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5.6 Idle Specifications (rpm and % CO)

Idle speed, ignition timing, oxygen sensor duty cycle, and idle mixture (% CO) must be checked and adjusted together. See 5.4 Idle Speed. Making these four checks and adjustments together is very important to both driveability and emission control. If the equipment necessary to accurately perform this work 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 at idle (% CO) is adjusted by raising or lowering the position of the control plunger relative to the air flow sensor plate lever, in order to adjust the amount of fuel delivered to the injectors for a given air flow. This is done by means of an adjusting screw inside the air flow sensor as shown in Fig. 5-25. 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.

Fig. 5-25. Cut-away view of air flow sensor showing idle mixture (% CO) adjusting screw.

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. Do not allow metal shavings to fall into the air flow sensor.

With the plug removed, the adjustment is made using a long 3 mm hex wrench as shown in Fig. 5-26. Turning the screw clockwise makes the mixture richer (higher % CO). Turning it counterclockwise makes the mixture leaner (lower % CO). Table h lists idle speed and % CO specifications. A new plug should be installed when the adjustment is complete.


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.


The U.S. government and some states consider it "illegal tampering" for the car to leave a commercial repair facility without the anti-tampering plug correctly installed.

Fig. 5-26. Engine idle mixture (% CO) being adjusted with 3 mm hex wrench (6).

Table h. Idle Specifications (CIS)

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