Evaporative Emissions System: Description and Operation

EVAP Control System:

EVAP 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 five major components:

- EVAP charcoal canister.
- EVAP solenoid.
- Throttle body.
- Fuel tank.
- Air separator.

EVAP charcoal canister
The cylindrical designed canister is filled with activated carbon that stores vapor transferred from the fuel tank. Integral to the canister is a trap which collects any liquid fuel (condensed vapor) from entering and contaminating the charcoal bed. The liquid is purged back to the tank whenever there is a vacuum exerted on the tank. The canister also has a volume compensator that maintains a tightly packed charcoal bed as the canister expands and contacts.

The two ports coming off the canister are identified as:

a. Purge.
b. Tank.

EVAP solenoid
The PCM (Powertrain Control Module) controlled, engine mounted solenoid regulates 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 (two 0.06" holes) 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. The fuel tank is made of High Density Polyethylene (HDPE) which is a high tech plastic material.

Air separator
The air separator is a labyrinth device which provides to the canister a clean and dry source of air.

During vehicle operation, vapor generated inside the fuel tank is transferred to and stored by the charcoal canister. When certain engine parameters are met the PCM commands the solenoid to energize and the stored vapors are consumed by the engine during the combustion process. Under cold or "Open Loop" conditions the PCM de-energizes the solenoid and canister purge is disabled.

The PCM shall enable canister purge when all of the conditions below are met:

- Maximum of 150 seconds after "Closed Loop" is enabled (time varies depending on coolant temperature at key "ON").
- Coolant temperature is above 80°C.
- When the PCM is not enabling injector shutoff for traction control and acceleration reduction.
- Throttle switch is open.
- Vehicle speed is above 20 mph
- Engine speed is greater than 1100 RPM.
- Engine metal over temp/low coolant condition is not present.

Should the EVAP solenoid fail closed, electrically or mechanically, the canister charcoal bed could become saturated allowing vapors to escape to the atmosphere. The vapors should not enter the passenger compartment because of the rear mounted canister.

RESULT OF INCORRECT OPERATION
Poor idle, stalling and poor driveability can be caused by:

- An improperly connected or faulty EVAP canister purge solenoid valve. This would usually be accompanied by a start that is not as fast as normal (once started, the engine does not build up speed as fast as it normally would) and puff of black smoke at the tailpipe.
- Damaged canister.
- Hoses split, cracked and/or not connected to the proper tubes.

Evidence of fuel loss or fuel vapor odor can be caused by:

- Liquid fuel leaking from front fuel lines.
- Cracked or damaged canister.
- Disconnected, misrouted, kinked, deteriorated or damaged hoses.
- Air cleaner or air cleaner gasket improperly seated.