Emission Control Systems: Description and Operation
EVAPORATION CONTROL SYSTEMEvaporative System Monitor Schematic-Typical:
The evaporation control system prevents the emission of fuel tank vapors into the atmosphere. When fuel evaporates in the fuel tank, the vapors pass through vent hoses or tubes to a charcoal filled evaporative canister. The canister temporarily holds the vapors. The Powertrain Control Module (PCM) allows intake manifold vacuum to draw vapors into the combustion chambers during certain operating conditions.
All engines use a duty cycle purge system. The PCM controls vapor flow by operating the duty cycle EVAP purge solenoid. Refer to Duty Cycle EVAP Canister Purge Solenoid.
When equipped with certain emissions packages, a Leak Detection Pump (LDP) will be used as part of the evaporative system for OBD II requirements. Also refer to Leak Detection Pump.
NOTE: The evaporative system uses specially manufactured lines/hoses. If replacement becomes necessary, only use fuel resistant hose.
ROLLOVER VALVES
Rollover Valve Locations:
The fuel tank is equipped with 2 interconnected rollover valves. The valves are located on the top of the fuel tank. These valves will prevent fuel flow through the fuel tank vent (EVAP) hoses in the event of an accidental vehicle rollover. The EVAP canister draws fuel vapors from the fuel tank through these valves.
The valves are not serviced separately. If replacement is necessary, the fuel tank must be replaced. Refer to Fuel Tank Removal/Installation in Fuel System.
EVAPORATION (EVAP) CANISTER
EVAP Canister Locations:
Two, maintenance free, EVAP canisters are used. The canisters are mounted to a common bracket located under the left side of vehicle behind the drivers door well. The EVAP canisters are filled with granules of an activated carbon mixture. Fuel vapors entering the EVAP canisters are absorbed by the charcoal granules.
Fuel tank pressure vents into the EVAP canisters. Fuel vapors are temporarily held in the canisters until they can be drawn into the intake manifold.
The duty cycle EVAP canister purge solenoid allows the EVAP canisters to be purged at predetermined times and at certain engine operating conditions.
DUTY CYCLE EVAP CANISTER PURGE SOLENOID
A duty cycle EVAP canister purge solenoid is used. The solenoid regulates the rate of vapor flow from the EVAP canisters to the throttle body. The PCM operates the solenoid.
During the cold start warm-up period and the hot start time delay, the PCM does not energize the solenoid. When de-energized, no vapors are purged. The PCM de-energizes the solenoid during open loop operation.
The engine enters closed loop operation after it reaches a specified temperature and the time delay ends. During closed loop operation, the PCM energizes and de-energizes the solenoid 5 or 10 times per second, depending upon operating conditions. The PCM varies the vapor flow rate by changing solenoid pulse width. Pulse width is the amount of time the solenoid energizes. The PCM adjusts solenoid pulse width based on engine operating condition.
EVAP Purge Solenoid and LDP Location:
The solenoid attaches to a bracket mounted to the left-inner wheelhouse (fender).
LEAK DETECTION PUMP (LDP)
EVAP System Test Port Location:
The Leak Detection Pump (LDP) is used only with certain emission packages. The LDP and LDP filter are attached to a common bracket. The assembly is located in the engine compartment on the left-inner wheelhouse (fender). The EVAP system test port is located in the engine compartment, below the brake power booster near the rear of the steering column.
The LDP is a device used to detect a leak in the evaporative system.
The pump contains a 3 port solenoid, a pump that contains a switch, a spring loaded canister vent valve seal, 2 check valves and a spring/diaphragm.
Immediately after a cold start, engine temperature between 40°F and 86°F, the 3 port solenoid is briefly energized. This initializes the pump by drawing air into the pump cavity and also closes the vent seal. During non-test test conditions, the vent seal is held open by the pump diaphragm assembly which pushes it open at the full travel position. The vent seal will remain closed while the pump is cycling. This is due to the operation of the 3 port solenoid which prevents the diaphragm assembly from reaching full travel. After the brief initialization period, the solenoid is de-energized, allowing atmospheric pressure to enter the pump cavity; This permits the spring to drive the diaphragm which forces air out of the pump cavity and into the vent system. When the solenoid is energized and de-energized, the cycle is repeated creating flow in typical diaphragm pump fashion. The pump is controlled in 2 modes:
PUMP MODE: The pump is cycled at a fixed rate to achieve a rapid pressure build in order to shorten the overall test time.
TEST MODE: The solenoid is energized with a fixed duration pulse. Subsequent fixed pulses occur when the diaphragm reaches the switch closure point.
The spring in the pump is set so that the system will achieve an equalized pressure of about 7.5 inches of water.
When the pump starts, the cycle rate is quite high. As the system becomes pressurized pump rate drops. If there is no leak the pump will quit. If there is a leak, the test is terminated at the end of the test mode.
If there is no leak, the purge monitor is run. If the cycle rate increases due to the flow through the purge system, the test is passed and the diagnostic is complete.
The canister vent valve will unseal the system after completion of the test sequence as the pump diaphragm assembly moves to the full travel position.
POSITIVE CRANKCASE VENTILATION (PCV) SYSTEM
PCV Valve/Hose-Typical:
All engines are equipped with a closed crankcase ventilation system and PCV valve.
This system consists of a crankcase PCV valve mounted on the cylinder head (valve) cover with a hose extending from the valve to the intake manifold.
A closed engine crankcase breather/filter, with a hose connecting it to the air cleaner housing, provides the source of air for system.
Typical Closed Crankcase Ventilation System:
The PCV system operates by engine intake manifold vacuum. Filtered air is routed into the crankcase through the air cleaner hose and crankcase breather/filter. The metered air, along with crankcase vapors, are drawn through the PCV valve and into a passage in the intake manifold. The PCV system manages crankcase pressure and meters blow by gases to the intake system, reducing engine sludge formation.
The PCV valve contains a spring loaded plunger. This plunger meters the amount of crankcase vapors routed into the combustion chamber based on intake manifold vacuum.
Engine Off or Engine Pop-Back-No-Vapor Flow:
When the engine is not operating or during an engine pop-back, the spring forces the plunger back against the seat. This Will prevent vapors from flowing through the valve.
High Intake Manifold Vacuum-Minimal Vapor Flow:
During periods of high manifold vacuum, such as idle or cruising speeds, vacuum is sufficient to completely compress spring. It will then pull the plunger to the top of the valve. In this position there is minimal vapor flow through the valve.
Moderate Intake Manifold Vacuum-Maximum Vapor Flow:
During periods of moderate manifold vacuum, the plunger is only pulled part way back from inlet. This results in maximum vapor flow through the valve.
CRANKCASE BREATHER/FILTER
Crankcase Breather/Filter:
The crankcase breather/filter is located on the cylinder head (valve) cover. The filter may be cleaned by washing it thoroughly in kerosene or an equivalent. Filter must be thoroughly drained. More frequent service may be necessary for vehicles operated extensively on short run, stop and go, extended engine idle service or extreme dust conditions.
VEHICLE EMISSION CONTROL INFORMATION (VECI) LABEL
VECI Label Location-Typical:
All vehicles are equipped with a combined VECI label. This label is located in the engine compartment and contains the following:
- Engine family and displacement
- Evaporative family
- Emission control system schematic
- Certification application
- Engine timing specifications (if adjustable)
- Idle speeds (if adjustable)
- Spark plug and gap
The label also contains an engine vacuum schematic. There are unique labels for vehicles built for sale in the state of California and the country of Canada. Canadian labels are written in both the English and French languages. These labels are permanently attached and cannot be removed without defacing information and destroying label.