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