EVAP System: Description

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 to the intake manifold. The ECM will change the duty cycle ratio of the purge VSV to control purge flow volume.

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

The ECM monitors the condition of both the key-off monitor and purge flow monitor to ensure proper operation of the EVAP system.

Component	Operation
Canister	Contains activated charcoal to absorb EVAP that is created in fuel tank.
Cut-off valve	Located in the fuel tank. Valve closes by its own weight when vehicle is overturned to prevent fuel from spilling out.
Purge VSV	Opens or closes line between canister and intake manifold. ECM opens and closes purge VSV to control EVAP purge flow. ECM opens purge valve to purge vapor gas that was absorbed by canister to intake manifold. ECM controls EVAP volume purged to intake manifold by duty cycle (current-carrying time) to purge valve. (ON: Open, OFF: Closed)
Roll-over valve	Located in fuel tank. Valve floats and closes when fuel tank is filled to 100%. Also, valve close by its own weight when vehicle is over turned to prevent fuel from spilling out.
Soak timer	Built into ECM. Soak timer counts power switch OFF time.
Fuel vapor-containment valve (FVCV)	Opens and closes line between fuel tank and canister. When vehicle is stopped, this valve closes to keep vapor in the tank and prevent it from being absorbed in canister. During refueling, valve opens and vapor is absorbed in canister. When driving, valve acts to change pressure in the tank slightly positive.
Fuel tank pressure sensor	Converts fuel tank pressure into voltage and outputs that voltage. ECM supplies a constant voltage of 5 V to pressure sensor and uses feedback from sensor to monitor fuel tank pressure, (fig. 3)
Canister pump module	(a) to (e) below are canister pump module components. 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) created in EVAP system to check EVAP leaks by closing purge VSV, turning on vent valve (closed) and operating vacuum pump (refer to Fig 3).
(b) Canister pressure sensor	Indicates EVAP pressure as voltage 5 V is applied by ECM. ECM detects EVAP system pressure using this voltage (refer to Fig 4).
(c) Check valve	Composed of nylon ball and spring. Valve blocks off atmosphere from the vacuum pump.
(d) Leak detection pump	Negative pressure (vacuum) is created in EVAP system for leak check.
(e) Reference orifice	Diameter is 0.02 inch. Negative pressure (vacuum) is created in reference orifice to measure reference pressure by closing purge VSV, turning off vent valve (vent) and operating leak detection pump. Reference pressure indicates criterion of EVAP small leak.

Key-off monitor 

This monitoring system checks for canister pump module malfunctions and leaks from the EVAP and closed tank systems. Be sure to leave the vehicle for at least 5 hours to sufficiently cool the fuel and stabilize fuel tank pressure. This makes EVAP system monitoring more accurate.

HINT:

If the engine coolant temperature is 35°C (95°F) or more, 5 hours after the power switch has been turned off, the ECM will begin performing a monitor check after another 2 hours. If the engine coolant temperature is still 35°C (95°F) or more, 7 hours after the power switch has been turned off, the ECM will begin performing a monitor check after another 2.5 hours.

There are two methods for monitoring the EVAP system.

1. If the fuel tank pressure is higher or lower than the atmospheric pressure, the system determines that there are no leaks in the closed tank system and the system will check for leaks from the piping and canister between the purge VSV and canister pump module. (Method A)
2. If the fuel tank pressure is almost the same as the atmospheric pressure, vacuum pressure will enter the fuel tank and the system will check for leaks from the fuel tank after checking for leaks from the canister. (Method B)

Sequence	Operation	Description	Duration
-	ECM activation	Activated by soak timer, 5 hours (7 or 9.5 hours) after power switch turned OFF.	-
A	Atmospheric pressure measurement	Vent valve turned OFF (vent) and EVAP system pressure is measured by ECM in order to register atmospheric pressure. If pressure in EVAP system is not between 70 kPa and 110 kPa (525 mmHg and 825 mmHg), ECM cancels EVAP system monitor.	10 seconds
B	First reference pressure measurement	In order to determine reference pressure standard, leak detection pump creates negative pressure (vacuum) through reference orifice and then ECM checks if leak detection pump and vent valve operate normally.	60 seconds
C	EVAP system pressure measurement	Vent valve is turned ON (closed) to shut EVAP system. Negative pressure (vacuum) is created in EVAP system, and EVAP system pressure is then measured. Write down measured values as they will be used in leak check. If EVAP pressure does not stabilize within 15 minutes, ECM cancels EVAP system monitor.	15 minutes
D	Purge VSV monitor	Purge VSV opens and then EVAP system pressure is measured by ECM. Large increase indicates normal.	10 seconds
E	Second reference pressure measurement	After second reference pressure measurement, leak check is performed by comparing first and second reference pressure standards. If stabilized system pressure is higher than second reference pressure standard, ECM determines that EVAP system has a leakage.	60 seconds
F	All actuator OFF	Fuel tank pressure is compared with atmospheric pressure. If thank pressure is higher than PH or lower than PL, ECM determines that EVAP system is normal and runs sequence I in method B. If tank pressure is around atmospheric pressure, ECM performs sequence G in method A.	5 seconds
G	Fuel tank pressure measurement	Vent valve is turned ON (closed). Fuel vapor-containment valve opens to allow negative pressure into the fuel tank and fuel tank pressure is measured. Write down measured values because they will be used in leak check. If fuel tank pressure does not stabilize within 15 minutes, ECM stops monitoring.	15 minutes*
H	Third reference pressure measurement	After 3rd reference pressure measurement, leak check of fuel tank is performed. If recorded fuel tank pressure is higher than 3rd reference pressure, ECM determines that EVAP system has leakage.	60 seconds
1	Fuel tank close valve close stuck check	Fuel vapor-containment valve is opened for a certain period of time to check whether the valve is stuck closed.	0.1 second
J	Final check	Atmospheric pressure is measured and then monitoring result is recorded by ECM.	-
HINT: *: If there is only a small amount of fuel in the fuel tank, stabilizing the EVAP pressure takes longer than usual.

Purge flow monitor 

If EVAP system pressure change is less than 7.5 mmHg (1 kPa) when the engine is running and the purge VSV is turned on (closed), the ECM determines that the purge flow is insufficient.