THREE WAY CATALYTIC CONVERTER (TWC) MONITORING
To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce the emissions of hydrocarbons, oxides of nitrogen and carbon monoxide. Normal vehicle operation or engine misfire can cause a catalyst to decay. A meltdown of the ceramic core can cause a reduction of the exhaust passage resulting in an increase in vehicle performance and deteriorate engine performance, driveability and fuel economy.
The catalyst uses dual oxygen sensor to monitor the efficiency of the converter because as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream (front) HO2 Sensor is used to detect the amount of oxygen in the exhaust gas before it enters the catalytic converter. A low voltage indicates a high oxygen content (lean air mixture). A high voltage indicates a low content of oxygen (rich mixture).
When the front HO2 Sensor detects a lean condition, there is an abundance in the oxygen in the exhaust has. A functioning converter would store this oxygen so it can use it for the oxidation of HC and CO. As the converter absorbs the oxygen, there will be a lack of oxygen downstream of the converter.
The output of the downstream (rear) HO2 Sensor will indicate limited activity in this condition. As the converter loses the ability to store oxygen, the condition can be detected from the operation of the rear HO2 Sensor. When the catalyst efficiency drops, no chemical reaction takes place. This means the concentration of oxygen will be the same downstream (rear) as upstream (front). The output voltage of the rear HO2 Sensor copies the voltage of the front HO2 Sensor. The only difference is a time lag (monitored by the ECM) between the switching of the HO2 Sensors.
To monitor the system, the lean-to-rich switches of front to rear HO2 Sensors is counted. The ratio of rear switches to front switches is used to determine whether the catalyst is operating properly. An effective catalyst will have fewer rear switches than it has front switches, that is, a ratio closer to zero. For a totally ineffective catalyst, this ratio will be one-to-one indicating that no oxidation occurs in the catalytic converter.
The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over the legal limit, the MIL will illuminate.
Scheme 119
Scheme 120
- Warm up engine to normal operating temperature.
- Run engine at idle.
- Disconnect PCV valve ventilation hose from cylinder head cover.
- Block PCV valve opening.
- Verify that vacuum is felt.
- Remove PCV valve.
- Blow through valve from port "A" and verify that air comes out of port "B".
- Blow through valve from port "B" and verify that no air comes out of port "A".
- Replace the PCV valve if necessary.
EVAP SYSTEM MONITORING: LEAKAGE DETECTION
The evaporative system is monitored by a pressure test. The canister close valve and fuel tank pressure sensor are used to close the system and observe tank pressure respectively.
Scheme 121
MONITORING CONDITIONS
The leak test is conducted when the vehicle is stopped during an idle condition.
MONITORING PROCESS
The canister close valve and the EVAP canister purge control valve are closed and an increase in fuel tank pressure caused by fuel evaporation occurs (compensation gradient). Afterwards, the canister close valve and the purge control valve are opened. The canister close valve will be shut if the purge control valve reached its final duty cycle.
Scheme 122
- Gross leak detection A gross leak is assumed if negative pressure is built-up below a certain threshold. In this case, the EVAP canister purge control valve is closed and the canister close valve is opened. Then, the diagnosis is completed.
- Small leak detection The EVAP canister purge control valve is closed as soon as the maximum vacuum level is achieved. The rate of vacuum loss will be corrected by the above compensation gradient. A small leak is assumed if this corrected rate of vacuum loss is above a certain threshold. Then, the canister close valve is opened and the diagnosis is completed.
MIL ILLUMINATION
If either a rough leak or a small leak is detected for two driving cycles, then a defective evaporative system is assumed and the MIL is illuminated.
AIR/FUEL MIXTURE CONTROL SYSTEM [MULTIPORT FUEL INJECTION (MFI) SYSTEM]
The MFI system is a system which uses the signals from the heated oxygen sensor to activate and control the injector installed in the manifold for each cylinder, thus precisely regulating the air/fuel mixture ratio and reducing emissions.
This in turn allows the engine to produce exhaust gases of the proper composition to permit the use of a three way catalyst. The three way catalyst is designed to convert the three pollutants (1) hydrocarbons (HC), (2) carbon monoxide (CO), and (3) oxides of nitrogen (NOx) into harmless substances. There are two operating modes in the MFI system.
- Open Loop air/fuel ratio is controlled by information programmed into the ECM.
- Closed Loop air/fuel ratio is adjusted by the ECM based on information supplied by the oxygen sensor.