Canister Purge Valve Diagnostic

Canister Purge Valve Diagnostic

The task of the canister purge valve diagnosis is to detect a defective purge control valve. The purge control valve is checked with regard to controllability of the flow rate such as permanently open as well as permanently closed. In this cases purge control valve is detected. Minor leaks or slightly blocked valves are not detected if the valve is still controllable to a large extent. A check for absolute tightness must be performed separately or it can be derived from a possibly given canister leak test.

The diagnosis is used in addition to the electrical diagnosis. Provided the electrical diagnosis has already detected a fault, the canister purge valve diagnosis remains inactive. If the electrical diagnosis should not yet have detected a fault it will be detected by the canister purge valve diagnosis.

There are two possibilities for an OK check:
1. From active check at idle. A deviation of the Lambda controller from its value prior to opening, the purge control valve indicates that the purge control valve can be controlled and thus is OK.
2. If a stoichiometric mixture is coming there is no deviation of the Lambda controller.
a. Only the reaction of the idle control, which closes the throttle valve, can be evaluated.
b. Indication for an OK check is the decrease of the air mass flowing through the throttle valve
c. If the valve cannot close any further the ignition angle efficiency is worsened. This is also detected.

There is one possibility for defective purge control valve check:
1. If neither a reaction of the Lambda controller or of the idle controller can be observed during the active check by controlling the purge control valve open. Then the purge control valve can no longer be controlled (jammed at closed or open position), so the purge control valve is defective. The canister purge valve diagnosis is depending on lambda controller, throttle angle and ignition efficiency.

Monitoring conditions
To carry out the purge valve diagnosis it is necessary that:
- Ambient temperature is above -20 Degrees C
- Engine temperature is above +65 Degrees C
- Altitude is less than (or equal to) 4000 meters
- Vehicle speed is 0 km/h
- Canister load is below 3
- Condition for Lambda closed loop control fulfilled
- Critical misfire or limp home on velocity pick-up signal not detected

With the following errors the purge control monitoring can not be performed. These errors will therefore disable the purge control diagnosis and the MIL (and the corresponding fault code) will be set. The disable conditions are:
- Condition for fault type "implausible signal" detected in the DM-TL module.
- Error on DM-TL change-over valve power stage, short circuit to ground.