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Aux. Emission Control Devices
Fuel and Evaporative Emission Hose/Pipe Connection Cleaning Notice
CAUTION
!
Clean all of the following areas before performing any disconnections in order to avoid possible
contamination in the system:
• The fuel pipe connections
• The hose connections
• The areas surrounding the connections
Evaporative Emission Control System
Description
EVAP System Operation
The evaporative emission (EVAP) control system limits
fuel vapors from escaping into the atmosphere. Fuel
tank vapors are allowed to move from the fuel tank, due
to pressure in the tank, through the vapor pipe, into the
EVAP canister. Carbon in the canister absorbs and
stores the fuel vapors. Excess pressure is vented
through the vent line and EVAP vent valve to
atmosphere. The EVAP canister stores the fuel vapors
until the engine is able to use them. At an appropriate
time, the control module will command the EVAP purge
valve ON, open, allowing engine vacuum to be applied
to the EVAP canister. With the EVAP vent valve OFF,
open, fresh air will be drawn through the valve and vent
line to the EVAP canister. Fresh air is drawn through the
canister, pulling fuel vapors from the carbon. The air/fuel
vapor mixture continues through the EVAP purge pipe
and EVAP purge valve into the intake manifold to be
consumed during normal combustion. The control
module uses several tests to determine if the EVAP
system is leaking.
Large Leak Test
This tests for large leaks and blockages in the EVAP
system. The control module will command the EVAP
vent valve ON, closed, and command the EVAP purge
valve ON, open, with the engine running, allowing
engine vacuum into the EVAP system. The control
module monitors the fuel tank pressure (FTP) sensor
voltage to verify that the system is able to reach a
predetermined level of vacuum within a set amount of
time. The control module then commands the EVAP
purge valve OFF, closed, sealing the system and
monitors the vacuum level for decay. If the control
module does not detect that the predetermined vacuum
level was achieved, or the vacuum decay rate is more
than a calibrated level on 2 consecutive tests, DTC
P0455 sets.
Precautions
General Description
Small Leak Test
The engine off natural vacuum, (EONV), diagnostic is
the small leak detection diagnostic for the EVAP system.
The EONV diagnostic monitors the EVAP system
pressure or vacuum with the key OFF. The EONV
utilizes the temperature changes and the resulting
naturally occurring vacuum or pressure in the fuel tank
immediately following a drive cycle. When the vehicle is
driven, the temperature in the fuel tank rises. When the
vehicle is parked with the engine OFF and key OFF, the
temperature in the fuel tank will continue to rise for a
period of time, and then begin to decrease. The EONV
diagnostic relies on this temperature change, and the
corresponding pressure change to determine if an EVAP
system leak is present. The EONV diagnostic is
designed to detect leaks as small as 0.51 mm (0.020 in).
The diagnostic can determine if a small leak is present
based on vacuum or pressure readings in the EVAP
system. When the EVAP system is sealed a finite
amount of pressure or vacuum will be observed. When a
0.51 mm (0.020 in) leak is present, little or no pressure
or vacuum is observed. If the test reports a failing value,
DTC P0442 will set.
Canister Vent Restriction Test
If the EVAP vent system is restricted, fuel vapors will not
be properly purged from the EVAP canister. The control
module tests this by commanding the EVAP purge valve
ON, open; and commanding the EVAP vent valve OFF,
open; and monitoring the FTP sensor for an increase in
vacuum. If vacuum increases more than a calibrated
value, DTC P0446 sets.
Aux. Emission Control Devices:
1B-1
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Troubleshooting
