Circuit Description

EVAP emission system is checked by applying vacuum to EVAP system and monitoring for a vacuum leak. PCM monitors vacuum level via fuel tank pressure sensor signal. When appropriate, EVAP canister purge valve and EVAP canister vent solenoid are turned on, allowing engine to draw a small vacuum on entire evaporative emission system. After desired vacuum level has been achieved, EVAP canister purge valve is turned off, sealing EVAP system.

A restricted or blocked EVAP canister vent path is detected by drawing a vacuum on EVAP system, turning off EVAP vent solenoid and EVAP canister purge valve and monitoring fuel tank vacuum sensor input. With EVAP vent solenoid open, any vacuum in system should decrease quickly unless vent path is blocked. Blockage can be caused by faulty EVAP vent solenoid, restricted vent hose, shorted EVAP vent solenoid driver circuit, or plugged evaporative canister. If blockage is detected, DTC P0446 will be set.

Conditions required to set DTC are:

• BARO is greater than 72.3 kPa.
• No MAP, TPS, IAT, ECT, EGR, EVAP, VSS or system voltage DTCs set.
• Intake air and coolant temperatures at startup are between 40-95°F (4.2-35°C).
• Difference between ECT and IAT at start up is less than 12.2°F (6.75°C).
• Fuel tank level sensor reads 15-85 percent.
• Fuel tank vacuum is greater than 8 in. H₂ O for 2 seconds.