Circuit Description

EVAP system includes fuel tank, fuel tank pressure sensor, EVAP pressure switching solenoid, fuel lines and hoses, vapor and purge lines, fuel cap, EVAP canister, and EVAP canister purge solenoid valve. Once every drive cycle, EVAP system will run a 25 to 40-second diagnostic where it monitors possible leaks at fuel tank pressure sensor, vent valve and canister purge valve.

This diagnostic has 5 phases and can set different DTC depending on which part of the diagnostic fails. The EVAP system is tested by applying pressure (vacuum) to the system and monitoring pressure decay. ECM monitors vacuum level through the fuel tank vapor pressure sensor. In the 5th phase of the diagnostics, EVAP vent solenoid is commanded closed (normally open valve) and EVAP canister purge valve is commanded to a fixed duty cycle, allowing engine to draw a vacuum on entire evaporative emission system. After a calibrated vacuum level is achieved, EVAP canister purge valve is turned off, sealing system. A leak is detected by monitoring for decrease in vacuum level over a calibrated period of time, while keeping all other variables constant.

DTC will set when:

• Vehicle speed is zero MPH.
• Engine at idle.
• ECT between 17-122°F (-8°C to 50°C).
• Battery voltage greater than 11 volts.
• Engine run time greater than 1.45 minutes.
• Altitude less than 9000 feet.
• Fuel level between 1.85-14.0 gallons.
• No TP, ECT, VSS, fuel tank pressure, O2S, idle control, purge valve, vent valve, battery voltage, misfire and EGR DTCs set.

When DTC sets, MIL will illuminate after 2 consecutive ignition cycles in which diagnostic runs with active fault. ECM will record operating conditions at time of diagnostic failure and store information in freeze frame buffer.