GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 1998 🢒 Seville 🢒 Engine 🢒 Engine Controls - 4.0L and 4.6L 🢒 Evaporative Emission Control System Operation Description

Enhanced EVAP Purpose

The main purpose of the Enhance EVAP system is to prevent fuel vapors from dispersing into the atmosphere. The Enhanced Evaporative Emission (EVAP) control system stores vapor generated by the vehicle and regulates its consumption during normal driving operation. The basic system used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the fuel vapors when the engine is not operating. When the engine is running, the fuel vapor is purged from the canister by intake airflow and consumed during the normal combustion process.

Evaporative Emissions System Purge/Pressure Diagnostic Station J 41413
Object Number: 20803 Size: SH
(1)Main Valve
(2)Evaporative Emission System Purge/Presure Diagnostic Station J 41413
(3)Nitrogen Cylinder
(4)Black Connecting Hose
(5)Threaded Fitting
(6)Gauge Set

The EVAP system purge/pressure diagnostic station is used to clear carbon released into the EVAP system by the canister. Carbon released into the EVAP system may cause EVAP system components to be inoperative. If diagnosis indicates that an EVAP system component needs to be replaced, the system should be checked for released carbon. A released carbon condition must be corrected before replacing any EVAP system component. Procedures for checking the EVAP system for released carbon and for correcting a released carbon condition are in the Diagnostic Aids for the applicable DTC tables.

Enhanced EVAP Operation

The EVAP purge valve allows manifold vacuum to purge the canister. The vapors should not enter the passenger compartment because of the rear mounted canister. The Powertrain Control Module (PCM) supplies a ground to energize the EVAP purge valve (purge ON ). The EVAP purge valve control is Pulse Width Modulated (PWM) or turned ON and OFF several times a second. The PCM controlled PWM output is commanded when the appropriate conditions have been met:

   • 

Engine coolant temperature above 25°C (77°F).

   • 

After the engine has been running about 2 1/2 minutes on a cold start or 30 seconds on a warm start.

   • 

The vehicle is operating in closed loop fuel control.

Canister purge PWM duty cycle varies according to operating conditions determined by mass air flow, fuel trim, and intake air temperature. Canister purge will be disabled if TP angle increases to above 70 percent. Canister purge will be re-enabled when TP angle decreases below 66 percent.

The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring vacuum decay. The fuel level sensor input to the PCM is used to determine if the fuel level in the tank is correct to run the EVAP diagnostic tests. To ensure sufficient volume in the tank to begin the various diagnostic tests, the fuel level must be between 15 percent and 85 percent. The PCM monitors system vacuum level via the fuel tank pressure sensor input.

EVAP System Components

The EVAP system includes the following components:

    • Fuel Tank
    • Fuel Level Sensor
    • Vent Solenoid (Fresh Air Solenoid)
    • Fuel Tank Pressure Sensor
    • Fuel pipes and hoses
    • Vapor lines
    • Fuel Cap
    • Evaporative Emission Canister
    • Purge lines
    • Purge Valve Solenoid
    • Enhanced Evaporative Emission (EVAP) Service Port
    • Throttle Body
    • Powertrain Control Module (PCM)
    • 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.
    • Fuel Level Sensor
        The fuel level sensor is an input to the PCM for the enhanced evaporative system diagnostic. The fuel level affects the rate of charge in the air pressure in the EVAP system. The fuel level sensor input to the PCM is used to determine if the fuel level in the tank is correct to run the EVAP diagnostic tests. To ensure sufficient volume in the tank to begin the various diagnostic tests, the fuel level must be between 15 percent and 85 percent.
        
    • Vent Solenoid (Fresh Air Solenoid)
        The Vent Solenoid (Fresh Air Solenoid) is the device that allows fresh air into the purge canister. The vent solenoid is an on/off device which is normally open to allow fresh air into the purge system. For diagnostic testing, the vent solenoid is closed and vacuum decay tests are run on the system. Consequently, the vent solenoid serves a dual purpose of supplying fresh air and acting as a diagnostic switch.
    • Fuel Tank Pressure Sensor
        The fuel tank pressure sensor is used to detect small leaks in the evaporative emission system. The diagnostic is based on a vacuum decay stratedy. At an appropriate time engine vacuum is used to draw a small vacuum on the entire evaporative emission system by mechanically sealing off the designed vent path.
    • Fuel Pipes and Hoses
        The fuel pipes and hoses carry the fuel to the various EVAP system components.
    • Vapor Lines
        The fuel vapor lines carry the fuel vapors to the various EVAP system components

       Notice: Use a fuel tank filler pipe cap with the same features as the original when a replacement is necessary. Failure to use the correct fuel tank filler pipe cap can result in a serious malfunction of the fuel system.

    • Fuel Cap
        The fuel cap is a screw-type cap that screws on to the fuel tank filler pipe. A loose or missing fuel cap will set a DTC.
    •  EVAP Emission Canister
        The 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 EVAP charcoal canister also has a volume compensator that maintains a tightly packed charcoal bed as the canister expands and contracts.
    •  EVAP Canister Purge Solenoid
        The EVAP canister purge solenoid allows manifold vacuum to purge the canister. The EVAP canister purge solenoid is normally closed. The Powertrain Control Module (PCM) supplies a ground to energize the EVAP canister purge solenoid valve (purge on). The EVAP canister purge solenoid control is Pulse Width Modulated (PWM) or turned on and off several times a second. The PCM controlled PWM output is commaned when the appropriate conditions have been met:
    • Engine coolant temperature above 25 degrees C (77 degrees F)
    • After the engine has been running about 2 ½ minutes on a cold start or 30 seconds on a warm start.
    • The vehicle is operating in closed loop fuel control.
    •  Canister purge PWM duty cycle varies according to operating conditions determined by mass air flow, fuel trim, and intake air temperature. Canister purge will be disabled if TP angle increases to above 70 percent. Canister purge will be re-enabled when TP angle decreases below 66 percent.
         The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring vacuum decay.
         The fuel level sensor input to the PCM is used to determine if the fuel level in the tank is correct to run the EVAP diagnostic tests. To ensure sufficient volume in the tank to begin the various diagnostic tests, the fuel level must be between 15% and 85%.
         The PCM monitors system vacuum level via the fuel tank pressure sensor input.
    • Purge Lines
        The purge lines carry the fuel vapors to the various EVAP system components.
    • Enhanced Evaporative Emission (EVAP) Service Port
        The Enhanced Evaporative Emission (EVAP) Service Port is located between the purge valve solenoid and the EVAP emission canister. The service port is identified by a green color cap. The port contains a schrader valve and fittings in order to allow the connection of the service tool kit Enhanced EVAP Pressure Purge Diagnostic Car J 41413.
    • Throttle Body
        The throttle body provides a intake manifold vacuum signal to purge the canister of its stored vapor. The purge ports 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.
    • Powertrain Control Module (PCM)
         The PCM commands the purge valve solenoid to open and allow engine vacuum to draw stored fuel vapors from the canister into the throttle body.

Object Number: 104758 Size: SH
(1)Vent Pipe
(2)Tank Vapor Pipe
(3)Purge Pipe

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 over temperature 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).