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Idle Air Bypass Adjustment

Idle speed adjustment is somewhat complicated, due to the sensitivity of emission controls and the effects of idle on emissions. Idle speed, ignition timing, oxygen sensor duty cycle, and idle mixture (% CO) must be checked and adjusted together. Check and, if necessary, adjust ignition timing first. See IGNITION. Then check and adjust idle speed, oxygen sensor duty cycle, and CO, repeating the adjustments until all four are simultaneously within specifications. For idle mixture (% CO) specifications, see Fig. 5-14 5.6 Idle Specifications (rpm and % CO). For more information on the oxygen sensor duty cycle, see EXHAUST SYSTEM AND EMISSION CONTROLS.

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.

When adjusting the idle speed, as a part of the procedure outlined above, the following requirements apply:
1. 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.
2. All electrical accessories should be off (including the radiator cooling fan-make adjustments only when the fan is not on).
3. Fuel pressure measuring equipment should not be connected.
4. The exhaust system must be free of leaks.
5. The oxygen sensor system must be functioning correctly.


If repairs have been made to the fuel system, raise the engine speed over 3000 rpm a few times, then let it idle for approximately 2 minutes. This will bleed any trapped air from the fuel system.

Make sure that the auxiliary air regulator is fully closed. The engine rpm should not change when the hose to the auxiliary air regulator is pinched shut. If the rpm drops when the hose is pinched, check the auxiliary air regulator as described below.

Fig. 5-15 shows additional preparations for idle adjustment. Pinch off the hose leading from the idle speed boost valve(s). Disconnect the crankcase ventilation hoses from the intake manifold and the air flow sensor. Remove the charcoal canister line "T" fitting from the air flow sensor boot and turn it to insert the side with the 1.5 mm (.059 in.) restrictor hole. If the car is not equipped with the "T" fitting, remove the hose and fit a restrictor plug with the proper size hole. Such a plug is available from Volkswagen, part no. 026 133 382D.

Connect a tachometer according to the instrument manufacturer's instructions. The ignition signal lead from the tachometer should be connected to terminal 1 of the coil.


Connect and disconnect the test leads only with the ignition off.

Idle speed should be between 800 and 1000 rpm. If not, adjust it to 900±30 rpm by turning the idle air bypass screw-clockwise to decrease idle speed, and counterclockwise to increase it. When all adjustments are complete, restore the hose and fitting connections described in Fig. 5-15 to their original configurations.

Fig. 5-15. Preparations for idle speed adjustment. Pinch shut idle boost valve hose (1), disconnect crank-case ventilation hose (2 and 3), remove "T" fitting (4) and reinsert restrictor. Idle air bypass adjusting screw is at 5.

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