Checking idle speed and exhaust gas analysis

The Audi 80 with the 5-cylinder engine has automatic idle speed control. There is no idle adjustment bolt, so there is nothing to adjust. If the idle speed deviates, it is simply checked. Under correct test conditions (described below), the following idle speed should be set: 720 – 860 rpm.

If you obtain other values:

PROCEDURE

Check all intake system parts (ducting, gaskets) for leaks. Unmetered air leaking in can change the idle speed so much that even the idle air control valve cannot correct it.

Read the fault memory at the workshop. There may be another fault in the system.

Test conditions
PROCEDURE

The test conditions for idle speed are correctly adjusted ignition, engine at normal operating temperature, and fault-free fuel injection and ignition systems.

Next, switch off all electrical consumers in the vehicle.

The cooling fan must not be running.

Remove the crankcase breather hose below the distributor and plug the hose to the intake manifold (use the plug from the carbon monoxide sampling tube).

Pull out the oil dipstick and cover the dipstick guide tube opening with a rag.

Tightly close the hose from the throttle valve housing to the fuel evaporation valve at the top of the thick air intake hose with a clamp or pliers.

Exhaust gas analysis
CO content is not normally checked at home, as this procedure is mandatory during the annual vehicle inspection. In addition, special equipment is required for this.

At the workshop, the control circuit current of the pressure regulator is also measured, because the pressure regulator, like the idle air control valve, must be within the specified range during basic adjustment. If this is not the case, then despite the specified CO value, there is a fault in the fuel injection system (in most cases a leak, as already mentioned).

Test under the same conditions as for idle adjustment.

PROCEDURE

Connect the CO measuring device to the special test port at the front of the engine compartment. Make sure the connection is tight.

Disconnect the electrical connector from the lambda sensor.

Start the engine and let it run at idle.

The CO content test in the exhaust gas should show 0.1–1.1 % CO.

If necessary, adjust the CO with the adjusting screw.

Remove the plug above the adjusting screw as follows: drill a 2.5 mm hole in the plug. Screw a 3 mm self-tapping screw into it and pull out the plug with pliers.

Important: while the adjusting tool is engaged with the adjusting bolt, never depress the accelerator pedal. Always remove the tool.

After each adjustment, briefly depress the accelerator pedal before reading the CO value.

Note: for all information on the exhaust gas emission test, catalytic converter function check, lambda sensor check and throttle cable, refer to the chapter «Mono-Motronic Injection System».