COMPONENT TESTING
The Evaporative Emission (EVAP) Controls are monitored by the Powertrain Control Module (PCM) and if found to be malfunctioning the PCM records the problem in the fault memory as a Diagnostic Trouble Code (DTC). If the problem persists or compromises the vehicle's emissions, the Malfunction Indicator Light (MIL) could be activated.
Evaporative Emissions (EVAP) Control Canister
This canister is used as a storage facility for fuel vapors that have escaped from components such as the fuel tank. This canister prevents these vapors from entering the atmosphere.
Generally, the only testing done to the canister is a visual inspection. Look the canister over and replace it with a new one if there is any evidence of cracks or other damage.
See Figure 2
Fig. Fig. 2: EVAP control canister-shown
Inspect all system hoses and tubes for signs of damage or cracks. Any damage or leakage must be repaired.
Evaporative Emissions Purge Control Solenoid Valve
The evaporative emissions control solenoid valve is located at between the canister and the intake manifold.
- Disconnect the vacuum hose from the valve.
- Connect a vacuum pump to the vacuum hose.
- Turn the ignition switch ON .
See Figure 3
Fig. Fig. 3: Installed view of the EVAP bypass solenoid valve and the EVAP two-way valve
- Apply vacuum to the hose.
- If the valve holds vacuum, the valve is OK.
- If the valve does not hold vacuum:
- Turn the ignition switch OFF .
- Detach the electrical connector from the valve.
- Check for continuity between the red/yellow wire and ground. If continuity is present, replace the valve. If no continuity is present, check for an open circuit in the red/yellow wire between the valve and the PCM.
- When finished testing, attach all disconnected hoses, connectors, and wires.
- Raise and support the vehicle.
- Remove the bolts retaining the Evaporative Emissions (EVAP) canister and bracket assembly.
- Label and disconnect the vapor hoses from the canister.
- Remove the canister from the bracket.
- Installation is the reverse of the removal procedure.
Evaporative Emissions Control Shut Valve
- Disconnect the vacuum hose from the EVAP shut valve.
- Remove the fasteners that hold the EVAP shut valve to the canister.
- Remove the EVAP control shut valve from the canister.
- Install the EVAP control shunt valve and fasteners.
- Connect all vacuum hoses.
See Figure 4
Fig. Fig. 4: The Control Canister Vent Shut Valve is mounted on top of the EVAP Canister on 9Models
- Raise and support the vehicle.
- Detach the electrical harness from the valve.
See Figure 5
Fig. Fig. 5: Exploded view of the EVAP two-way valve
- Disconnect the fuel vapor hoses and remove the valve.
- Installation is the reverse of the removal procedure.
OPERATION
Changes in atmospheric temperature cause fuel tanks to breathe, that is, the air within the tank expands and contracts with outside temperature changes. If an unsealed system was used, when the temperature rises, air would escape through the tank vent tube or the vent in the tank cap. The air which escapes contains gasoline vapors.
The Evaporative Emission Control System provides a sealed fuel system with the capability to store and condense fuel vapors. When the fuel evaporates in the fuel tank, the vapor passes through the EVAP emission valve, through vent hoses or tubes to a carbon filled evaporative canister. When the engine is operating the vapors are drawn into the intake manifold and burned during combustion.
A sealed, maintenance free evaporative canister is used. The canister is filled with granules of an activated carbon mixture. Fuel vapors entering the canister are absorbed by the charcoal granules. A vent cap is located on the top of the canister to provide fresh air to the canister when it is being purged. The vent cap opens to provide fresh air into the canister, which circulates through the charcoal, releasing trapped vapors and carrying them to the engine to be burned.
Fuel tank pressure vents fuel vapors into the canister. They are held in the canister until they can be drawn into the intake manifold. The canister purge valve allows the canister to be purged at a pre-determined time and engine operating conditions.
Vacuum to the canister is controlled by the canister purge valve. The valve is operated by the PCM. The PCM regulates the valve by switching the ground circuit on and off based on engine operating conditions. When energized, the valve prevents vacuum from reaching the canister. When not energized the valve allows vacuum to purge the vapors from the canister.
During warm up and for a specified time during hot starts, the PCM energizes the valve-preventing vacuum from reaching the canister. The EVAP purge control solenoid begins to operate when the engine coolant temperature reaches a predetermined operating temperature.
Once the proper coolant temperature is achieved, the PCM controls the ground circuit to the valve. When the PCM opens the ground, this allows vacuum to flow through the canister and vapors are purged from the canister into the throttle body. During certain idle conditions, the PCM may energize the purge valve to control fuel mixture calibrations.
The fuel tank is sealed with a pressure-vacuum relief filler cap. The relief valve in the cap is a safety feature, preventing excessive pressure or vacuum in the fuel tank. If the cap is malfunctioning, and needs to be replaced, ensure that the replacement is the identical cap to ensure correct system operation.
The following components are part of and affect the operation of the EVAP (Evaporative Emission) Control system:
See Figure 1
Fig. Fig. 1: Schematic of the EVAP Control System-1998-99
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