Evaporative Emissions System: Description and Operation

DESCRIPTION
Fuel evaporation is a normal process in any fuel system. The purpose of the evaporative emission system ([1][2]EVAP) is to limit the amount of fuel vapors (hydrocarbons) escaping into the atmosphere. To accomplish this, the EVAP system transfers the fuel vapors that accumulate in the fuel tank into an activated carbon storage container - the EVAP canister. The EVAP canister will store these vapors until the powertrain control module (PCM) determines that the engine is able to burn this stored fuel vapor. When the PCM determines that the engine is able to use this stored fuel vapor, it is pulled, or purged, along with fresh air, into the intake air flow by opening the purge solenoid. It is then consumed in the normal combustion process.

OPERATION
The powertrain control module (PCM) monitors the ability of the system to maintain vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks and a Pass is reported to the diagnostic executive. If there is a leak, the system either will not achieve a specific vacuum or a vacuum cannot be maintained. A fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests.

The canister purge solenoid allows [1][2]intake manifold pressure (vacuum) to purge the stored vapors in the charcoal canister. The PCM supplies a ground to energize (open) the normally closed purged solenoid which allows vacuum to purge the canister. The ground is pulse width modulated (PWM), pulsed on and off several times per second, to control the flow of purge vapors from the canister. The PCM calculates a PWM duty cycle for canister purge flow control using the following operating parameters: engine load, throttle position, coolant temperature and ambient temperature.

COMPONENTS
The components that make up the enhanced evaporative emission system consist of the following:
1. Canister Vent Solenoid, located on the upper end of the fuel filler pipe, just behind the left rear wheelhouse liner.
2. Fuel Tank.
3. Evaporative Emission System Canister, nested on the top of the fuel tank at the left, rear corner.
4. Fuel Tank Pressure Sensor, inserted on the top of the right, rear portion of the fuel tank.
5. System fuel lines and hoses.
6. Evaporative System Service Diagnostic Port, located in the canister purge line under driver's side floorpan behind the left, front wheel.
7. Canister Purge Solenoid, mounted below the [1][2]intake manifold on the side of the cylinder block and toward the rear of the block.

THEORY OF OPERATION
Evaporative systems are not new to the automotive industry. The first systems were introduced in the industry in the 1970's as a part of the overall scheme to reduce vehicle hydrocarbon emissions. They have not changed appreciably until government mandates as part of OBD II diagnostics and tightening emission regulations.

Enhanced EVAP systems are designed to reduce emissions from the fuel system:
^ While the vehicle is operating or parked by continually monitoring the system for leaks and to activate the Service Engine Soon lamp if a problem is detected.

The PCM monitors the control circuits to the vent and purge solenoids and the tank pressure circuitry for possible faults. These faults are as follows:
^ DTC P0443 - Canister Purge Solenoid Circuit Fault
^ DTC P0449 - Canister Vent Solenoid Circuit Fault
^ DTC P0452 - Tank Pressure Sensor Circuit Fault - Low Voltage
^ DTC P0453 - Tank Pressure Sensor Circuit Fault - High Voltage

Refer to the appropriate DTC charts for further explanation.
Testing and Inspection

As required by law, the diagnostic routine must be able to detect a leak as small as 0.040 inches in diameter. The integrity of the entire vapor handling portion of the fuel system, including the purge solenoid, purge lines, fuel tank, EVAP canister, vent line, vent solenoid, fuel filler pipe and fuel cap is continually tested.