MOT No. 7
6-cylinder engines are equipped with automatic control of the engine speed in idle mode. There is no adjustment bolt, so there is nothing to adjust. The idle speed is simply checked. Under the right test conditions (more on that below) the following speed should be set to 680-820 rpm (2.6 liter engine) or 700-800 rpm (2.8 liter engine). Otherwise:
Check all elements of the intake system (piping, gaskets) for tightness. Penetrating unaccounted air can change the idle speed so much that even the idle speed control valve cannot compensate for it.
Interrogate the memory of the fault memory in the workshop. Maybe there is another problem.
Verification conditions
The conditions for checking the speed in idling mode are the temperature of the engine warmed up to operating temperature, as well as serviceable injection and ignition systems.
Next, you need to turn off all consumers of electricity in the car.
The cooling fan should also not be running.
Remove both crankcase vent lines, plug hose ends.
Turn off the air conditioner.
Set the automatic transmission shift lever to position «N» or «R».
Warm up the engine to operating temperature (oil temperature min. 60°C).
The air filter element must be in a spotless condition.
For CO analysis, an exhaust device must not be connected to the exhaust system.
Exhaust gas analysis
At home, checking the CO content is usually not carried out, since this procedure is mandatory when passing the annual technical inspection. In addition, this requires a special device. The description of the work to be performed is checked only after the injection system has been brought into the so-called basic position using the VAG 1551 fault memory reader. What does disabling certain adjustment functions mean? (e.g. lambda regulation) to obtain reasonable comparable values.
Connect an exhaust gas meter to the right measuring tube at the front in the engine compartment. Ensure tight fit.
Start the engine and leave it to idle.
The measuring device should show 0.15-1.0% CO (2.6 liter engine) or 0.3-1.2% CO (2.8 liter engine).
On models with a 2.8-liter engine, the CO content can be adjusted using the adjusting screw on the air mass meter.
With a 2.6-liter engine, there is no possibility of regulation. If the measurement showed a different CO content, it is necessary to look for the cause using the fault memory.
The CO indicator differs from the normative
With different CO values, it is necessary to look for leaks in the air supply and exhaust system (a faulty spark plug increases the CO content by 1.5%).
Too high CO:
- Check injection nozzles
- Fuel pressure too high
- Faulty temperature sensor
Too low CO:
- Unaccounted air enters the air supply system
Note: Everything you need to know about the CO test, catalytic converter performance test, lambda probe test can be found in chapter «Mono-Motronic injection system». When troubleshooting, the lambda probe is checked according to the method described here.
Disconnecting the battery clears the idle speed correction factor for the injection system. The system must therefore slowly re-adjust to the correct speed, a process that takes several minutes. So don't immediately panic if the car suddenly doesn't work perfectly after changing the battery, you just need to drive a little until the regulation is completed.
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