Evaporative Emissions Control Systems (EVAP)
•Purpose•Enhanced and Non-Enhanced•Components•Function•Finding Faults
Purpose: To trap and hold gasoline vapors (HC) To deliver vapors to engine airflow for complete
burning Provides system ventilation to allow purging Provides pressure relief to prevent excess pressure
inside fuel tank
2 Types of Systems Non-Enhanced
Diagnostic ability is limited to purge detection and monitoring
Enhanced Monitors purge flow Monitors system integrity Leak checking 1996-2000: leak checking to .040 in. 2000-up: leak checking to .020 in.
Components: Carbon canister
Traps and holds fuel vapors for burning by engine Purge valve/ Purge solenoid
Controlled by PCM to allow canister purging during certain conditions
Vacuum lines Connect tank to canister Connect canister to engine
Fuel tank filler cap Seals fuel tank Allows for pressure relief Prevents excessive vacuum from collapsing tank
Fuel expansion tank Prevents liquid fuel from entering canister
Fuel Tank Pressure Sensor (Enhanced) Vent valve (Enhanced)
Provides fresh air for system purging Can be used to seal system to check for leaks
Function: As fuel heats up: vapors form and are
vented to canister Canister is filled with activated charcoal Fuel vapors attach to carbon surface 1 gram of charcoal contains surface area
equal to ¼ acre Holds 1/3 of its own weight in fuel vapors
As system is purged, vapor molecules are pulled from carbon surface
Function: Pressure build-up is caused by:
Volatility rates Fuel tank size Fuel level inside tank Fuel slosh Temperature Fuel return
Function: Purging is closely controlled
Excess fuel vapors can upset air/fuel ratio Purging is allowed:
Closed loop Cruising speeds
Purging is not allowed: Open loop Idle or deceleration WOT
Finding Faults Two types of complaints:
Fuel vapor smell MIL illumination
Retrieve DTC’s Check freeze frame data for ECT and VSS
Check TSB‘s, recalls and PCM reflashes
DTC’s: Purge control circuit failures Purge flow failures Leak detection DTC’s
Finding Faults Follow manufacturer’s diagnostic
troubleshooting procedure Purge control circuit failures: can be
checked with bidirectional scan tool Command purge valve on and listen for
clicking sound With valve commanded on check for air flow
through valve Check power and ground wires with
voltmeter Purge valve can be manually activated
with jumper wires
Finding Faults Purge flow failures: can be caused by
leakage or restriction preventing purge Smoke machine: insert probe into test
port and watch machine’s flow meter Proper flow eliminates possibility of
restriction in system Improper flow indicates system is
restricted: isolate system and continue checking
Finding Faults Leak checks: connect smoke machine
and close vent solenoid Vent solenoid can be closed with scan
tool, manually cycled with jumper wires or vent pinched with suitable tool
Smoke machine should indicate no flow if system is free from leaks
If leaks are present, smoke will be visible
Locating Leaks Smoke testing: uses low pressure smoke
introduced to system from test port Allows for visual leak detection
Nitrogen gas pressurization Very low pressure (> 1 psi) Uses sound amplification for detecting
leaks
Toyota Evaporative Emissions