Except for the 1990-93 Passat GL and CL using the 2.0L 9A engine equipped with the CIS E Motronic fuel injection system, the idle speed and fuel mixture are cannot be adjusted and are controlled by the Engine Control Module (ECM). The CIS-E Motronic system does allow the base idle fuel mixture to be adjusted, however the idle speed on these models is controlled bye the ECU and cannot be adjusted.
This adjustment should only be performed by certified technicians as it requires removal of a tamper resistant plug (hardened steel ball) in the upper sensor plate housing, which, if removed, must be reinstalled, a long 3 mm hex tool and the use of an approved exhaust gas analyzer. During this procedure the exhaust gas analyzer must be connected to the exhaust system before the catalytic converter and the Oxygen sensor electrical connector disconnected.
This adjustment is extremely sensitive, such that 1 / 8 of a turn clockwise (rich) or counterclockwise (lean) will make a dramatic difference.
Because the idle speed and air/fuel mixture are controlled by the ECM, routine adjustments are unnecessary. If the engine is not in proper running order, a faulty sensor or other component is usually the cause and should be repaired or replaced as necessary.