EVAP System: Description

Fig 1: Identifying EVAP Hose (To Intake Manifold) & Purge VSV

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

Fig 2: Identifying Canister Pump Module & Intake Manifold

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

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

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

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

1. Key-off monitor 

   This monitor checks for EVAP (Evaporative Emission) system leaks and canister pump module malfunctions. The monitor starts 5 hours* after the ignition switch is turned off. At least 5 hours are required for the fuel to cool down which stabilizes the EVAP pressure, thus making the EVAP system monitor more accurate.

   The leak detection pump creates negative pressure (vacuum) in the EVAP system and the pressure is measured. Finally, the ECM monitors for leaks from the EVAP system, and malfunctions in both the canister pump module and purge VSV based on the EVAP pressure.

   HINT: 

   *: If the engine coolant temperature is not below 35°C (95°F) 5 hours after the ignition switch is turned off, the monitor check starts 2 hours later. If it is still not below 35°C (95°F) 7 hours after the ignition switch is turned off, the monitor check starts 2.5 hours later.

2. Purge flow monitor 

   The purge flow monitor consists of 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 VSV (Vacuum Switching Valve) is ON (open), the ECM monitors the purge flow by measuring the EVAP pressure change. If negative pressure is not created, the ECM begins the 2nd monitor.

   • The 2nd monitor

     The vent valve is turned OFF (open) and the EVAP pressure is measured. If the variation in the pressure is less than 0.15 kPa-g (1.12 mmHg-g), the ECM interprets this as the purge VSV being stuck closed, 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 pressure, before and after conduction of the purge flow monitor, is measured by the ECM.

   Fig 3: Identifying Soak Timer, ECM & Canister

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
   TEXT IN ILLUSTRATION
   
   

   *1	Canister	*2	to Intake Manifold
   *3	ECM	*4	Soak Timer
   *5	Purge VSV (On)	*6	Fuel Tank Cap Assembly
   *7	Canister Filter	*8	Fuel Tank
   *9	Leak Detection Pump (Off)	*10	Canister Pressure Sensor
   *11	Reference Orifice (0.02 Inches)	*12	Vent Valve (Off)
   *13	Canister Pump Module	-	-
   *a	EVAP Purge Flow	-	-

   Component	Operation
   Canister	Contains activated charcoal to absorb EVAP (Evaporative Emissions) generated in fuel tank.
   Cut-off valve	Located in fuel tank. Valve floats and closes when fuel tank 100% full.
   Purge VSV (Vacuum Switching Valve)	Opens or closes line between canister and intake manifold. ECM uses purge VSV to control EVAP purge flow. In order to discharge EVAP absorbed by canister to intake manifold, ECM opens purge VSV. EVAP discharge volume to intake manifold controlled by purge VSV duty cycle (current flow time) (Open: ON; Closed: OFF).
   Roll-over valve	Located in fuel tank. Valve closes by its own weight when vehicle overturns to prevent fuel from spilling out.
   Soak timer	Built into ECM. To ensure accurate EVAP monitor, measures 5 hours (+/-15 min) after ignition switch turned off. This allows fuel to cool down, stabilizing EVAP pressure. When approximately 5 hours have elapsed, ECM activates (refer to fig. 3 in Fig 6).
   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 closed. When ECM turns valve OFF, EVAP system vented. Negative pressure (vacuum) created in EVAP system to check for EVAP leaks by closing purge VSV, turning on vent valve (closing it) and operating leak detection pump (refer to fig. 1 in Fig 4).
   (b) Canister pressure sensor	Indicates pressure as voltage. ECM supplies regulated 5 V to canister pressure sensor and uses feedback from sensor to monitor EVAP system pressure (refer to fig. 2 in Fig 5).
   (c) Leak detection pump	Creates negative pressure (vacuum) in EVAP system for leak check.
   (d) Reference orifice	Opening with 0.02 inch diameter. Vacuum produced through orifice by closing purge VSV, turning off vent valve and operating leak detection pump to monitor reference pressure. Reference pressure used to detect small leak of EVAP.

   Fig 4: Identifying Canister Pump Module

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
   TEXT IN ILLUSTRATION
   
   

   *1	Airflow	*2	Leak Detection Pump: Off
   *3	Condition: Purge Flow	*4	Condition: Leak Check
   *5	Vent Valve: Off (Vent)	*6	to Canister Filter (Atmosphere)
   *7	Canister	*8	Reference Orifice (0.02 Inches)
   *9	Canister Pressure Sensor	*10	Leak Detection Pump: On
   *11	Vent Valve: On (Closed)	-	-
   *a	Canister Pump Module (fig. 1 in Fig 4)	-	-

   Fig 5: Canister Pressure Sensor Specification Diagram

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
   Fig 6: Identifying ECM Soak Timer Circuit

   Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002