The evaporative emissions (EVAP) control system used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon, charcoal, storage device, canister, to hold the vapors when the vehicle is not operating. When the engine is operating, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.
The evaporative system includes the following components:
The EVAP purge solenoid valve is mounted on the top of the intake manifold. The EVAP purge solenoid valve isolator (2) and mounting fastener (3) must never be substituted. The EVAP purge solenoid valve allows manifold vacuum to purge the canister through the vacuum port (4). The powertrain control module (PCM) supplies a ground to energize the EVAP purge solenoid valve, purge ON. The EVAP purge solenoid control is pulse width modulated (PWM) or turned ON and OFF several times a second.
The EVAP canister purge pulse width modulation (PWM) duty cycle varies according to the operating conditions determined by the mass air flow, the fuel trim, the engine coolant temperature, and the intake air temperature . For certain EVAP tests, the diagnostic is disabled if the TP angle increases to above 75 percent.
The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring for vacuum decay.
The fuel level sensor input to the PCM is used to determine if the fuel level in the tank is within a range that allows the EVAP diagnostic tests to run. To ensure sufficient volume in the tank to begin the various diagnostic tests, the fuel level must be between 15 percent and 85 percent.
The PCM monitors the fuel tank pressure vacuum level via the fuel tank pressure sensor input.
The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring vacuum decay.
The PCM monitors vacuum level via the fuel tank pressure sensor input. At an appropriate time, the EVAP purge solenoid and the EVAP vent solenoid (2) are turned ON, allowing engine vacuum to draw a small vacuum on the entire evaporative emission system. After the desired vacuum level has been achieved, the EVAP purge solenoid is turned OFF, sealing the system. A leak is detected by monitoring for a decrease in vacuum level over a given time period, all other variables remaining constant. A small leak in the system causes DTC P0442 to be set.
If the desired vacuum level cannot be achieved in the test described above, a large leak or a faulty EVAP purge solenoid is indicated. This can be caused by the following conditions:
Any of the above conditions sets DTC P0440.
A restricted or blocked EVAP canister vent path is detected by drawing vacuum into the EVAP system. The PCM turns OFF the EVAP vent solenoid and the EVAP purge solenoid, EVAP vent solenoid Open, EVAP purge PWM 0 percent. The PCM monitors the fuel tank pressure sensor input. With the EVAP vent solenoid open, any vacuum in the system should decrease quickly unless the vent is blocked. A blockage is caused by the following conditions:
If any of the above conditions are present, DTC P0446 sets.
The PCM checks for conditions that cause the EVAP system to purge continuously by commanding the EVAP vent solenoid ON and the EVAP purge solenoid OFF, EVAP vent solenoid CLOSED, EVAP purge PWM 0 percent. If fuel tank pressure level increases during the test, a continuous purge flow condition is indicated. This can be caused by the following conditions:
If any of the above conditions are present, DTC P1441 sets.
Refer to the appropriate DTCs for further diagnostic procedures regarding the EVAP system.
