Evaporative Emissions System: Description and Operation

Canister:

PURPOSE
The Evaporative Emission (EVAP) Control System stores vapor generated by the vehicle and regulates its consumption during normal driving operation. The main purpose of the EVAP system is to prevent fuel vapor from dispelling into the atmosphere.

OPERATION
The EVAP System has four major components: The Charcoal Canister with Solenoid, Throttle Body, Fuel Tank, and Remote Vent Hose.

Charcoal Canister (with solenoid)
The 1.5 liter cylindrical inverted function canister is filled with activated carbon that stores vapor transferred from the fuel tank. A single tube extends down the center of the canister to a liquid trap at the canister base. Fuel vapor/condensed liquid from the tank enters a port on the canister cover. The vapor/liquid travels down a center tube where the vapor is absorbed by the charcoal and the liquid is stored in the lower well. Both are contained until purged through the normal combustion process. The three ports coming off the canister are identified as:

a. Fuel vapor Port (from fuel tank)
b. EVAP Port (purge line to throttle body)
c. Atmospheric Port (from fresh air source via the body mounted vent hose).

The EVAP solenoid which is integrally mounted, is controlled by the Powertrain Control Module (PCM) to regulate vapor flow from the canister to the throttle body. The solenoid is normally closed and when certain engine parameters are met, the solenoid opens allowing engine vacuum to purge the canister.

Throttle Body
The throttle body provides a ported vacuum signal to purge the canister of its stored vapor. At idle or close throttle conditions the purge ports are not exposed to the manifold. As the blade uncovers, the port vacuum is available to purge the canister. The EVAP routine is controlled by the PCM via the electronic solenoid.

Fuel Tank
Fuel vapor generated inside the fuel tank is released to the canister for containment.

Remote Vent Hose
In the event of the EVAP solenoid malfunctioning, a remote vent hose, which is connected to the atmospheric port and routed to the engine coolant fan left, vents vapor down preventing any vapor from entering the passenger compartment via the car's ventilation system.

When a purge mode is commanded, stored fuel vapor is drawn into the single dip tube at the base of the canister. Any liquid trapped in the well is also drawn into the tube. Fresh air is drawn into the canister through the remote vent hose.

The PCM operates a solenoid valve which purges the charcoal canister with ported vacuum at the throttle body. Under cold engine, "OPEN LOOP" or idle conditions, the solenoid is de-energized by the PCM, which does not allow vacuum to the canister through the normally closed solenoid.

The canister will be enabled (PCM energizes the solenoid) when:

^ Coolant is above 80°C (176°F).
^ Closed Loop has been achieved for at least 30 seconds.
^ Throttle Position switch open.
^ Vehicle speed greater than 10 mph.
^ Engine speed RPM above a threshold.
OR
^ Code P013, P017, P042, P043, P044, or P045 is present.

When the solenoid is closed (is not receiving voltage, or has a stuck plunger) the canister will not purge to the intake manifold. This will prevent the canister from purging vapors and could result in a fuel odor complaint in the engine compartment due to a saturated canister.