversion, will show greatly increased activity in the output voltage from the post-catalyst heated oxygen sensor (2). The degraded catalyst post-catalyst HO2S output voltage will therefore appear similar to the typically active output voltage of the pre-catalyst heated oxygen sensor (1).

Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft rotational velocity, reference period, variations. The powertrain control module determines the crankshaft rotational velocity using the crankshaft position sensor and the camshaft position sensor. When a cylinder misfires the crankshaft actually slows down momentarily. By monitoring the crankshaft and the camshaft position sensor signals, the control module can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic will be required to report a misfire that is present within 1000-3200 engine revolutions.

For a catalyst damaging misfire, the diagnostic will respond to a misfire that is within 200 engine revolutions.

Rough roads may cause a false misfire detection. A rough road will cause torque to be applied to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity and cause a false misfire detection.

On automatic transaxle equipped vehicles, the torque converter clutch (TCC) will be disabled whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs (torque) on the crankshaft rotation.

When the TCC has been disabled as a result of a misfire detection, the TCC will be re-enabled after approximately 3200 engine revolutions with no misfire is detected. The TCC will remain disabled whenever a misfire is detected. This allows the misfire diagnostic to evaluate the system.

Fuel Trim System Monitor Diagnostic Operation

The fuel system monitor diagnostic averages of short-term and long-term fuel trim values. If these fuel trim values stay at the fuel trim limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of the short-term fuel trim values and the long-term fuel trim values to the rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If both values are outside the acceptable thresholds, a rich or lean DTC will be recorded.

In order to meet OBD ll requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if the fuel trim counts in the weighted fuel trim cells exceed the specifications. A vehicle that has a fuel trim problem that is causing a concern under certain conditions but operates fine under other conditions may not set a fuel trim DTC. For example an engine that is idling high due to a small vacuum leak or an engine that is running rough due to a large vacuum leak may set an idle speed DTC or an HO2S DTC but not a fuel trim DTC.

A fuel trim DTC may be triggered by many different vehicle faults. Use all of the diagnostic information available when diagnosing a fuel trim fault.

Scheme 78

The camshaft position (CMP) actuator system enables the powertrain control module (PCM) to change camshaft timing while the engine is running. The CMP actuator assembly (1) varies camshaft position in response to directional changes in oil pressure. The CMP actuator solenoid valve controls the oil pressure that is applied to advance or retard the camshaft. Modifying camshaft timing under changing engine demand provides better balance between the following performance concerns

1. Engine power output
2. Fuel economy
3. Lower tailpipe emissions

The CMP actuator solenoid valve (5) is controlled by the PCM. The crankshaft position (CKP) sensor (4) and the CMP sensor (3) are used to monitor changes in camshaft position. The PCM uses information from the following sensors in order to calculate the desired camshaft position

1. The engine coolant temperature (ECT) sensor
2. The mass air flow (MAF) sensor
3. The throttle position (TP) sensors
4. The vehicle speed sensor (VSS)

Scheme 79

The CMP actuator assembly has an outer housing (4) that is driven by the engine timing chain. Inside the assembly is a wheel (2) with fixed vanes that is attached to the intake camshaft (3). The CMP actuator assembly also contains a lock pin (1) that prevents movement between the outer housing and the wheel vane assembly. The lock pin is released by oil pressure before any movement in the CMP actuator assembly takes place.

Scheme 80

The CMP actuator solenoid valve directs the oil flow that controls the camshaft phase movement. The PCM commands the CMP solenoid to move the solenoid plunger (2) and spool valve (1) until oil flows from the advance passage (7) or the retard passage (8). In most operating conditions the opposite oil return passage is open at the same time as the oil supply passage.

Operation

The PCM operates the CMP actuator solenoid valve by pulse width modulation (PWM) of the solenoid coil. The higher the PWM duty cycle, the larger the change in camshaft timing. Oil pressure that is applied to the advance side of the fixed vanes will rotate the camshaft in a clockwise direction. The clockwise movement of the camshaft will advance the timing up to a maximum of 40 degrees. When oil pressure is applied to the return side of the vanes, the camshaft will rotate counterclockwise until returning to 0 degrees.

Scheme 81

Oil flowing to the CMP actuator housing (2) from the CMP solenoid advance passage (5) applies pressure to the advance side of the vane wheel (1) in the CMP actuator assembly. At the same time the CMP solenoid retard passage (7) is open, allowing oil pressure to decrease on the retard side of the vane wheel. These two simultaneous actions cause the vane wheel (1) to rotate clockwise, advancing camshaft advance timing.

Scheme 82

When the oil flowing to the CMP actuator housing (2) is from the CMP solenoid retard passage (7), oil pressure is applied to the retard side of the vane wheel (1). Because the solenoid advance passage (5) is open, allowing oil pressure to decrease on the advance side of the vane wheel (1), the camshaft position retards.

The PCM can also command the CMP actuator solenoid valve to stop oil flow from both passages in order to hold the current camshaft position. The PCM is continuously comparing CMP sensor input with CKP sensor input in order to monitor camshaft position and detect any system malfunctions. The following table provides camshaft phase commands for common driving conditions

CMP Actuator System Operation

Fuel System Description

The fuel system is comprised of two subsystems. The fuel supply system and the fuel metering system. The correct amount of fuel can not be delivered to the engine cylinders without a constant supply of air from the air intake system. The air intake system provides measured clean air to each cylinder for proper mixture with the pressurized fuel. The powertrain control module (PCM), along with the information sensors and switches, controls the activity between all three systems.

Scheme 83

Scheme 84

The fuel supply system is a returnless on-demand design, that reduces evaporative emissions. The fuel metering system is a sequential fuel injection (SFI) design. The function of the fuel metering system is deliver the correct amount of fuel to the engine under all operating conditions. The main component of the vehicle's fuel metering system is the fuel injectors. There is one fuel injector for each cylinder. The fuel injectors are controlled sequentially by the PCM. The PCM bases the control of the fuel injectors on several important engine parameters. These engine parameters include the following

1. The mass air flow (MAF) sensor
2. The intake air temperature (IAT) sensor
3. The engine coolant temperature (ECT) sensor
4. The throttle position (TP) sensor
5. The accelerator pedal position (APP) sensor, FWD only
6. The engine speed (RPM)
7. The heated oxygen sensor (HO2S)

The engine is designed to use only unleaded fuel. Unleaded fuel must be used for proper emission control system operation. Using unleaded fuel will also decrease spark plug fouling and extend engine oil life. Leaded fuel can damage the emission control system. The use of leaded fuel can result in the loss of emission warranty coverage.

The vehicle fuel system is equipped with an evaporative emission (EVAP) control system that minimizes the release of fuel vapors to the atmosphere. For more information on the EVAP system, refer to Evaporative Emission Control System Description .

Scheme 85

The supply of fuel is stored in the fuel tank. An electric fuel pump pressurizes the fuel to the fuel filter, out to the fuel lines and thru the fuel pulsation damper to the fuel rail. The electric fuel pump is suspended from the top of the fuel tank as part of the fuel sender assembly. The fuel pump is designed to provide fuel at a pressure above the regulated pressure needed by the fuel injectors. The fuel pressure regulator keeps the fuel supplied to the fuel injectors at the regulated pressure.

The fuel pressure regulator is located inside of the fuel tank and is part of the fuel sender assembly. Locating the fuel pressure regulator in the fuel tank eliminates the need for a return line from the fuel rail. A returnless system reduces the internal temperature of the fuel tank resulting in lower evaporative emissions.

The fuel system is also equipped with an enhanced evaporative emission system and an on-board refueling vapor recovery (ORVR) system. Together the 2 systems minimize the release of fuel vapors to the atmosphere.

Scheme 86

Scheme 87

Fuel Injection Operation

The PCM controls the amount of fuel the fuel injector supplies to each cylinder by controlling the ON time, or length of pulse, of each individual injector. The delivery timing of the fuel into the cylinder head intake port by the fuel injector is controlled by the PCM. The timing and pulse of the fuel injectors is carefully calculated with inputs from the various sensors so that a suitable air/fuel mixture is supplied to the engine for every driving condition.

There are two types of injection timing. One is synchronous injection, when fuel injection is synchronous with the ignition signal or the signal from the camshaft position (CMP) sensor. The other is asynchronous injection, when fuel injection takes place independently of the ignition signal or the signal from the CMP sensor.

The PCM first calculates the correct timing of the fuel injectors by factoring the engine speed and the air volume together. Then the PCM applies certain compensations that are based on the information provided by various sensors which detect the state of the engine and the current driving conditions.

Air/Fuel Ratio Feedback Compensation-Closed Loop Operation

In order to obtain the most efficient engine performance with a good balance of power and fuel economy, the air/fuel mixture must be kept as close to the theoretical air/fuel ratio of 14.7:1 as possible. Maintaining the 14.7:1 air/fuel ratio also provides efficient operation of the 3-way catalytic converter (TWC) and a high clarification rate of CO, HC and NOx in the exhaust gas stream, In order to accomplish this the PCM first compares the input voltage from the heated oxygen sensor 1 (HO2S 1) with a specified reference voltage.

The HO2S 1 sensors are a wide band or wide range design. As with all oxygen sensors, the wide band HO2S measures the amount of oxygen in the exhaust system, but provides more information than the typical switching style HO2S. The wide band sensor consists of an oxygen sensing cell, an oxygen pumping cell, and a heater. The exhaust gas sample passes through a diffusion gap between the sensing cell and the pumping cell. The powertrain control module (PCM) supplies a voltage to the HO2S and uses this voltage as a reference to the amount of oxygen in the exhaust system. An electronic circuit within the PCM controls the pump current through the oxygen pumping cell in order to maintain a constant voltage in the oxygen sensing cell. The PCM monitors the voltage variation in the sensing cell and attempts to keep the voltage constant by increasing or decreasing the amount of current flow, or oxygen ion flow, to the pumping cell. By measuring the amount of current required to maintain the voltage in the sensing cell, the PCM can determine the concentration of oxygen in the exhaust. The HO2S input signal is displayed as a voltage within the range of 1-5 volts, with a steady value of 3.2-3.3 volts equal to the air fuel ratio of 14.7:1. If the HO2S 1 input voltage is lower than the specified reference voltage, the PCM determines that the air/fuel ratio is richer than the theoretical air/fuel ratio and reduces the fuel. If the input voltage from the HO2S 1 is higher than the specified reference voltage, the PCM determines that the air/fuel ratio is leaner and increases the fuel. By repeating these operations, the PCM can adjust the air/fuel ratio in order to be closer to the theoretical air/fuel ratio. Control of the fuel delivery system as just described is known as Closed Loop operation.

The HO2S signal is also displayed as a lambda value. A lambda value of 1 is equal to the stoichiometric air fuel ratio of 14.7:1. Under normal operating conditions, the lambda value will remain around 1. When the fuel system is lean and the oxygen level high, the lambda signal will be high or more than 1. When the fuel system is rich and the oxygen level low, the lambda signal will be low or less than 1.

Closed Loop Fuel Control Operation

The Closed Loop fuel control operation will not take place under any of the following conditions

1. At engine start up
2. When the fuel injection is increased just after engine start up.
3. When the engine coolant temperature (ECT) sensor is indicating a low coolant temperature.
4. When the engine is operating under a high demand, such as at wide open throttle (WOT).
5. During fuel cutoff
6. When the HO2S 1 is cold-Open Loop operation.

Control of the air supply that is mixed with the metered fuel is detailed in the description of the throttle actuator control (TAC) system and the air intake system. Refer to Throttle Actuator Control (TAC) System Description and Air Intake System Description .

EVAP System Operation

When the engine is operating in Closed Loop and the engine coolant temperature is more than 74°C (165°F), the powertrain control module (PCM) initiates purging of the stored fuel vapors. In order to purge the fuel system vapors, the PCM commands the purge valve ON, OPEN, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent valve OFF, OPEN, fresh air will be drawn through the EVAP canister air filter and vent valve into the EVAP canister. Manifold vacuum continues to pull fresh air through the canister pulling fuel vapors from the carbon. The air/fuel vapor mixture passes through the purge line and purge valve into the intake manifold to be consumed during the normal combustion process.

Scheme 88

Atmospheric pressure (4) is allowed into the canister in order to ensure that purge flow is consistently maintained whenever purge vacuum is applied to the canister. The PCM will change the duty ratio cycle of the purge solenoid valve, controlling the purge flow volume. Purge flow volume is determined by manifold pressure, the purge solenoid duty ratio cycle, and the restrictor passage (3) in the canister. The restrictor passage prevents a large volume of vapor from suddenly entering the intake manifold and affecting the airfuel ratio.

The powertrain control module (PCM) uses reference pulses from the crankshaft position (CKP) sensor in order to determine the engine speed. The PCM cannot operate the ignition system or the fuel injectors without the engine speed signal from the CKP. The PCM controls the ignition timing by controlling the ignition coils.

Each ignition coil has a built-in ignition module that controls the current flow in the primary coil winding. When the current flow is interrupted, the electrical field around the primary coil collapses and a high voltage is induced in the secondary coil. The secondary coil voltage travels from the coil output terminal, through the spark plug boot, and across the spark plug gap to the engine block. As a fail-safe function the ignition control module sends an ignition confirmation signal back to the PCM whenever the primary field collapses.

The camshaft position (CMP) sensor input is used to detect an engine misfire. The PCM also uses the CMP sensor signal as an input for modifying the fuel injection timing and for modifying the ignition timing.

The PCM receives information on the engine status from various engine sensors and then selects the most appropriate ignition timing settings from within the PCM's programming. The following are the most important inputs for determining ignition timing requirements

1. The engine speed
2. The accelerator pedal position (APP) sensor
3. The throttle position (TP)
4. The intake air volume
5. The engine coolant temperature (ECT)
6. The knock sensor (KS) input

Scheme 89

The crankshaft position (CKP) sensor (1) is located in the front cover of the cylinder block near the crankshaft pulley (2). The CKP sensor produces an AC signal that increases in both frequency and amplitude as the engine speed increases. The CKP sensor signal is sent to the PCM in order to indicate the RPM and the crankshaft position. The PCM uses the CKP sensor signal along with the camshaft position (CMP) sensor signal for the following purposes

1. To calculate the fuel injector pulse
2. To establish the piston position top dead center (TDC) for the ignition timing
3. To start the ignition coil and fuel injection sequencing

The CKP sensor reluctor (2) has 34 teeth and is mounted on the crankshaft (3) behind the timing chain cover and the crankshaft pulley. When the crankshaft rotates, the CKP sensor signal rotor teeth pass by the CKP sensor (1) causing a fluctuation in the sensors magnetic field. The fluctuation in the magnetic field induces a voltage in the CKP sensor circuitry that corresponds to every 10 degrees of crankshaft rotation. The reluctor has one wide tooth that indicates top dead center (TDC) of the number 1 cylinder. With the information that the PCM receives from the CKP sensor, the PCM determines engine speed and crankshaft position.

Scheme 90

The camshaft position (CMP) sensor (1) is located in the cylinder head near the number 4 fuel injector. The CMP sensor is a signal generator that is composed of a magnet and a coil with an iron core. The PCM relies on the AC signal provided by the CMP sensor in order to perform the following

1. To optimize the ignition timing
2. To optimize the fuel delivery
3. To monitor CMP actuator solenoid valve operation

Scheme 91

The CMP sensor signal rotor (2) is part of the intake camshaft (1) and has 3 teeth located on the outer circumference. When the CMP sensor signal rotor (2) rotates past the CMP sensor (3), electrical signals are generated. The AC signals that are generated by the CMP sensor are sent to the PCM.

Scheme 92

The knock sensor (KS) (1) is located below the intake manifold (2), on the engine block between cylinder 2 and cylinder 3. The KS detects engine detonation and sends a signal to the PCM. The PCM uses the input from the KS to adjust the ignition timing in order to control detonation. For detailed information on the operation of the KS refer to Knock Sensor (KS) System Description .

Knock Sensor (KS) System Description

The 2.4L engine is equipped with a knock sensor (KS) (1) that is located on the engine block behind the intake manifold (2). Ignition timing determines the relationship between the time the spark plug is fired and the time that the piston reaches the end of the pistons upward travel in the cylinder (TDC). If the spark plug ignites the compressed air/fuel mixture too late, not all of the air/fuel mixture has time to burn while the fuel is highly compressed. Late ignition causes a decrease in fuel efficiency, decreased power, and increased exhaust emissions. If the spark plug fires too soon, too much of the air/fuel mixture starts burning before the piston reaches the top of the compression stroke. Early ignition of the air/fuel mixture causes detonation, commonly referred to as spark knock. Constant spark knock in the motor is undesirable. Excessive spark knock can reduce engine performance. If severe enough, detonation can cause engine damage.

Every engine has an optimum ignition timing value. The optimum ignition timing is usually the earliest or most advanced firing of the spark plug that is possible without causing detonation. An engines optimum ignition timing is designed to be the most advanced ignition timing possible during the most demanding conditions. The optimum ignition timing is affected by all of the following variables

1. The engine load
2. The engine temperature
3. The atmospheric pressure
4. The fuel quality
5. The fuel's octane rating

Ignition systems equipped with a knock sensor (KS) can be engineered for optimum ignition timing. A KS detects when the engine is experiencing detonation and then signals the PCM to reduce the spark advance until detonation is no longer detected.

Scheme 93

The 2.4L engine is equipped with a flat design non-resonant type KS. The non-resonant KS enables the powertrain control module (PCM) to adjust the ignition timing in order to adapt to any of the variables that affect the optimal ignition timing.

Scheme 94

The KS detects when the engine is experiencing detonation. The sensor then signals the PCM to reduce the spark advance until detonation is no longer detected. A conventional resonant type KS uses a vibration plate that has the same resonance point as the expected knocking frequency of the engine. The non-resonant type KS has a steel weight (1) with an insulator (4) separating the weight from a piezoelectric element (3). The vibration caused by engine detonation is transferred to the weight, whose inertia applies pressure to the piezoelectric element. The weights action against the element, generates an electromotive force that is modified and transmitted to the PCM. The non-resonant design sensor is able to detect detonation vibration over a wide frequency band, from 6 kHz to 15kHz. In comparison, the resonant type KS detects only vibrations that are within a narrow frequency band.

In response to the KS signal the PCM retards the spark advance in order to reduce the detonation. The amount of timing retard that the PCM applies is based on the engine speed and the length of time that the engine detonation is detected. Once the spark timing is retarded, the KS circuitry in the PCM performs calculations in order to determine how much spark advance should be re-introduced. Normally the ignition timing advance is increased until zero retard, or normal ignition timing, is re-established. If detonation occurs again, the whole cycle will repeat. The alteration of the ignition timing by the KS often occurs continuously while the engine is running, even though no detonation is heard by the vehicle's operator.

Results of Faulty Knock Sensor Operation

Loss of the KS signal causes the PCM to operate in fail safe mode. In fail safe mode the PCM commands maximum spark retard. A KS that falsely indicates detonation can cause the PCM to retard the ignition timing unnecessarily. Reduced spark advance can cause any of the following conditions

1. Poor fuel economy
2. Sluggish engine performance
3. Higher exhaust emissions

A KS that fails to detect detonation can cause the PCM to control the ignition timing as if no detonation were occurring. Failure of the PCM to retard the ignition timing when necessary could cause any of the following concerns

1. An excessive engine detonation
2. Engine damage during heavy engine loads
3. Higher exhaust emissions

Scheme 95

The compressed combustion gas which escapes past the piston rings into the crankcase is known as blow-by gas. Blow-by gas contains large amounts of CO and HC. The positive crankcase ventilation (PCV) system prevents the blow-by gas from being emitted into the atmosphere. The PCV system routes the crankcase blow-by gas back into the intake system where the blow-by gas becomes part of the combustion process. The PCV system consists of the following components

1. The PCV valve
2. The crankcase vent (breather) or air inlet tube
3. The air inlet filter or separator, if used
4. Any hoses or couplers

The primary control of engine crankcase blow-by gas is the positive crankcase ventilation (PCV) valve. The PCV valve meters the flow of blow-by gas according to the manifold vacuum signal.

Scheme 96

One side of the PCV valve is exposed to manifold vacuum and the other side is exposed to crankcase gases. The PCV valve allows some vacuum pressure to pass through the valve orifice and create a low pressure condition in the crankcase. The blow-by gases in the crankcase are then drawn into the intake system and consumed in the normal combustion process. The volume of blow-by gas entering the intake manifold is precisely controlled In order to maintain idle quality. The correct PCV valve with the proper calibration must be used. The relationship between the flow of blow-by gas and the engine manifold vacuum is charted in the following table

PCV Valve Operation

Results Of Incorrect Operation

A plugged valve or hose may cause any of the following conditions

1. A rough engine idle
2. Engine stalling or low engine idle speed
3. High engine crankcase pressure
4. Engine oil leaks
5. Engine oil in the air cleaner
6. Oil sludge in the engine
7. Engine oil consumption
8. Excessive exhaust emissions

A leaking valve or hose may cause any of the following conditions

1. A rough engine idle
2. Engine stalling
3. High engine idle speed
4. Incorrect engine crankcase pressure
5. Excessive exhaust emissions

Scheme 97

The air induction system provides air and oxygen for the combustion process. The air cleaner (ACL) keeps dirt from entering the engine. Outside air is drawn into a forward mounted air cleaner. The air first passes through a carbon filter (3) that is located in the upper ACL housing (1). The carbon filter absorbs fuel vapors that may be emitted from the combustion chamber after the engine is turned off. Next the intake air flows through the air filter element (2) and past the mass air flow (MAF) sensor to the throttle body. The air then flows through the intake manifold and into the cylinder head. Finally in the cylinder head, the air travels through the intake runner, ending in the combustion chamber.

The following components are directly or indirectly a part of the air supply system.

Scheme 98

The mass air flow (MAF) sensor (1) measures the changes in the intake air volume that result from the changes in throttle opening and the air density. The airflow measurements are used by the PCM in order to determine the engine fueling requirements.

The MAF sensor is a hot-wire design. A platinum hot-wire (2) and a thermistor (3) are located in the intake air bypass passage of the MAF sensor housing. The temperature of the platinum hot-wire is affected by exposure to air flow and by exposure to air temperature. The platinum hot-wire is maintained at a set temperature by controlling the current flow through the wire. The MAF sensor converts the changes in current flow to a voltage signal. The voltage signal from the MAF sensor enables the PCM to detect changes in the air density and changes in air volume.

The MAF sensor also contains the IAT sensor. The IAT sensor cannot be serviced separately from the MAF sensor.