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7.6 Idle Mixture Specifications (% CO)

For best results, ignition timing, idle speed, and idle fuel mixture (% CO) must be checked and adjusted together, and in that order. Final adjustment to idle mixture should be made only after idle speed is adjusted to specification. Final adjustments to idle speed should only be made after ignition timing is adjusted to specification. See IGNITION and 7.3 Idle Speed.

Making these checks and adjustments together is very important to both driveability and emissions 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.

To check idle mixture (% CO):
1. Remove the cap from the CO tap tube that comes from the exhaust manifold and connect an exhaust gas analyzer.
2. Disconnect the coolant temperature sensor harness connector as shown in Fig. 7-17 or Fig. 7-18.

Fig. 7-17. Coolant temperature sensor connector (arrow) on 1987-1988 Digifant II system (engine codes RV and PF) disconnected for test.

Fig. 7-18. Coolant temperature sensor connector (arrow) on 1989 and later Digifant II system (engine codes RV and PF) disconnected for test.

3. Raise the oil dipstick slightly (to vent the crankcase).
4. Disconnect and plug the crankcase ventilation hose as shown in Fig. 7-19, or clamp the hose tightly to close it off. Be careful that the clamp does not damage the hose.
5. Start the engine. Briefly raise the engine speed to at least 2100 rpm, and let it return to idle. Repeat this step four times to cancel the control unit's hot-start fast idle function.

Fig. 7-19. Crankcase ventilation hose disconnected from emission control valve (arrow) and plugged for idle mixture (% CO) adjustment.

6. Check idle speed and adjust if necessary. With the crankcase ventilation hose plugged and the coolant temperature sensor disconnected, idle speed should be 800±50 rpm. At idle, the CO reading should be within the specifications on the graph in Fig. 7-20. Notice that the correct value varies with altitude above sea level.

Fig. 7-20. Idle mixture (% CO) checking tolerance vs. altitude.

The idle mixture (% CO) is adjusted by turning the idle mixture screw, changing the amount of air allowed to bypass the air flow sensor flap. This changes the opening force acting on the flap, which changes the potentiometer signal to the control unit. Since the injector opening time (and therefore the amount of fuel delivered) is influenced by the potentiometer signal, the idle mixture is changed.

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 small plug is removed by drilling with a 2.5 mm (3/32 in.) drill bit and threading a screw into the plug so that it can be extracted with pliers.

With the plug removed, adjust the idle mixture using a 5 mm hex wrench as shown in Fig. 7-21. Turning the screw clockwise makes the mixture richer (higher % CO). Turning it counterclockwise makes the mixture leaner (lower % CO). Adjust idle mixture to achieve the correct % CO for the average altitude above sea level according to the graph in Fig. 7-22.

Fig. 7-21. Idle mixture (% CO) being adjusted by turning air flow sensor adjusting screw with 5 mm hex wrench.

Fig. 7-22. Idle mixture (% CO) adjusting specification vs. altitude.

Reconnect the crankcase breather hose to the emission control valve, and replace the dipstick. Check and adjust idle speed as described in 7.3 Idle Speed.


If the % CO drastically increases when the crankcase ventilation hose is reconnected, it is not due to incorrect adjustment. It is probably caused by engine oil contaminated by fuel due to short distance driving. Changing the oil is a temporary solution.

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