EVAPORATIVE EMISSION CONTROL SYSTEM OPERATION
The basic Evaporative (EVAP) Emission control system used is charcoal canister storage method. This method transfers fuel vapor from fuel tank to an activated carbon (charcoal) storage device (canister) to hold vapors when vehicle is not operating. When engine is running, fuel vapor is purged from carbon element by intake airflow and consumed in normal combustion process.
Gasoline vapors from fuel tank flow into tube labeled TANK. These vapors are absorbed into carbon. The canister is purged by powertrain control module (PCM)/engine control module (ECM) when engine has been running for a specified amount of time. Air is drawn into canister and mixed with vapor. This mixture is then drawn into intake manifold.
The PCM/ECM supplies a ground to energize EVAP emission canister purge solenoid valve. This valve is Pulse Width Modulated (PWM) or turned on and off several times a second. The EVAP emission canister purge PWM duty cycle varies according to operating conditions determined by mass airflow, fuel trim, and intake air temperature.
Poor idle, stalling and poor driveability can be caused by following conditions
- An inoperative EVAP emission canister purge solenoid valve.
- A damaged canister.
- Hoses that are split, cracked, or not connected to proper tubes.
POSITIVE CRANKCASE VENTILATION SYSTEM OPERATION
A Positive Crankcase Ventilation (PCV) system is used to provide complete use of crankcase vapors. Fresh air from air cleaner is supplied to crankcase. The fresh air is mixed with blowby gases which are then passed through a vacuum hose into intake manifold.
Periodically inspect hoses and clamps. Replace any crankcase ventilation components as required. A restricted or plugged PCV hose may cause following conditions
- Rough idle.
- Stalling or low idle speed.
- Oil leaks.
- Oil in air cleaner.
- Sludge in engine.
A leaking PCV hose may cause following conditions
- Rough idle.
- Stalling.
- High idle speed.
Results of Incorrect Operation
Too much EGR flow tends to weaken combustion, causing engine to run roughly or to stop. With too much EGR flow at idle, cruise, or cold operation any of following conditions may occur
- The engine stops after a cold start.
- The engine stops at idle after deceleration.
- The vehicle surges during cruise.
- Rough idle.
If EGR valve stays open all time engine may not idle. Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause following conditions
- Spark knock (detonation).
- Engine overheating.
- Emission test failure.
FUEL CONTROL SYSTEM OPERATION
The function of fuel metering system is to deliver correct amount of fuel to engine under all operating conditions. The fuel is delivered to engine by individual fuel injectors mounted into intake manifold near each cylinder.
The two main fuel control sensors are Manifold Absolute Pressure (MAP) sensor and Oxygen Sensor (02S).
The MAP sensor measures or senses intake manifold vacuum. Under high fuel demands MAP sensor reads a low vacuum condition, such as wide open throttle. The powertrain control module (PCM)/engine control module (ECM) uses this information to richen mixture, thus increasing fuel injector on-time, to provide correct amount of fuel. When decelerating, vacuum increases. This vacuum change is sensed by MAP sensor and read by PCM/ECM, which then decreases fuel injector on-time due to low fuel demand conditions.
IDLE AIR SYSTEM OPERATION
The idle air system operation is controlled by base idle setting of throttle body and Idle Air Control (IAC) valve.
The powertrain control module (PCM)/engine control module (ECM) uses IAC valve to set idle speed , dependent on conditions. The PCM/ECM uses information from various inputs, such as coolant temperature, manifold vacuum, etc., for effective control of idle speed.
IGNITION SYSTEM OPERATION
This ignition system does not use a conventional distributor and coil. It uses a crankshaft position sensor input to powertrain control module (PCM/engine control module (ECM). The PCM/ECM then determines Electronic Spark Timing (EST) and triggers direct ignition system ignition coil.
This type of distribututorless igniting system uses a "waste spark" method of spark distribution. Each cylinder is paired with cylinder that is opposite it (1-4 or 2-3). The spark occurs simultaneously in cylinder coming up on compression stroke and in cylinder coming up on exhaust stroke. The cylinder on exhaust stroke requires very little of available energy to fire spark plug. The remaining energy is available to spark plug in cylinder on compression stroke.
These systems use EST signal from PCM/ECM to control electronic spark timing. The PCM/ECM uses following information
- Engine load (manifold pressure or vacuum).
- Atmospheric (barometric) pressure.
- Engine temperature.
- Intake air temperature.
- Crankshaft position.
- Engine speed (rpm).
COMPREHENSIVE COMPONENT MONITOR DIAGNOSTIC OPERATION
Comprehensive component monitoring diagnostics are required to monitor emissions-related input and output powertrain components.
Misfire Monitor Diagnostic Operation
The misfire monitor diagnostic is based on crankshaft rotational velocity (reference period) variations. The powertrain control module (PCM)/engine control module (ECM) determines crankshaft rotational velocity using Crankshaft Position (CKP) sensor and Camshaft Position (CMP) sensor. When a cylinder misfires, crankshaft slows down momentarily. By monitoring CKP and CMP sensor signals, PCM/ECM can calculate when a misfire occurs.
For a non-catalyst damaging misfire, diagnostic will be required to monitor a misfire present for between 1000-3200 engine revolutions.
For catalyst-damaging misfire, diagnostic will respond to misfire within 200 engine revolutions. Rough roads may cause false misfire detection. A rough road will cause torque to be applied to drive wheels and drive train. This torque can intermittently decrease crankshaft rotational velocity. This may be falsely detected as a misfire.
A rough road sensor, or G sensor, works together with misfire detection system. The G sensor produces a voltage that varies along with intensity of road vibrations. When PCM/ECM detects a rough road, misfire detection system is temporarily disabled.
Fuel Trim System Monitor Diagnostic Operation
This system monitors averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares averages of short-term fuel trim values and long-term fuel trim values to rich and lean thresholds. If either value is within thresholds, a pass is recorded. If both values are outside their thresholds, a rich or lean DTC will be recorded.
The fuel trim system diagnostic also conducts an intrusive test. This test determines if a rich condition is being caused by excessive fuel vapor from Evaporative (EVAP) Emission canister. In order to meet OBD II requirements, control module uses weighted fuel trim cells to determine need to set a fuel trim DTC. A fuel trim DTC can only be set if fuel trim counts in weighted fuel trim cells exceed specifications. This means that vehicle could have a fuel trim problem which is causing a problem under certain conditions (i.e. engine idle high due to a small vacuum leak or rough idle due to a large vacuum leak) while it operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or H02S DTC may set). Use a scan tool to observe fuel trim counts while problem is occurring.
A fuel trim DTC may be triggered by a number of vehicle faults. Make use of all information available (other DTCs stored, rich or lean condition etc.) when diagnosing a fuel trim fault.