2. Modern EVAP systems are quite complex in design. The principle of their work is as follows. When the gasoline contained in the fuel tank is heated on a hot day, it evaporates and fuel vapors are formed. These vapors, which are raw unburned hydrocarbons, increase the pressure inside the sealed fuel tank. If it were not provided for the possibility of their removal from the fuel tank, eventually there would be a leak somewhere in the tank. The pressure inside the back is monitored by a vapor pressure sensor, which informs the powertrain control module about the pressure level (RSM). When the PCM senses that the pressure is above a prescribed threshold, it activates a pressure switching valve that opens to allow steam to pass into the EVAP canister. The adsorber retains these vapors until the PCM activates the purge valve, which opens and purges the EVAP system, i.e. allows the vacuum present in the intake manifold to draw vapors from the adsorber into the intake manifold.
3. The vapor pressure sensor is located on top of the fuel tank. This sensor monitors the fuel vapor pressure inside the tank. When the vapor pressure exceeds the upper threshold, the vapor pressure sensor sends a signal to the PCM, which opens the vacuum switching valve (VSV) for the pressure switch valve, allowing fuel vapor to pass to the EVAP canister where it is stored until purge is performed.
4. When the engine is cold or still warm, accumulated fuel vapors are not allowed to exit the EVAP canister. After warming up the engine (up to 75°C) The PCM puts the system into closed loop operation. It activates the canister purge valve, which regulates the vapor flow from the canister to the intake manifold. The vapor flow is controlled by the purge valve in response to commands from the PCM. The PCM module controls the duration of the valve, which means that the opening of the valve can be controlled more accurately (those. there is not only a fully open or fully closed state), which allows you to adjust the volume of blown vapors with an appropriate cut so that the air-fuel mixture does not become too rich. On four-cylinder models, the EVAP canister purge valve is located on the bulkhead behind the throttle body. On V6 models, the EVAP canister purge valve is located in the engine compartment, on the right side of the engine (where is the timing belt located).
Note. On earlier Toyota and Lexus models, the purge valve was referred to as the vacuum changeover valve (VSV) for EVAP or VSV for EVAP. However, in this book it is referred to as the canister purge valve, or simply the purge valve.
5. When the EVAP system is purged and accumulated vapors are drawn out of the canister by the vacuum present in the intake manifold, vacuum could quickly build up inside both the canister and the fuel tank if they were not vented to atmosphere. Therefore, during the purge process, atmospheric air is drawn through the air filter housing, the fresh air line and then enters the adsorber. Canister shut-off valve opens and closes the EVAP system fresh air line in response to signals from the powertrain control module (RSM). When commanded by the PCM, the canister shutoff valve also closes the fresh air supply line to the EVAP canister for a check test. The canister shut-off valve is located in the engine compartment, on the underside of the air filter housing.
Note. On earlier Lexus and Toyota models, this device is referred to as the vacuum switch valve (VSV) for canister stop valve (CCV). However, in this book it is simply referred to as the canister shut-off valve.
EVAP system monitoring function
6. The EVAP system diagnostic monitoring function is an OBD-II test that the PCM performs to check the EVAP system and fuel tank for leaks. Before the monitoring sequence starts to work, some conditions must be satisfied. First you must start the engine. If the engine is cold, the engine coolant temperature and intake air temperature are approximately equal. PCM carefully monitors the warm-up process. Once the oxygen sensors and catalytic converters have warmed up sufficiently to allow the system to enter closed loop operation, the PCM will activate the EVAP system purge sequence. The purge valve opens, the canister check valve opens, and the contents of the EVAP canister are purged, i.e., introduced into the intake manifold. When warming up at fast idle, the vacuum in the intake system is high and «overenrichment» mixture caused by scavenging vapors built up in the EVAP canister actually helps to smooth out the engine at idle.
7. When purge, the pressure in the EVAP system can be said to be neutral because the open canister shut-off valve allows atmospheric pressure to enter the canister while vapors are drawn out of the canister and into the intake manifold. During this initial period of operation, the PCM also monitors the pressure in the fuel tank using a vapor pressure sensor. Once the purge process is complete, the PCM closes the canister shut-off valve. When the canister shut-off valve closes first, the pressure switch valve and purge valve are still open and therefore a vacuum is created in the purge line from the air intake to the canister and in the EVAP line from the canister to the fuel tank (relative). The PCM then closes the purge valve to create a vacuum (relative) in the line between the tank and the purge valve. It then controls all pressure changes (using a vapor pressure sensor), to check the EVAP system for a leak. If there is a leak, the malfunction indicator lamp lights up (MIL) or Check engine warning light (check engine) and PCM generates div gnostic trouble code (DTC), which indicates a malfunction in the system (see paragraph 2).
8. At a certain point in the monitoring sequence, the PCM closes the canister shutoff valve and opens the pressure switch valve, which causes a pressure drop in the EVAP system. The PCM keeps the purge valve open until the pressure in the EVAP system drops to a prescribed threshold, at which point the PCM closes the purge valve. If the pressure does not drop or drops too much. The PCM turns on the malfunction indicator lamp (MIL) or the Check engine warning lamp and generates the CTC code (see paragraph 2), which indicates an incorrect flow rate when purging the EVAP system.
9. The PCM then controls the operation of the canister shut-off valve and the ventilation function of the system (air intake). When the vapor pressure rises to a prescribed threshold, the PCM opens the canister shut-off valve. The pressure in the system quickly rises due to the introduction of air into the system. If the PCM detects no increase in pressure, or the pressure is below the prescribed increased value, it decides that either the canister check valve is malfunctioning or there is a capacity restriction somewhere in the line connecting the system to the atmosphere. In this case, the DTC is generated again (see paragraph 2) and the malfunction indicator lamp turns on (MIL) or check engine light.
10. Finally, the PCM closes the pressure switching valve, which prevents atmospheric air from entering the system from the fuel tank side. When the pressure switch valve is working properly, it should create a slight increase in pressure inside the tank (because the fuel inside the tank is still slowly heating up). But if no pressure change occurs, the PCM decides that the pressure switching valve is not closing and generates the DTC again (see paragraph 2) and turns on the malfunction indicator lamp (MIL) or check engine light. The monitoring sequence is now complete. The PCM immediately repeats the entire sequence again if conditions permit and continues to do so as long as the engine is running in closed loop mode.