Because idle speed and mixture are adaptive, routine checking and adjustment is unnecessary. There is no idle or mixture screw, and the adjustment procedure is greatly simplified.
Volkswagen specifies that the first step in any Digifant I system troubleshooting must be to check the OBD fault memory, and to find and correct any faults that may be indicated.
The fault memory should always be checked and any faults corrected before checking any of the basic engine settings such as air/fuel mixture. This especially applies to a car that may still be protected by any of Volkswagen's extensive warranties.
Do not try to adjust the sealed screws on the throttle body housing and air flow sensor.
When checking and adjusting idle speed and mixture, the following conditions are required:
The engine must be at normal operating temperature (oil temperature at least 176°F (80°C)). The radiator fan should have cycled on and off at least once.
All electrical accessories should be off (including the radiator cooling fan-make adjustments only when the fan is not running).
The idle stabilizer valve must be operating correctly. (It should vibrate and hum slightly when the ignition is on).
The throttle valve potentiometer must be operating correctly. (Idle speed changes noticeably when the potentiometer harness connector is disconnected and reconnected).
The exhaust system must be free of leaks.
The oxygen sensor must be operating correctly.
Connect a tachometer according to the instrument manufacturer's instructions in order to accurately measure engine rpm. The ignition signal lead from the tachometer should be connected to terminal 1 of the coil. Also connect an approved CO tester (SUN 105 or equivalent) to the CO tap tube on the exhaust manifold.
Do NOT allow terminal 1 of the ignition coil to be shorted to ground when connecting test equipment, as this will cause failure of the Digifant I control unit.
Connect and disconnect the tachometer test leads only with the ignition switched off.
Ignition timing should be within specifications, as described in IGNITION before making final adjustments to idle speed and mixture.
To check and adjust idle speed and mixture:
With the engine idling, check idle speed and mixture (% CO). Table t lists idle specifications.
Table t. Idle Specifications (Digifant I)
If specifications are OK, turn the engine off and disconnect all test equipment. If specifications are incorrect, adjust as follows.
Crankcase ventilation hose disconnected from emission control valve (arrow) and plugged for idle mixture (% CO) adjustment.
With the engine off, disconnect the crankcase ventilation hose from the emission control valve atop the valve cover, then plug the hose. See earlier Fig. 7-19.
Start the engine and let it idle.
Coolant temperature sensor connector (arrow) on 1989 and later Digifant II system (engine codes RV and PF) disconnected for test.
Disconnect the blue coolant-temperature sensor harness connector. See earlier Fig. 7-18.
Let the engine idle for 1 minute, then reconnect the harness connector.
Do not reconnect the coolant temperature sensor harness connector while the radiator cooling fan is running.
Turn the engine off and reconnect the crankcase ventilation hose.