2. Modern EVAP systems are quite complex in design. The principle of their operation is as follows. When gasoline contained in the fuel tank heats up on a hot day, it evaporates, creating fuel vapor. These vapors, which are raw, unburned hydrocarbons, increase the pressure inside the sealed fuel tank. If there were no way to remove them from the fuel tank, a leak would eventually develop somewhere in the tank. The pressure inside the tank is monitored by a vapor pressure sensor, which reports the pressure level to the powertrain control module (PCM). When the PCM registers that the pressure exceeds a prescribed threshold, it activates a pressure selector valve, which opens, allowing vapor to pass into the EVAP canister. The canister stores these vapors until the PCM activates the purge valve, which opens and purges the EVAP system, allowing the vacuum present in the intake manifold to pull the vapors out of the canister and 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 an upper threshold, the vapor pressure sensor signals the PCM, which opens the vacuum selector valve (VSV) to the pressure selector valve, allowing the fuel vapor to pass to the EVAP canister where it is stored until purging occurs.
4. When the engine is cold or still warming up, the accumulated fuel vapors are not allowed to escape from the EVAP canister. Once the engine has warmed up (to 75°C), the PCM puts the system into closed loop operation. It activates the canister purge valve, which regulates the flow of vapors from the canister to the intake manifold. The flow of vapors is regulated by the purge valve in response to commands from the PCM. The PCM controls the duration of the valve operation, which means that the valve opening can be controlled more precisely (i.e. there is not only a fully open or fully closed state), which allows the volume of vapors blown out to be adjusted appropriately so that the air-fuel mixture does not become too rich. On models with a four-cylinder engine, the EVAP canister purge valve is located on the bulkhead behind the throttle body. On models with a V6 engine, the EVAP canister purge valve is located in the engine compartment, on the right end 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 Switching Valve (VSV) for the Evaporative Emission System 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, a vacuum would quickly develop inside both the canister and the fuel tank if they were not vented to the atmosphere. Therefore, during the purging process, atmospheric air is drawn through the air cleaner housing, the fresh air line, and then into the canister. The canister shutoff valve opens and closes the EVAP system fresh air line in response to signals from the powertrain control module (PCM). When commanded by the PCM, the canister shutoff valve also closes the fresh air line to the EVAP canister for a check. The canister shutoff valve is located in the engine compartment, on the underside of the air cleaner housing.
Note: On earlier Lexus and Toyota models, this device is referred to as the Vacuum Switching Valve (VSV) for the Canister Check Valve (CCV). However, in this book, it is simply called the Canister Check Valve.
EVAP system monitoring function
6. The EVAP diagnostic monitoring function is an OBD-II test that the PCM runs to check the EVAP system and fuel tank for leaks. Certain conditions must be met before the monitoring sequence will begin. First, the engine must be running. If the engine is cold, the engine coolant temperature and intake air temperature are approximately equal. The PCM closely monitors the warm-up process. Once the oxygen sensors and catalytic converters are warm enough to enter closed loop operation, the PCM initiates the EVAP purge sequence. The purge valve opens, the canister shutoff 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 the "over-rich" mixture caused by purging the vapors stored in the EVAP canister actually helps smooth out the engine's idle performance.
7. During the purge, the EVAP system is essentially pressure neutral because the open canister shutoff valve allows atmospheric pressure to enter the canister while vapor is being drawn out of the canister into the intake manifold. During this initial period of operation, the PCM also monitors the fuel tank pressure via the vapor pressure sensor. Once the purge process is complete, the PCM closes the canister shutoff valve. When the canister shutoff valve first closes, the pressure selector valve and the 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 monitors all pressure changes (using a vapor pressure sensor), to check the EVAP system for leaks. If there is a leak, the Malfunction Indicator Lamp (MIL) or Check Engine Light will illuminate (check the engine) and the PCM generates a diagnostic trouble code (DTC) that 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 selector valve, which causes the pressure in the EVAP system to drop. The PCM holds the purge valve open until the pressure in the EVAP system drops to a specified threshold, at which point the PCM closes the purge valve. If the pressure does not drop or drops too much, the PCM illuminates the Malfunction Indicator Lamp (MIL) or Check Engine Light and sets a CTC code (see paragraph 2), which indicates 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 inlet). When the vapor pressure increases to a prescribed threshold, the PCM opens the canister shutoff valve. The system pressure rises rapidly due to air being introduced into the system. If the PCM detects no increase in pressure or the pressure is below the prescribed increased value, it determines that either the canister shutoff valve is malfunctioning or there is a restriction in the bleed capacity somewhere in the line connecting the system to the atmosphere. In this case, the DTC (see paragraph 2) and the malfunction indicator lamp (MIL) or Check engine light comes on.
10. Finally, the PCM closes the pressure switching valve, which prevents atmospheric air from entering the system from the fuel tank. When the pressure switching valve is working properly, it should create a slight increase in pressure inside the tank (because the fuel inside the back is still slowly heating up). But if no pressure change occurs, the PCM decides that the pressure switching valve is not closing and generates a DTC again (see paragraph 2) and illuminates the Malfunction Indicator Lamp (MIL) or Check Engine Lamp. The monitoring sequence is now complete. The PCM immediately repeats the entire sequence again if conditions allow, and continues to do so as long as the engine is operating in closed loop.