GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1998 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 5.7L 🢒 Evaporative Emission Control System Operation Description

Object Number: 12722 Size: LH
(1)Purge Line
(2)Vapor From Fuel Tank
(3)Air

The Evaporative Emission (EVAP) Control 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 vapors when the vehicle is not operating. When the engine is operating, the fuel vapor is purged from the carbon element by intake air flow and consumed in the normal combustion process.

EVAP Canister

EVAP Canister
Object Number: 11795 Size: LH
(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 System

The evaporative system includes the following components:

    • Fuel tank
    • Evaporative emission canister vent solenoid
    • Fuel tank pressure sensor
    • Fuel pipes and hoses
    • Vapor lines
    • Fuel cap
    • Evaporative emission canister
    • Purge lines
    • Purge valve solenoid

EVAP System Operation
Object Number: 218763 Size: SH

The EVAP purge solenoid valve allows manifold vacuum to purge the canister. The Powertrain Control Module (PCM) supplies a ground to energize the EVAP purge solenoid valve (purge on). The EVAP purge solenoid control is Pulse Width Modulated (PWM) or turned on and off several times a second.


Object Number: 18839 Size: SH

The EVAP canister purge PWM duty cycle varies according to the operating conditions determined by the mass air flow, the fuel trim, the engine coolant temperature, and the intake air temperature . For certain EVAP tests, the diagnostic will be disabled if the TP angle increases to above 75%.

The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring for 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%.

Results of Incorrect Operation
    •  Poor idle, stalling and poor driveability can be caused by the following:
       -  Malfunctioning purge solenoid.
       -  Damaged canister.
       -  Hoses/lines split, cracked and/or not connected properly.
    • Evidence of fuel loss or fuel vapor odor can be caused by the following:
       - Liquid fuel leaking from the fuel lines
       - Cracked or damaged canister
       - Inoperative canister control valve
       - Vacuum hoses that are:
   - Disconnected
   - Mis-routed
   - Kinked
   - Deteriorated
   - Damaged

Object Number: 228634 Size: SH

The evaporative leak detection diagnostic strategy is based on applying vacuum to the EVAP system and monitoring vacuum decay.

The PCM monitors vacuum level via the fuel tank pressure sensor input. At an appropriate time, the EVAP purge solenoid and the EVAP vent solenoid are turned on, allowing engine vacuum to draw a small vacuum on the entire evaporative emission system. After the desired vacuum level has been achieved, the EVAP purge solenoid is turned off, sealing the system. A leak is detected by monitoring for a decrease in vacuum level over a given time period, all other variables remaining constant. A small leak in the system causes DTC P0442 to be set.

If the desired vacuum level cannot be achieved in the test described above, a large leak or a faulty EVAP purge solenoid is indicated. This can be caused by the following conditions:

    •  Disconnected or faulty fuel tank pressure sensor.
    •  Missing or faulty fuel cap.
    •  Disconnected, damaged, pinched, or blocked EVAP purge line.
    •  Disconnected or damaged EVAP vent hose.
    •  Disconnected, damaged, pinched, or blocked fuel tank vapor line.
    •  Disconnected or faulty EVAP canister solenoid.
    •  Disconnected or faulty EVAP vent solenoid.
    •  Open ignition feed circuit to the EVAP vent or purge solenoid.
    •  Damaged EVAP canister.

Any of the above conditions sets DTC P0440.

A restricted or blocked EVAP canister vent path is detected by drawing vacuum into the EVAP system. The PCM turns off the EVAP vent solenoid and the EVAP purge solenoid (EVAP vent solenoid Open, EVAP purge PWM 0%). The PCM monitors the fuel tank pressure sensor input. With the EVAP vent solenoid open, any vacuum in the system should decrease quickly unless the vent is blocked. A blockage is caused by the following conditions:

    •  Faulty EVAP vent solenoid (stuck closed).
    •  Plugged, kinked, or pinched vent hose.
    •  Shorted EVAP vent solenoid driver circuit.
    •  Plugged evaporative canister.

If any of the above conditions are present, DTC P0446 sets.

The PCM tests for conditions that cause the EVAP system to purge continuously by commanding the EVAP vent solenoid on and the EVAP purge solenoid off (EVAP vent solenoid CLOSED, EVAP purge PWM 0 %). If fuel tank pressure level increases during the test, a continuous purge flow condition is indicated. This can be caused by the following conditions:

    •  EVAP purge solenoid leaking.
    •  EVAP purge and engine vacuum source lines switched at the EVAP purge solenoid.
    •  EVAP purge solenoid driver circuit grounded.

If any of the above conditions are present, DTC P1441 sets.

Refer to the appropriate DTCs for further diagnostic procedures regarding the EVAP system.

Visual Check of Evaporative Emission Canister
    • Cracked or damaged, replace the canister.
    • Fuel leaking from the canister, replace canister and inspect lines and line routing.