Evaporative Emissions System: Description and Operation

Evaporative Emissions

The Evaporative Emission (EVAP) system consists of the following:

- EVAP canister assembly
- EVAP canister bracket
- EVAP canister purge valve
- EVAP canister vent solenoid
- Fresh air hose (pickup vehicles only)
- Fuel Tank Pressure (FTP) sensor
- Vapor tubes

The EVAP system:

- prevents hydrocarbon emissions from reaching the atmosphere.
- stores, in the EVAP canister, fuel vapors that are generated during vehicle operation and fuel filling (On-Board Refueling Vapor Recovery (ORVR) only) 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 PCM which uses various sensor inputs to calculate the desired amount of 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(s) through a vapor tube.
- to the engine when the EVAP canister purge valve is opened by the PCM.

The FTP sensor:

- monitors the vapor pressure levels in the fuel tank.
- communicates the pressure reading to the PCM during the OBD II leak test.
- is part of the vapor tube assembly on pickup vehicles (located above fuel tank).
- is located on the fuel pump module flange on chassis cab vehicles and is serviced with the fuel pump module only, it is not serviced separately. For additional information, refer to Fuel Tank and Lines - Gasoline and Diesel Service and Repair.

The EVAP canister assembly:

- is mounted above the spare tire on pickup vehicles and is connected to a fresh air hose routed into the box sill.
- is mounted mid frame on chassis cab vehicles equipped with an aft-of-axle fuel tank.
- is mounted on the No. 6 frame crossmember on chassis cab vehicles equipped with a midship tank.
- contains activated carbon.
- stores fuel vapors.

The fuel tank filler cap:

- relieves system vacuum below negative 4.7 kPa (19 in H2O).

The canister vent solenoid:

- is located on the EVAP canister assembly.
- is normally open.
- seals the EVAP system for the OBD II leak and pressure tests.
- is repaired as a separate item.

The EVAP canister purge valve:

- is located in the engine compartment.
- is normally closed.
- regulates the purging of the EVAP canister.
- is controlled by the PCM.

The EVAP system monitor:

- is a self-test strategy within the PCM which tests the integrity of the EVAP system. For additional information on the operation of the system monitor, refer to Computers and Control Systems.

Vehicle Emission Vacuum Routing

Pickup









Chassis Cab with Midship Tank









Chassis Cab with Aft Tank