Evaporative Emissions System: Description and Operation
Evaporative Emissions System Monitor
The leak test monitor is designed to find any evaporative leak between 40 thou, (the EVAP reduces to 20 thou on V8 Sedan normally aspirated at 2001 model year) and a gross leak. The 40 thou test operates whilst the vehicle is moving and includes checks for canister closure valve stuck closed (restricted airflow on the fuel tank breather) and the EVAP canister purge valve stuck open (leaking). The EVAP canister closure valve stuck open and EVAP canister purge valve stuck closed is part of the gross leak judgment. The 20 thou leak test is an additional test, which is carried out at idle.
DTCs
P0442 40 thou (or larger) leak detected
P0443 EVAP canister purge valve malfunction EVAP canister purge valve leaking
P0444 EVAP canister purge valve circuit low electrical circuit check
P0445 EVAP canister purge valve circuit high electrical circuit check
P0446 EVAP canister closure valve malfunction restricted airflow through tank breather
P0447 EVAP canister closure valve open circuit electrical circuit check
P0448 EVAP canister closure valve short circuit electrical circuit check
P0450 Fuel Tank Pressure (FTP) sensor malfunction no change in output
P0452 FTP sensor low input electrical circuit check
P0453 FTP sensor high input electrical circuit check
P0455 gross leak
P0456 20 thou leak
Leak Test Operation
The leak test will be initialized when a number of entry conditions are satisfied. They will include ECT, IAT, engine load, vehicle speed, vapor concentration and purge amount.
40 Thou Leak Test
When the entry conditions are satisfied the EVAP canister purge valve will be closed and the EVAP canister closure valve will then close. The EVAP system is now sealed, the FTP sensor will take the initial value of pressure (P1). After 15 seconds the FTP sensor will take a further reading (P2). The difference between P1 and P2 becomes the first pressure rise.
The EVAP canister purge valve will then be ramped open to pull the FTP down to - 2.00 kPa; the EVAP canister purge valve will then close. If the pressure rises too quickly then a second pull down will occur. The FTP sensor then takes a further reading of the tank pressure (P3). After a further 15 seconds a final pressure reading (P4) is taken. The difference between P3 and P4 becomes the second pressure rise.
The EVAP canister closure valve is then opened and the leak value is calculated and compared with the pass/fail threshold. The result may be discarded if the vapor concentration is too high, the first pressure rise is too high or the fuel movement in the tank causes excessive vapor. If the pressure in the tank does not return close to atmospheric within a few seconds of the EVAP canister closure valve opening then the test will continue and may flag DTC P0446.
If the EVAP canister purge valve is ramped open and the tank is not pulled down to -2.00 kPa, a gross leak will be flagged. If however during the second pressure rise the tank pressure rises to a value, which would indicate that it couldn't be a gross leak.
Failure to pull the tank pressure down resulting in flagging P0455 can be due to a gross leak, vapor pipe detached / fuel cap left off etc. or the EVAP canister purge valve stuck closed or the EVAP canister closure valve stuck open.
Leak Test Diagram 40 Thou Test
20 Thou Leak Test
The 20 thou leak test is similar to the diagram above, with the exception that the tank is pulled down to -1.25 kPa rather than -2.00 kPa as above. In addition the 20 thou test is carried out at idle or with the vehicle moving at less then 9 mph. Component faults P0443, P0446, P0450 & P0455 cannot be determined from the 20 thou test.
