Evaporative Emissions System: Description and Operation
Evaporative Emission Control System Description
EVAP System Operation
The Evaporative (EVAP) emission control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, trough the vapor pipe, into the EVAP canister.
Active carbon in the canister absorbs and stores the fuel vapors.
The EVAP canister stores the fuel vapors until the engine is able to use them. At appropriate time the Engine Control Module (ECM) will command the EVAP purge solenoid valve ON allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF fresh air is drawn through the vent valve and the vent line to the EVAP canister.
Fresh air is drawn through the canister pulling fuel vapors from the active carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion.
The ECM uses several tests to determine if the EVAP system is leaking.
Large Leak Test
This tests for large leaks and blockages in the EVAP system. The ECM commands the EVAP vent solenoid valve ON and commands the EVAP purge solenoid valve ON with the engine running allowing engine vacuum into the EVAP system. The control module monitors the fuel tank pressure sensor voltage to verify that the system is able to reach a predetermined level of vacuum within a set amount of time. The ECM then commands the EVAP purge solenoid valve OFF. This action seals the system and allows the ECM to monitor the vacuum level for decay. If the control module does not detect that the predetermined vacuum level was achieved, or the vacuum decay rate is more than a calibrated level on two consecutive tests, DTC P0455 will set.
Small Leak Test
The engine off natural vacuum diagnostic is the small leak detection diagnostic for the EVAP system. While previous leak detection methods were performed with the engine running the ECM monitors the EVAP system pressure or vacuum with the ignition OFF. It may be normal for the control and the EVAP solenoid valve to remain active for up to 40 min. after the engine is turned OFF (this is important to remember when performing a parasitic draw test). The small leak test utilizes the temperature changes in the fuel tank immediately following a drive cycle to use the natural existing vacuum or pressure in the fuel tank. When the vehicle is driven the temperature rises in the tank. After the vehicle is parked the temperature in the fuel tank continues to rise for a period of time then starts to drop. The small leak test diagnostic is designed to detect leaks as small as 0.50 mm (0.02 in). The diagnostic can determine if a small leak is present based on vacuum or pressure readings in the EVAP systems. When the system is sealed a final amount of pressure or vacuum will be observed. When a leak is present often little or no pressure or vacuum is observed. If the test reports a failing value DTC P0442 will set.
Canister Vent Restriction Test
After the small leak test has passed, a vacuum is retained in the evaporative emission (EVAP) system. The ECM tests for a restricted vent path by commanding the purge valve OFF closed and the vent valve OFF open. The fuel tank pressure sensor is monitored for a decrease in EVAP system vacuum. If the vacuum does not decrease to near 0 kPa (0 PSI) in a calibrated time DTC P0446 will set.
Purge Valve Leak Test
If the evaporative emission (EVAP) purge valve does not seal properly, fuel vapors could enter the engine at an undesired time causing driveability concerns. The ECM tests for this by commanding the EVAP purge valve OFF closed and vent valve OFF OPEN, and monitors the fuel tank pressure for an increase in vacuum. If the ECM detects that EVAP system vacuum increases above a calibrated value, a DTC sets.
Evaporative Emission System Tester
The Evaporative Emission System Tester is used to help locate leaks in the evaporative emission (EVAP) system. The Evaporative Emission System Tester provides a clean, dry, regulated supply of nitrogen to pressurize the EVAP system. It also provides smoke to help in locating the leak source. Refer to the directions on the cart for proper operation.
The Fuel Tank Cap Adapter is used to adapt the to the fuel filler neck. Pressurizing, or inducing smoke to the EVAP system at the fuel filler neck allows testing of the filler neck and the fuel fill cap on vehicles with onboard refueling vapor recovery.
The High Intensity White Light is used to help locate the leak source.
On Board Refueling Vapor Recovery System
This vehicle is equipped with an On Board Refueling Vapor Recovery System. This system has developed and equipped to meet enhanced evaporative emission (EVAP) control requirements during vehicle moving, parking, and refueling. The On Board Refueling Vapor Recovery system operates in the following manner:
* One canister collects evaporative vapors while the vehicle is moving, parking, and refueling.
* Collected vapor flows into the engine through the intake manifold, where it is consumed during vehicle operation.
* Fuel flowing through a reduced diameter section in the filler pipe creates suction in the filter neck.
The On Board Refueling Vapor Recovery system operates by the liquid trap, liquid seal, system which ensures long-term durability. The On Board Refueling Vapor Recovery system has been designed with the following functional features:
* Collection and routing of refueling vapors to the canister.
* Nozzles that are compatible with conventional and Stage II vapor recovery nozzles.
* Fuel shut-off signal.
* Prevention of liquid fuel entering the canister during normal driving and during vehicle rollover.
* Fuel tank overpressure prevention.
* Fuel tank venting to canister during vehicle operation.
* Fuel vapor dome overfill protection.
This vehicle is also equipped with an On Board Diagnostic II (OBD II) system. This system identifies failures or malfunctions of the On Board Refueling Vapor Recovery system and warns the driver through the malfunction indicator lamp (MIL) on the instrument cluster.
The On Board Refueling Vapor Recovery system requires no special refueling procedures and maintenance.