Evaporative Emissions System: Description and Operation: Overview

Overview
The Evaporative Emission (EVAP) system prevents fuel vapor build-up in the sealed fuel tank. Fuel vapors trapped in the sealed tank are vented through the vapor valve assembly on top of the tank. The vapors leave the valve assembly through a single vapor line and continue to the EVAP canister (located in the engine compartment, in the rear of vehicle near luggage compartment area or along the frame rail) for storage until the vapors are purged to the engine for burning.

There are two types of Evaporative Emission (EVAP) systems:
^ The Enhanced Evaporative Emission (EVAP) system.
^ The On-Board Refueling Vapor Recovery (ORVR) Evaporative Emission (EVAP) system.

NOTE: Refer to the Evaporative Emission System components for information for: fuel vapor control valve, fuel vapor vent valve assembly and fuel vapor separator assembly, liquid/vapor fuel discriminator, fuel filler pipe check valve, fill limit valve assembly, fill limit vent valve assembly, fuel filler pipe flapper valve, fuel vapor control valve (fuel tank mounted), ORVR T-connector assembly and EVAP canister.

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, using various sensor inputs, calculates 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.
^ has an evaporative emission test port for test purposes.

The fuel vapors are routed:
^ from the fuel tank through the fuel vapor vent valve.
^ 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 Evaporative Emissions (EVAP) canister:
^ is located under the rear of the vehicle above the rear axle.
^ contains activated carbon.
^ stores fuel vapors.

The fuel tank filler cap:
^ relieves system pressure above 14 kPa (56.21 inches H2O).
^ relieves system vacuum below 3.8 kPa (15.26 inches H2O).

The canister vent solenoid:
^ is normally open.
^ seals the EVAP system for the inspection and maintenance (I/M 240) test and OBD II leak and pressure tests.
^ is mounted to the EVAP canister.

The Evaporative Emission (EVAP) canister purge valve:
^ is normally closed.
^ regulates the purging of the EVAP canister.
^ is controlled by the PCM.

The Evaporative Emission (EVAP) system test port:
^ is attached to the EVAP canister purge valve.
^ is used to connect the Evaporative Emissions System Leak Tester to the EVAP system.

The Evaporative Emission (EVAP) system monitor:
^ is a self-test strategy within the PCM which tests the integrity of the EVAP system.
^ monitors the EVAP system for leaks.
^ monitors electronic EVAP components for irrationally high or low voltages.
^ monitors for correct EVAP system operation.
^ uses negative and positive leak test methods to test and activate the EVAP system.

The EVAP Running Loss System leak test:
^ utilizes intake manifold vacuum to test the EVAP system and involves several stages.