Purging System With Leak Test Function

Purge System Function Requirements with Leak Test, General

The evaporative emission (EVAP) system performed the control when the following conditions are satisfied.

Stop the vehicle at idle
Shifting to D or R range (from TCM)
Blocking time after engine start elapsed (approx. 3 minutes) or self adjustment of the mixture formation occurs
Intake air temperature via charge air temperature sensor upstream of throttle valve less than 45 deg.C (113 deg.F)
Lambda control released
Coolant temperature over coolant temperature for engine start less than 100 deg.C (212 deg.F)
Air pressure more than 78 kPa (0.795 kg/cm ² , 10.87 psi) (which means that no test takes place above an altitude of about 2500 meters)
Low loading of activated EVAP canister
Fuel level in fuel tank between of about 1/4 and 3/4, signal (from BCM)
No malfunction on sensor element of heated oxygen sensor
No malfunction on sensor element of Air fuel ratio (A/F) sensor
No malfunction in EVAP canister vent control valve
No malfunction on fuel level sensor indicator
No malfunction in the EVAP purge control valve
Battery voltage more than 11 V

Purge System with Leak Test, General

The legislator requires that no fuel vapors reaches the open air.

The leak test for the evaporative emission control system must take place in three stages (function chain) and recognizes the following leak:

Gross leak (a leak the same or greater than a 2.5 mm dia.)
Fine leak (a leak the same or greater than a 1 mm dia.)
Very fine leak (a leak the same or greater than a 0.5 mm dia.)

Purge System with Leak Test, Function Sequence

The function sequence is subdivided into the following steps:

Function sequence for gross leak-fine leak check
Function sequence for gross leak-very fine leak check

Function Sequence for Gross Leak-Fine Leak Check

Function sequence for gross leak-fine leak checkTo check for a gross leak the activated EVAP canister vent control valve closes and the EVAP purge control valve opens. The vacuum in the intake manifold reaches the fuel tank. Vacuum then builds up in the fuel tank, and is measured by the fuel tank pressure sensor. If no vacuum builds up in the fuel tank (approx. -6 mbar within approx. 12 seconds) there is a major leak (e.q. tank cap open, hose line loose).The test will be interrupted and the error Gross Leak stored in the malfunction memory. If the ECM recognizes a system which has a gross leak, the fuel reserve warning lamp is actuated in combination meter. The additional message CHECK FILLER CAP is output in the malfunction display if a refueling volume of > 25 l is detected. Otherwise only the error Gross Leak is saved in the malfunction memory. While checking for fine leaks, a vacuum of about -6 mbar is built up again and the system is made air proof. The vacuum is them measured for approx. 30 seconds. The vacuum must not reduce more rapidly than 0.3 to 0.5 mbar per second (depends on fuel level). If the vacuum drops more rapidly, there is a fine leak. The test will be interrupted and the error Fine Leak is stored.

NOTE: If no leak can be found, although a Gross Leak malfunction is indicated (if the leak diameter is more than 2.5 mm, there should be a loud hiss), replace the fuel tank pressure sensor.

Function Sequence for Gross Leak-Very Fine Leak Check

If no Fine Leak has been detected, purging is briefly enabled and a vacuum of about -6 mbar is then built up again. In a closed system, the vacuum must not drop faster than 0.10 to 0.15 mbar per second (depending on fuel level). If the vacuum dissipates more rapidly, the Very Fine Leak malfunction is stored. The reduction in vacuum depends on the fuel level in the fuel tank and on fuel outgassing. Fuel outgassing is measured shortly before fuel tank pressure sensor is checked, and taken into account for malfunction indication. The activated EVAP canister vent control valve is reopened after the tests. The test for fine and very fine leaks is aborted if an excessive lean correction of the lambda control takes place during the buildup of the vacuum.The function of the EVAP purge control valve is checked at the same time by the activation. If the closed EVAP purge control valve jams, the Gross Leak malfunction is stored. If there are malfunction recognized, then these are stored by the ECM in malfunction memory and the MIL is actuated in the combination meter.