Evaporative Emissions System: Description and Operation

EVAPORATIVE EMISSIONS

Evaporative Emission System Components - Midship Fuel Tank:

Evaporative Emission System - Aft-Of-Axle Fuel Tank:

The evaporative emission (EVAP) system:
- prevents hydrocarbon emissions from reaching the atmosphere.
- stores fuel vapors in the EVAP canister that are generated during vehicle operation or hot soak, until they can be consumed by the engine during normal engine operation.
- routes the stored fuel vapors to the engine during engine operation.
- is controlled by the powertrain control module (PCM) which uses various sensor inputs to calculate the desired amount of EVAP purge flow. The PCM regulates the purge flow, induced by the application of intake manifold vacuum, by varying the duty cycle applied to the canister purge valve.

The fuel vapors are routed:
- from the fuel tank through the fuel vapor vent valves.
- to the EVAP canister through a vapor line.
- to the engine when the EVAP canister purge valve is opened by the PCM.

The fuel tank pressure (FTP) sensor:
- monitors the pressure levels in the fuel tank.
- communicates the pressure reading to the PCM during the OBD II leak test.

The EVAP canister:
- is located along the LH frame rail for vehicles with rear fuel tanks or on the rear crossmember for vehicles with a midship fuel tank.
- contains activated carbon.
- stores fuel vapors.

The fuel tank filler cap:
- relieves system vacuum below 3.5 kPa (15.09 inches H2O).

The canister vent solenoid:
- is normally open.
- seals the EVAP system for the OBD II leak and pressure tests.
- is mounted to the EVAP canister.
- is repaired as a separate item.

The evaporative emission canister purge valve:
- is normally closed.
- regulates the purging of the EVAP canister.
- is controlled by the PCM.