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6.7 Idle Specifications

Idle fuel mixture adjustment must only be made in accordance with differential pressure regulator current, and only after confirming that ignition timing and idle speed are within specifications. See IGNITION and 6.3 Idle Speed. For information on measuring differential pressure regulator current see 6.1 Air Flow Measurement and Fuel Metering.

If the equipment necessary to accurately perform these checks and adjustments is not available, we suggest turning the job over to an authorized Volkswagen dealer or other qualified repair shop. In a properly equipped shop, these checks and adjustments can be made quickly, accurately, and at reasonable cost.

Fuel mixture is controlled by the control unit and the differential pressure regulator. The system adjusts fuel pressure in the fuel distributor to adjust mixture in response to oxygen sensor signals. A small CO test pipe connection to the exhaust manifold provides a place to connect a CO meter. The pipe is about 8 in. long with a blue rubber plug on the end.

If necessary, a small idle mixture screw inside the air flow sensor, shown in Fig. 6-53, raises or lowers the position of the control plunger relative to the air flow sensor plate lever. In response to this adjustment, the system changes the differential regulator current to increase or decrease fuel delivery and correct the fuel mixture. The purpose of this screw is to adjust the system so that the differential pressure regulator is operating near the middle of its range. When properly adjusted, the idle mixture screw does not affect the actual fuel mixture (% CO remains within specifications).

The idle mixture screw can only be reached and adjusted after removing the anti-tampering plug in the top of the air flow sensor housing. This is a small plug with a soft aluminum top and a hard steel bottom, removed by drilling a 2.5 mm (3/32 in.) hole in the aluminum and using a sheet metal screw to extract the plug.


Do not drill through the steel bottom. Drill only about 3.5 to 4.0 mm (9/64 to 5/32 in.) deep. Do not allow metal shavings to fall into the air flow sensor.

Fig. 6-53. Cut-away view of air flow sensor showing idle mixture (differential pressure regulator current) adjusting screw.

With the plug removed, the adjustment is made using a long 3 mm hex wrench as shown in Fig. 6-54. Turning the screw clockwise lowers the differential pressure current. Turning it counterclockwise increases differential pressure regulator current.


Do not accelerate the engine or push on the adjusting wrench while it is in place. Doing so may damage the air flow sensor. Remove the wrench and briefly accelerate the engine after each adjustment.

Fig. 6-54. Engine idle mixture (differential pressure regulator current) being adjusted with 3 mm hex wrench. (Volkswagen special tool no. P377, order no. TPO 377 000 25 ZEL, shown.)

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