If only a small amount of fuel is in the fuel tank it takes longer for the EVAP pressure to stabilize

Operation A: Atmospheric Pressure Measurement

Purge VSV: OFF\

Operation B, E:

0.02 Inch Leak Criterion Measurement

Canister Pressure Sensor

Fuel Tank

Vent Valve: OFF (vent) Canister Pump Module

-Canister Filter

Leak Detection Pump: OFF

OFF (vent)

Operation C: EVAP System Pressure Measurement

ON (closed)

Reference Orifice (0.02 Inch)

Operation D: Purge VSV Monitor



Atmospheric Pressure

Negative Pressure


ON (closed)


(a)Purge VSV stuck open

In operation C, the leak detection pump creates negative pressure (vacuum) in the EVAP (Evaporative Emission) system. The EVAP system pressure is then measured by the ECM using the canister pressure sensor. If the stabilized system pressure is higher than [second 0.02 inch leak criterion x 0.2], the ECM interprets this as the purge VSV (Vacuum Switching Valve) being stuck open. The ECM illuminates the MIL and sets the DTC (2 trip detection logic).

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

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