EVAPORATIVE EMISSION CONTROLS
INDEX
page page
DESCRIPTION AND OPERATION
CRANKCASE VENT FILTER................ 16
EVAPORATION CONTROL SYSTEM......... 13
EVAPORATIVE (EVAP) CANISTER........... 13
LEAK DETECTION PUMP................. 14
POSITIVE CRANKCASE VENTILATION (PCV)
SYSTEMS............................ 15
PRESSURE-VACUUM FILLER CAP.......... 14
PROPORTIONAL PURGE SOLENOID......... 13ROLLOVER VALVE....................... 13
VEHICLE EMISSION CONTROL INFORMATION
LABEL............................... 16
DIAGNOSIS AND TESTING
LEAK DETECTION PUMP................. 16
PCV VALVE TEST....................... 16
REMOVAL AND INSTALLATION
LEAK DETECTION PUMP REPLACEMENT.... 16
ROLLOVER VALVES..................... 17
DESCRIPTION AND OPERATION
EVAPORATION CONTROL SYSTEM
The evaporation control system prevents the emis-
sion 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 evap-
orative canister. The canister temporarily holds the
vapors. The Powertrain Control Module (PCM) allows
intake manifold vacuum to draw vapors into the com-
bustion chambers during certain operating condi-
tions.
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
Purge Solenoid in this section.
NOTE: The evaporative system uses specially man-
ufactured hoses. If they need replacement, only use
fuel resistant hose.
ROLLOVER VALVE
All vehicles have a rollover valve. The valve also
prevents fuel flow through the fuel tank vent valve
hoses should the vehicle rollover. All vehicles pass a
360É rollover.
The charcoal filled evaporative canister stores the
vapors. The rollover valve is not a serviceable item.
EVAPORATIVE (EVAP) CANISTER
All vehicles use a sealed, maintenance free, evapo-
rative (charcoal) canister. The canister is attached to
the frame under the driver's seat (Fig. 1).
Fuel tank vapor vents into the canister. The canis-
ter temporarily holds the fuel vapors until intake
manifold vacuum draws them into the combustion
chamber. The canister proportional purge solenoidallows the canister to be purged at predetermined
intervals and engine conditions.
PROPORTIONAL PURGE SOLENOID
All vehicles use a Proportional purge solenoid. The
solenoid regulates the rate of vapor flow from the
EVAP canister to the throttle body. The PCM oper-
ates the solenoid.
During the cold start warm-up period and the hot
start time delay, the PCM does not energize the sole-
noid. When de-energized, no vapors are purged. The
PCM de-energizes the solenoid during open loop oper-
ation.
Fig. 1 Evaporative Canister
NSEMISSION CONTROL SYSTEMS 25 - 13
The proportional purge solenoid operates at a fre-
quency of 200hz and is controlled by an engine con-
troller circuit that senses the current being applied
to the proportional purge solenoid and then adjusts
that current to achieve the desired purge flow. The
proportional purge solenoid controls the purge rate of
fuel vapors from the vapor canister and fuel tank to
the engine intake manifold.
PRESSURE-VACUUM FILLER CAP
CAUTION: Remove the fuel filler cap to relieve fuel
tank pressure. The cap must be removed prior to
disconnecting any fuel system component or ser-
vicing the fuel tank.
A pressure-vacuum relief cap seals the fuel tank
(Fig. 3). Tightening the cap on the fuel filler tube
forms a seal between them. The relief valves in the
cap are a safety feature. They prevent possible exces-
sive pressure or vacuum in the tank. Excessive fuel
tank pressure could be caused by a malfunction in
the system or damage to the vent lines.
The seal between the cap and filler tube breaks
when the cap is removed and relieves fuel tank pres-
sure.
If the filler cap needs replacement, only use the
correct part.
LEAK DETECTION PUMP
The leak detection pump 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, when the engine
temperature is between 40ÉF and 86ÉF, the 3 portsolenoid 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 posi-
tion. 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 assem-
bly from reaching full travel. After the brief initial-
ization 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-ener-
gized, 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.
Fig. 2 Proportional Purge Solenoid
Fig. 3 Pressure Vacuum Filler Cap
25 - 14 EMISSION CONTROL SYSTEMSNS
DESCRIPTION AND OPERATION (Continued)
