Evaporative Emissions: Notes

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

• EVAP canister assembly (located in front of the fuel tank)
• EVAP canister purge valve and hose assembly (located on the back side of the intake manifold)
• EVAP canister vent solenoid and dust separator assembly (attached to the EVAP canister)
• EVAP canister vent solenoid and dust separator assembly quick connect coupling fitting (attached to the EVAP canister vent solenoid and dust separator assembly)
• Fuel Tank Pressure (FTP) sensor and vapor tube assembly (located on top of the fuel tank)
• Vapor tubes

The 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 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 EVAP canister purge valve.

The fuel vapors are routed:

• from the fuel tank through the fuel vapor/grade 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 FTP sensor:

• measures the pressure levels in the fuel tank continuously.
• serves as input to the PCM.

The EVAP canister:

• contains activated carbon.
• stores fuel vapors.

The Easy Fuel™ capless fuel tank filler assembly:

• relieves system vacuum below 3.8 kPa (15.26 in H₂ O).

The canister vent solenoid:

• is controlled by the PCM.
• is normally open.
• seals the EVAP system for the OBD II leak and pressure tests.
• is mounted to the EVAP canister.
• is repaired through removal/replacement of the canister vent solenoid and dust separator assembly. The canister vent solenoid must not be serviced separately from the canister vent solenoid and dust separator assembly.

The EVAP canister purge valve:

• is normally closed.
• regulates the purging of the EVAP canister.
• is controlled by the PCM.

The Engine On EVAP leak-check monitor:

• uses intake manifold vacuum to reach a target vacuum on the EVAP system. The FTP sensor is used by the Engine On EVAP leak-check monitor to determine if the target vacuum necessary to carry out the leak-check on the EVAP system has been reached. Once target vacuum on the EVAP system is achieved, the change in EVAP system vacuum over a calibrated period of time determines if a leak exists.

The Engine Off Natural Vacuum (EONV) EVAP leak-check monitor is executed:

• once the Engine On EVAP leak-check monitor is completed and the key is turned off. The EONV EVAP leak-check monitor determines if a leak is present when the naturally occurring change in the fuel tank pressure or vacuum does not exceed a calibrated limit during a calibrated amount of time. A separate, low-power consuming microprocessor in the PCM manages the EONV leak-check.