EVAP System: Description

Fig 1: Identifying EVAP Components Location

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HINT: 

The canister pressure sensor, the canister filter, the leak detection pump and the vent valve are built into the canister pump module.

Fig 2: EVAP System Component Location

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While the engine is running, if a predetermined condition (closed-loop, etc.) is met, the purge valve is opened by the ECM and fuel vapors stored in the charcoal canister assembly are purged to the intake manifold. The ECM changes the duty cycle ratio of the purge valve to control purge flow volume.

The purge flow volume is also determined by the intake manifold pressure. Atmospheric pressure is allowed into the charcoal canister assembly through the vent valve to ensure that the purge flow is maintained when negative pressure (vacuum) is applied to the charcoal canister assembly.

The following two monitors run to confirm appropriate EVAP system operation.

1. Purge flow monitor 

   The purge flow monitor consists of the 2 monitors. The 1st monitor is conducted every time and the 2nd monitor is activated if necessary.

   • The 1st monitor

     While the engine is running and the purge valve (Vacuum Switching Valve) is on (open), the ECM monitors the purge flow by measuring the EVAP pressure change. If the variation in the pressure is less than threshold, the ECM begins the 2nd monitor.

   • The 2nd monitor

     The vent valve is turned ON (closed) and the EVAP pressure is then measured. If the variation in the pressure is less than threshold, the ECM interprets this as the purge valve being stuck closed, and illuminates the MIL and stores DTC P0441 (2 trip detection logic).

   • Atmospheric pressure check:
     • In order to ensure reliable malfunction detection, the variation between the atmospheric pressures, before and after of the purge flow monitor, is measured by the ECM.
   Fig 3: Checking Atmospheric Pressure

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
   TEXT IN ILLUSTRATION
   
   

   *1	ECM	*2	Soak Timer
   *3	Purge Valve (on)	*4	Fuel Tank Cap Assembly
   *5	Fuel Tank	*6	Canister Filter
   *7	Leak Detection Pump (off)	*8	Canister Pressure Sensor
   *9	Charcoal Canister Assembly	*10	Vent Valve (off)
   *11	Canister Pump Module	*12	Reference Orifice (0.02 inch)
   *a	EVAP Purge Flow	*b	to Intake Manifold

   Component	Operation
   Charcoal Canister Assembly	Contains activated charcoal to absorb EVAP (Evaporative Emissions) generated in fuel tank.
   Cut-off valve	Located in fuel tank. Valve closes by its own weight when vehicle overturns to prevent fuel from spilling out.
   Purge valve (Vacuum Switching Valve)	Opens or closes line between charcoal canister assembly and intake manifold. ECM uses purge valve to control EVAP purge flow. In order to discharge EVAP absorbed by charcoal canister assembly to intake manifold, ECM opens purge valve. EVAP discharge volume to intake manifold controlled by purge valve duty cycle (current-carrying time). (Open: on, Close: off)
   Fuel tank vent valve	Located in the fuel tank. Passes evaporative emissions occurring during fueling to the charcoal canister assembly. Additionally, this mechanism also detects when the fuel tank is full.
   Soak timer	Built into ECM. To ensure accurate EVAP monitor, measures 5 hours (+/-15 min) after ignition switch is turned off. This allows fuel to cool down, stabilizing EVAP pressure. When approximately 5 hours elapsed, ECM activates (refer to fig. 3).
   Canister pump module	Consists of (a) to (d) below. Canister pump module cannot be disassembled.
   (a) Vent valve	Vents and closes EVAP system. When ECM turns valve on, EVAP system is closed. When ECM turns valve off, EVAP system is vented. Negative pressure (vacuum) is created in EVAP system to check for EVAP leaks by closing purge valve, turning on vent valve (closing it) and operating leak detection pump (refer to fig. 1).
   (b) Canister pressure sensor	Indicates pressure as voltages. ECM supplies regulated 5 V to pressure sensor, and uses feedback from sensor to monitor EVAP system pressure (refer to fig. 2).
   (c) Leak detection pump	Creates negative pressure (vacuum) in EVAP system for leak check.
   (d) Reference orifice	Has opening with 0.02 inch diameter. Vacuum is produced through orifice by closing purge valve, turning off vent valve and operating leak detection pump, to monitor reference pressure. Reference pressure indicates a small leak of EVAP.

   Fig 4: Vacuum Is Produced Through Orifice By Closing Purge VSV

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
   TEXT IN ILLUSTRATION
   
   

   *1	Canister	*2	Reference Orifice (0.02 Inch)
   *3	Canister Pressure Sensor	-	-
   *a	Canister Pump Module (fig. 1)	*b	Airflow
   *c	Condition: Purge Flow	*d	Condition: Leak Check
   *e	Vent Valve: off (vent)	*f	to Canister Filter (Atmosphere)
   *g	Leak Detection Pump: off	*h	Vent Valve: on (closed)
   *i	Leak Detection Pump: on	-	-

   Fig 5: Canister Pressure Sensor Specification

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   Fig 6: Canister Pressure Sensor Circuit Diagram

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