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 solenoid to
atmosphere. The EVAP canister stores the fuel vapors until the engine is
able to use them. At an appropriate time, the PCM will command the EVAP purge
valve open, allowing engine vacuum to be applied to the EVAP canister. With
the EVAP vent valve 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 EVAP system requires the PCM be able to detect
a leak as small as 0.040 inch in the EVAP system. The PCM uses several tests
to determine if the EVAP system is sealed.
Weak Vacuum Test
This tests for large leaks and blockages in the EVAP system. The PCM
will command ON (close) the EVAP vent valve and command ON (open) the EVAP
purge valve with the engine running, allowing engine vacuum into the EVAP
system. The PCM monitors the fuel tank pressure (FTP) sensor to verify that
the system is able to reach a predetermined level of vacuum within a set amount
of time. If the PCM does not detect the predetermined vacuum level on 2 consecutive
tests, a DTC P0440 will set.
Vacuum Decay Test
If the weak vacuum test passes, the PCM will command OFF the EVAP purge
valve, sealing the EVAP system. The PCM tests for vacuum decay in the EVAP
system by monitoring the fuel tank pressure (FTP) sensor for a change in voltage
over a period of time. If the decay rate is more than a calibrated value,
the PCM will rerun the test. If the test fails again, a 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 PCM tests this by commanding the EVAP
purge valve CLOSED and commanding the EVAP vent valve OPEN and monitoring
the FTP sensor for an increase in vacuum. If an increase in vacuum is detected,
DTC P0446 will set.
Purge Valve Leak Test
If the EVAP purge valve does not seal properly, fuel vapors could enter
the engine at an undesired time causing driveability concerns. The PCM tests
for this by commanding the EVAP purge and vent valves closed, sealing the
system, and monitoring the FTP for an increase in vacuum. If the PCM detects
an increase in vacuum, DTC P1441 will set.
EVAP System Components
EVAP System Overview
