EVAP Purge/Vent Control Diagnosis

EVAP Purge / Vent Control Diagnosis

Circuit Description
The evaporative emission (EVAP) system includes the following components:
- Fuel Tank
- EVAP vent solenoid
- Fuel tank pressure sensor
- Fuel pipes and hoses
- Fuel vapor lines
- Fuel filler cap
- EVAP canister
- Purge lines
- EVAP canister purge valve
- EVAP service port

The evaporative emission system is checked by applying vacuum to the EVAP system and monitoring for a vacuum decay. The ECM monitors the vacuum level through the fuel tank pressure sensor signal. At the appropriate time, the EVAP canister purge valve and the EVAP vent solenoid are turned ON allowing the engine to draw a small vacuum on the entire EVAP system. After the desired vacuum level has been achieved, the EVAP canister purge valve is turned OFF, sealing the system. If a sufficient vacuum level cannot be achieved or the vacuum level decreases, a leak is indicated.

Diagnostic Aids
The scan tool has the ability to command the EVAP canister purge and EVAP vent solenoids ON and OFF.

A grounded or shorted wire may cause the solenoids to energize when the ignition is turned ON. A restricted or sticking solenoid valve may cause improper EVAP system vacuum readings. When diagnosing intermittent complaints, be sure to thoroughly check for backed-out terminals, improper mating, broken locks, damaged terminals and wiring.

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EVAP Purge Solenoid Diagnosis