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 (charcoal) storage device (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 inches 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 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:
Diagnostic Test
PCode
Possible Causes
Weak Vacuum Test (Large Leak Test)
PO440
A Large Leak (above .060 in.) or Blockage Somewhere in the System.
Vacuum Decay Test (Small Leak Test)
P0442
A Small Leak somewhere in the system (.020-.060 in.).
Canister Vent Restriction Test
P0446
A Restriction In The EVAP Vent System.
Purge Valve Leak Test
P1441
A Stuck Open or Partially Open Purge Valve.
The fuel tank pressure (FTP) sensor measures the difference between the air pressure (or vacuum) in the fuel tank and the outside air pressure.
The sensor mounts at the top of the fuel tank sending unit. The PCM supplies a 5 volt reference voltage and ground to the sensor. The sensor provides a signal voltage between 0.1-4.9 volts to the PCM. When the air pressure in the fuel tank is equal to the outside air pressure, such as when the fuel fill cap is removed, the output voltage of the sensor will measure 1.3-1.7 volts.
When the air pressure in the tank is 4.5 inches H2O (1.25 kPa), the sensor output voltage should measure 0.5 ± 0.2 volts (1.25 kPa).
The sensor voltage increases to approximately 4.5 volts at 14 inches of H2O (-3.75 kPa).
When energized, the evaporative canister purge valve allows the fuel vapor to flow from the EVAP canister to the engine. The normally closed valve is pulse width modulated by the PCM in order to precisely control the vapor flow. The valve opens during the Enhanced Evaporative Diagnostic Test in order to create a vacuum in the fuel tank and then closed in order to seal the system.
The evaporative canister vent valve not only allows the fresh outside air to the EVAP canister during the purge modes, but also allows the diagnostic to pull a vacuum on the fuel tank by closing the vent valve. This valve is normally open.
The EVAP canister, filled with charcoal pellets, stores the fuel vapors from the fuel tank. Vacuum is pulled from the fuel tank through the Vapor pipe (2). Fresh air (3) is mixed with the vapors and pulled to the intake manifold from the canister through the purge line (1). The engine vacuum purges the EVAP canister during normal driving.
Important: Use the J 41413 EVAP Pressure/Purge Diagnostic Station in order to provide a clean, dry, low pressure gas source. Do not substitute any other pressurized gas source. Damage may result to the EVAP system.
