Evaporative Emissions Control Systems (EVAP)PurposeEnhanced and
Non-EnhancedComponentsFunctionFinding Faults
Purpose:To trap and hold gasoline vapors (HC)To deliver vapors
to engine airflow for complete burningProvides system ventilation
to allow purgingProvides pressure relief to prevent excess pressure
inside fuel tank
2 Types of SystemsNon-EnhancedDiagnostic ability is limited to
purge detection and monitoringEnhancedMonitors purge flowMonitors
system integrityLeak checking1996-2000: leak checking to .040
in.2000-up: leak checking to .020 in.
Components:Carbon canisterTraps and holds fuel vapors for
burning by enginePurge valve/ Purge solenoidControlled by PCM to
allow canister purging during certain conditionsVacuum linesConnect
tank to canister Connect canister to engineFuel tank filler
capSeals fuel tankAllows for pressure reliefPrevents excessive
vacuum from collapsing tankFuel expansion tankPrevents liquid fuel
from entering canisterFuel Tank Pressure Sensor (Enhanced)Vent
valve (Enhanced)Provides fresh air for system purgingCan be used to
seal system to check for leaks
Function:As fuel heats up: vapors form and are vented to
canisterCanister is filled with activated charcoalFuel vapors
attach to carbon surface1 gram of charcoal contains surface area
equal to acreHolds 1/3 of its own weight in fuel vaporsAs system is
purged, vapor molecules are pulled from carbon surface
Function:Pressure build-up is caused by:Volatility ratesFuel
tank sizeFuel level inside tankFuel sloshTemperatureFuel return
Function:Purging is closely controlledExcess fuel vapors can
upset air/fuel ratioPurging is allowed:Closed loopCruising
speedsPurging is not allowed:Open loopIdle or decelerationWOT
Finding FaultsTwo types of complaints:Fuel vapor smellMIL
illuminationRetrieve DTCsCheck freeze frame data for ECT and
VSSCheck TSBs, recalls and PCM reflashesDTCs:Purge control circuit
failuresPurge flow failuresLeak detection DTCs
Finding FaultsFollow manufacturers diagnostic troubleshooting
procedurePurge control circuit failures: can be checked with
bidirectional scan toolCommand purge valve on and listen for
clicking soundWith valve commanded on check for air flow through
valveCheck power and ground wires with voltmeterPurge valve can be
manually activated with jumper wires
Finding FaultsPurge flow failures: can be caused by leakage or
restriction preventing purgeSmoke machine: insert probe into test
port and watch machines flow meter Proper flow eliminates
possibility of restriction in systemImproper flow indicates system
is restricted: isolate system and continue checking
Finding FaultsLeak checks: connect smoke machine and close vent
solenoidVent solenoid can be closed with scan tool, manually cycled
with jumper wires or vent pinched with suitable toolSmoke machine
should indicate no flow if system is free from leaksIf leaks are
present, smoke will be visible
Locating LeaksSmoke testing: uses low pressure smoke introduced
to system from test portAllows for visual leak detectionNitrogen
gas pressurizationVery low pressure (> 1 psi) Uses sound
amplification for detecting leaks
