System Diagnosis - EVAP Control

CIRCUIT DESCRIPTION
The evaporative system includes the following components:
- The fuel tank.
- The [1][2]EVAP vent solenoid.
- The fuel tank pressure sensor.
- The fuel pipes and hoses.
- The vapor lines.
- The fuel cap.
- The evaporative emission canister.
- The purge lines.
- The EVAP purge solenoid.

The EVAP purge solenoid valve allows manifold vacuum to purge the canister. The Powertrain Control Module (PCM) supplies a ground to energize the solenoid valve (purge ON). The EVAP purge solenoid control is Pulse Width Modulated (PWM) or turned ON and OFF several times a second. The duty cycle (pulse width) is determined by engine operating conditions including load, throttle position, coolant temperature and ambient temperature. The duty cycle is calculated by the PCM and the output is commanded when the appropriate conditions have been met.

The [1][2]EVAP Vent solenoid is used to seal the EVAP system during the EVAP leak diagnostic tests. The fuel tank pressure sensor is used to monitor vacuum in the system during the diagnostic tests.

A stuck closed EVAP purge solenoid, stuck open [1][2]EVAP vent solenoid, disconnected or damaged EVAP purge line or vent hose, leaking fuel cap, or fill neck should set a DTC P0440 EVAP System. A small leak in the EVAP system should set a DTC P0442 EVAP Control System Small Leak Detected. An EVAP vent solenoid stuck closed, a restricted vent hose, or a blocked EVAP canister should set a DTC P0446 EVAP Canister Vent Blocked. A continuous purge condition with no purge commanded by the PCM (EVAP purge solenoid stuck open or leaking) should set a DTC P1441 EVAP System Flow During Non-Purge. Refer to the DTC tables for further diagnostic procedures regarding the EVAP system.

DIAGNOSTIC AIDS
Check for the following conditions:
- Poor connection at PCM.
- Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal-to-wire connection.
- Damaged harness.
- Inspect the wiring harness for damage. If the harness appears to be OK, observe the Fuel Tank Pressure display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the malfunction.
- Incorrect [1][2]EVAP purge or vacuum source line routing.
- Verify that the source vacuum and [1][2]EVAP purge lines to the canister purge valve are not switched.

IMPORTANT: Although these diagnostics are considered type A, they act like type B diagnostics under certain conditions. Whenever the [1][2]EVAP diagnostics report that the system has passed, or if the battery has been disconnected, the diagnostic must fail during two consecutive cold start trips before setting a DTC. The initial failure is not reported to the diagnostic executive or displayed on a scan tool. A passing system always reports to the diagnostic executive immediately.

TEST DESCRIPTION
Number(s) below refer to the step number(s) on the Diagnostic Table:
5. The PCM will command the [1][2]EVAP purge and vent solenoids closed with the scan tool Seal System EVAP output control function activated. Fuel tank pressure should not decrease under these conditions.
6. The PCM commands the EVAP purge solenoid off (open) and the vent solenoid off (closed) with the scan tool System Perf. [1][2]EVAP output control function activated. Engine vacuum should cause fuel tank pressure to decrease when System Perf. is activated.