Purge flow monitor

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

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 vent valve is turned OFF (open) and the EVAP pressure is then measured. If the variation in the pressure is less than 0.5 kPa (3.75 mmHg), the ECM interprets this as the purge VSV being stuck closed, and illuminates the MIL and sets 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 conduction of the purge flow monitor, is measured by the ECM.

EVAP Purge Flow:

Reference Orifice (0.02 inch)

Fuel Cap

Refueling Valve

Reference Orifice (0.02 inch)

Fuel Cap

Refueling Valve

To Intake Manifold

To Intake Manifold

Soak Timer

Vent Valve (OFF) Canister Pump Module




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 ratio (current-carrying time). (Open: ON, Close: OFF)

Refueling valve

Controls EVAP pressure from fuel tank to canister. Valve consists of diaphragm, spring and restrictor (diameter: 0.08 inch). When fuel vapor and pressure inside fuel tank increase, valve opens. While EVAP purged, valve closes and restrictor prevents large amount of vacuum from affecting pressure in fuel tank. Valve opened while refueling. When valve open, adding fuel into fuel tank possible.

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 to OFF. This allows fuel to cool down, stabilizing Fuel Tank Pressure (FTP). When approximately 5 hours elapsed, ECM activates.

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 (closed) and operating leak detection pump (refer to fig. 1).

(b) Canister pressure sensor

Indicates pressure as voltages. ECM supplies regulated 5 V to canister 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 produced through orifice by closing purge VSV, turning off vent valve and operating leak detection pump, to monitor 0.02 inch leak criterion. 0.02 inch leak criterion indicates small leak of EVAP.

Canister Pump Module (fig. 1):

Canister Pump Module (fig. 1):

Condition: Purge Flow

: Airflow


Reference Orifice


Canister Pressure Sensor / From Refueling Valve Leak Detection Pump: OFF

To Canister Filter


Vent Valve: ON (closed)

Condition: Leak Check

Leak Detection Pump: ON

Canister Pressure Sensor Specification (fig. 2):


Standard atmospheric pressure is 101.3 kPa (759.8mmHg)

Soak Timer Circuit (fig. 3):

Ignition Switch

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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