Post by s54smg2 on Mar 1, 2014 23:39:51 GMT -5
The evaporative emission (EVAP) control system limits the fuel vapors from escaping into the
atmosphere. The EVAP transfers the fuel vapors from the sealed fuel tank to an activated carbon
storage device, the EVAP canister. The EVAP canister stores the vapors until the engine is able to use
the extra fuel vapor. When the engine is able to use the extra fuel vapor, the intake air flow purges the
fuel vapor from the carbon element and then the normal combustion process consumes the fuel vapor.
The system is required in order to detect the evaporative fuel system leaks as small as 0.040 inch
between the fuel filler cap and the EVAP canister purge valve. The system can test the evaporative
system integrity by applying a vacuum signal, ported or manifold, to the fuel tank in order to create a
small vacuum.
The powertrain control module (PCM) then monitors the ability of the system to maintain the vacuum. If
the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS is
reported by the PCM. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot
be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and
driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts
sub-tests in order to detect the fault conditions. If the diagnostic fails a sub-test, the PCM stores a
diagnostic trouble code (DTC) in order to indicate the type of fault detected.
The EVAP diagnostic tests include the following tests:
Weak Vacuum Test: This tests for large leaks and blockages in the EVAP system. The fuel tank
pressure (FTP) sensor detects a low fuel tank pressure resulting from a large vacuum leak or a
restriction in the vacuum supply to the fuel tank. The FTP sensor value should increase over a period
of time. If the PCM does not detect an increase, a malfunction is indicated by setting DTC P0440.
Evaporative Emissions System (EVAP)
atmosphere. The EVAP transfers the fuel vapors from the sealed fuel tank to an activated carbon
storage device, the EVAP canister. The EVAP canister stores the vapors until the engine is able to use
the extra fuel vapor. When the engine is able to use the extra fuel vapor, the intake air flow purges the
fuel vapor from the carbon element and then the normal combustion process consumes the fuel vapor.
The system is required in order to detect the evaporative fuel system leaks as small as 0.040 inch
between the fuel filler cap and the EVAP canister purge valve. The system can test the evaporative
system integrity by applying a vacuum signal, ported or manifold, to the fuel tank in order to create a
small vacuum.
The powertrain control module (PCM) then monitors the ability of the system to maintain the vacuum. If
the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS is
reported by the PCM. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot
be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and
driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts
sub-tests in order to detect the fault conditions. If the diagnostic fails a sub-test, the PCM stores a
diagnostic trouble code (DTC) in order to indicate the type of fault detected.
The EVAP diagnostic tests include the following tests:
Weak Vacuum Test: This tests for large leaks and blockages in the EVAP system. The fuel tank
pressure (FTP) sensor detects a low fuel tank pressure resulting from a large vacuum leak or a
restriction in the vacuum supply to the fuel tank. The FTP sensor value should increase over a period
of time. If the PCM does not detect an increase, a malfunction is indicated by setting DTC P0440.
Evaporative Emissions System (EVAP)