Evaporative Emissions System: Description and Operation

EVAPORATIVE EMISSIONS (EVAP) SYSTEM COMPONENTS









EVAP System Description

The evaporative emission (EVAP) system:

- is equipped with an on-board refueling vapor recovery (ORVR) system
- prevents hydrocarbon emissions from reaching the atmosphere
- stores fuel vapors in the EVAP canister that are generated during vehicle operation or hot soak, or vehicle refueling, until they can be consumed by the engine
- 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 purge flow. The PCM regulates the purge flow, induced by the application of intake manifold vacuum, by varying the duty cycle applied to toe EVAP canister purge valve
- has an evaporative emission test port for test purposes

EVAP System Operation

The fuel vapors are routed:

- from the fuel tank through the fill limiting vent valve and 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










PCM

- Calculates a variable duty cycle based on a monitored FTP sensor signal input and the desired amount of purge vapor flow to the intake manifold for a given engine condition. It can then output the duty cycle to the solenoid on the EVAP canister purge valve
- Uses the EVAP Running Loss System inputs to evacuate the system using the EVAP canister purge valve, seals the EVAP Running Loss System from atmosphere using the CV solenoid and observes total vacuum bleed-up lost for a period of time using the FTP sensor

Canister Vent Solenoid

- Mounted on the EVAP canister
- Receives a variable duty cycle signal (between 0 and 100%) from the PCM
- Seals the EVAP canister from atmospheric pressure during the EVAP Running Loss System test monitor. This allows the EVAP canister purge valve to obtain the target vacuum in the fuel tank during the monitor run
- Is repaired as a separate item

EVAP Canister Purge Valve

- Normally closed valve
- Controls the flow of vapors (purging) from the EVAP canister to the intake manifold during various engine operating modes

Fuel Tank Pressure Sensor

- Is located in the fuel vapor valve tube and is serviced as part of the fuel vapor valve tube assembly
- Monitors the fuel tank pressure during engine operation and continuously transmits an input signal to the PCM
- Monitors the fuel tank pressure or vacuum bleed-up during the EVAP monitor testing
- Controls excessive fuel tank pressure by forcing the system to purge

Fuel Vapor Control Valve/Vent Valve Assembly

- Control the flow of fuel vapor entering the engine
- Prevent fuel tank overfilling during refueling operation
- Prevent liquid from entering the EVAP canister and the EVAP canister purge valve under any vehicle altitude, handling or roll over condition






Evaporative Emission (EVAP) Canister

- Stores fuel vapors from the gas tank
- When engine is running above idle, fuel vapors are purged from the EVAP canister into the engine for combustion

Fuel Pump and Tank Assembly

- Consists of the fill limiting vent valve and fuel vapor vent valve
- Prevents suspended liquid fuel from being drawn into the evaporative emission canister along with the fuel vapors