Evaporative Emission Control System. Cadillac Concours 1996

For 1990-2009 cars

### EVAP 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 dispersing into the atmosphere.

### EVAP Operation

(1) | EVAP Canister Purge Solenoid
(2) | Bolt
(3) | Throttle Body
(4) | Vapor Line to Throttle Body
(5) | EVAP Canister Purge Solenoid
(6) | Vapor Line to Canister
(7) | RT Cam Cover (Rear)
(8) | Throttle Body

The Evaporative Emission Control (EVAP) System helps to reduce evaporated hydrocarbons (fuel) that enter the atmosphere. The most important component of this system is the EVAP charcoal canister. The EVAP canister draws the evaporated fuel from the fuel tank and stores it. When several conditions are met, the PCM commands the EVAP solenoid to open and allow engine vacuum to draw stored fuel from the EVAP canister into the throttle body. This is called purging the canister. Once the evaporated fuel enters the throttle body, it is burned in the combustion process.

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.

### Major EVAP System Components

• | EVAP Charcoal Canister

| The 2.0 litre rectangular 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 contracts.

• | EVAP Canister Purge Solenoid

• | Throttle Body

| The throttle body provides a intake manifold vacuum signal to purge the canister of its stored vapor. The purge ports (two 0.06 in. holes) are always exposed to the manifold side of the throttle blades, making intake vacuum available to purge the canister. EVAP canister purge is controlled by the PCM via the purge solenoid.

• | Fuel Tank

| The 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.

• | EVAP Vacuum Switch

| The purge vacuum switch is a normally open switch which closes when vacuum is applied. It is an input to the PCM to signal that the EVAP solenoid has opened, allowing manifold vacuum to draw fuel vapors from the charcoal canister into the intake manifold.

#### EVAP Canister

(1) | Purge Tube to Solenoid
(2) | Buffer Tube
(3) | Charcoal Bed
(4) | EVAP Charcoal Canister
(5) | Volume Compensator
(6) | Fresh Air Inlet from Air Separator
(7) | Liquid Fuel Trap
(8) | Vapor Tube from Fuel Tank

### EVAP Control

The PCM controls the EVAP solenoid using a pulse width modulated (PWM) signal. By using this type of control, the PCM can vary the rate (duty cycle) at which it opens or closes the solenoid. As mentioned, a strict set of conditions must be met before the PCM will enable canister purge. This optimizes driveability and evaporative emission reduction.

Once the PCM decides to enable the EVAP solenoid, it monitors rear oxygen sensor data to determine if the air/fuel mixture is correct. If the PCM determines the air/fuel mixture is incorrect, it will adjust the EVAP solenoid duty cycle to provide the correct amount of fuel vapor from the EVAP canister. This process is called duty cycle learning.

### Enable/Disable Conditions

All of the following conditions must be met before the PCM will enable the EVAP solenoid. Once enabled, failure to meet any of these conditions will cause the PCM to disable the EVAP solenoid.

1. PCM fuel control must be in closed loop operation and commanded Air/Fuel ratio must be 14.7 to 1 for a certain period of time before the EVAP solenoid will be enabled. This delay will ensure that the engine is sufficiently warmed up and ready. The delay period is based on engine coolant temperature (ECT) at engine start-up.
2. Fuel is not being disabled to any cylinder due to traction control system operation or acceleration reduction fuel control.
3. The PCM determines that no engine overtemperature condition exists. The EVAP solenoid will not be enabled if an overtemperature condition exists in order to help reduce engine temperature.
4. Vehicle speed is present and MAF sensor input is at least 16.0 grams per second or Engine running for at least 17 minutes.

### Duty Cycle Learning

When all of the conditions to enable the EVAP solenoid have been met, the PCM will adjust the duty cycle if closed loop fuel control is operating free of limitations (short and long term fuel trim not at maximum or minimum limits).