The Evaporative Emission Control (EVAP) System helps to reduce evaporated hydrocarbons (fuel) that enter the atmosphere. The most important component f 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.
The Evaporative Emission Control (EEC) or (EVAP) system stores fuel vapor generated by the vehicle and regulates its consumption during normal driving operation. The main purpose of the EEC system is to prevent fuel vapor from dispelling into the atmosphere.
The EEC routine is controlled by the PCM via the electronic solenoid.
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.
The canister is filled with activated carbon that stores vapor transferred from the fuel tank. The tank also stores the vapor that is emitted from the engine's induction system while the engine is not running. When the engine is running, the stored vapor is purged from the carbon storage device by the intake airflow and then consumed in the normal combustion process.
The 3 ports coming off the canister are identified as:
- Fuel vapor port (from fuel tank)
- EEC port (purge line to throttle body)
- Atmospheric port (from fresh air source via the body mounted vent hose)
The EVAP Canister purge valve controls engine vacuum to the canister. Under the appropriate conditions., the PCM commands the purge valve open. This allows engine vacuum to draw fresh air into the canister through the EVAP Vent Valve. The fuel vapors exit the canister and are consumed during the normal combustion process.EVAP Canister Vent Valve
The EVAP Canister Vent Valve is used for certain EVAP system performance tests performed by the PCM. The PCM can close the vent that effectively seals the system. The PCM can then evaluate pressure changes within the system by monitoring the Fuel Tank Pressure Sensor signal.Fuel Tank
Fuel vapor generated inside the fuel tank is released to the canister for containment.Fuel Tank Pressure Sensor
The Fuel Tank Pressure Sensor is used for certain EVAP system performance tests performed by the PCM. The Fuel Tank Pressure sensor contains a diaphragm that changes resistance based on pressure. When EVAP system pressure is low (during purge) sensor output voltage is low. When the system pressure is high, sensor output voltage is high. The PCM monitors pressure changes within the system using the Fuel Tank Pressure Sensor signal. This information can be used to detect leads within the system or to verify the operation of the system components.
The PCM operates a solenoid valve that purges the canister with ported vacuum at the throttle body. Under cold engine operation `OPEN LOOP' or idle conditions the solenoid is de-energized by the PCM, this does not allow vacuum to the canister through the normally closed solenoid.
The canister will be enabled, the PCM will energize the solenoid when:
When the solenoid is closed (is not receiving voltage or has a stuck plunger) the canister will not purge to the intake manifold. This will prevent the canister from purging vapors and could result in a saturated canister.
Late Model Vehicles
During specific operating conditions, the PCM performs various tests on the evaporative emission system. The system tests consist of the following series of events. After ensuring that the EVAP purge solenoid valve duty cycle has dropped to 0%, indicating that the valve is closed, the PCM commands the EVAP canister vent solenoid valve closed, sealing the system. The PCM monitors the accumulation of vapor pressure within the fuel tank via the fuel tank pressure sensor. The EVAP test is aborted if the vapor pressure is too high. If any vapor pressure is measured, it will be used later to compensate a pressure reading during the small leak detection test. If a vacuum is measured which exceeds a calculated limit during vapor accumulation, DTC P0440 will set. The EVAP canister purge solenoid valve is then opened. Simultaneously, the Vent solenoid is opened. If the vapor pressure does not bleed off or bleeds off too slowly, DTC P0440 is set. Once the EVAP purge solenoid valve reaches its position for the remainder of the diagnostic test, the Vent solenoid will be closed again. This causes vacuum to be applied to the entire EVAP system. The PCM monitors the vacuum level within the system. If the desired vacuum level can not be achieved, or if the vacuum level is reached but it took too much time, DTC P0455 will set. Once the desired vacuum level is reached, the purge solenoid is closed, sealing the system, The PCM continues to monitor the fuel tank pressure sensor signal, measuring the rate of vacuum decay. If the system holds vacuum, as it should, the vent solenoid is opened and the test is completed. In addition to the system tests, the PCM monitors the circuit integrity of the purge solenoid (DTC P0443), the vent solenoid (DTC P0446), and the fuel tank pressure sensor (DTC P0450).Early Model Vehicles
- Attach a clean length of hose to the fuel tank vapor line connection on the canister and attempt to blow through the purge control valve. It should be difficult or impossible to blow through the valve. If air passes easily, the valve is stuck open and should be replaced.
- Connect a hand-held vacuum pump to the top vacuum line fitting of the purge control valve. Apply a vacuum of 15 in. Hg (51 kPa) to the purge valve diaphragm. If the vacuum reading is less than 10 inches after 10 seconds, the diaphragm is leaking and the TPCV must be replaced. If the diaphragm holds vacuum, try to blow through the hose connected to the lower tube while vacuum is still being applied. An increase flow of air should be observed. If not, replace the TPCV.
- Attach a hose to the lower port of the canister and attempt to blow through it. Air should pass through and into the canister. If not, replace the canister.
- Detach the electrical connector at the canister purge solenoid. Attach a hose to the upper port on the canister and attempt to blow air through it. Air should not pass through into the canister. If air passes, replace the purge solenoid.
- Measure the resistance of the purge control solenoid. The resistance should be greater than 20 ohms and less than 100 ohms. If the solenoid resistance is not in this range, replace the solenoid.
- Reattach all purge hoses and the solenoid electrical connector.
Check for the following conditions.