Access to the idle speed adjustment screw
The idle speed adjustment screw is accessible through a recess in the bottom of the air cleaner housing.
Using a socket wrench to adjust the fuel mixture quality
1. Install the exhaust gas analyzer into the exhaust pipe.
2. Start the engine in idle mode. On cars with automatic transmission, set the selector lever to the PARK position. Check that all electrical consumers, including the air conditioner, are turned off.
3. The idle speed adjustment screw is accessible through a recess in the bottom of the air cleaner housing (see Fig. Access to the idle speed adjustment screw).
4. Turn the adjusting screw to set the desired idle speed. Increase the engine speed briefly, then re-measure and, if necessary, adjust the idle speed.
5. To adjust the CO content in the exhaust gases, remove the air filter housing, remove the mixture adjustment screw plug and reinstall the air filter housing.
[Article was obtained from the portal mercedesman]
6. Start the engine at idle speed, then increase the engine speed to 2,000 rpm for 10 seconds and return it to idle speed again.
7. Check the CO content. To adjust the CO content, insert the socket wrench into the mixture quality screw tube and press the wrench until the regulator engages with the adjusting screw (see Fig. Using a socket wrench to adjust the fuel mixture quality).
8. Adjust the CO content by turning the adjusting screw. Turning the screw clockwise enriches the fuel mixture, and counterclockwise leans it.
9. Increase the engine speed to 2,000 rpm briefly, then return the engine to idle and re-measure the CO content. When making adjustments, turn the mixture quality adjusting screw only half a turn each time.
10. Remove all control and measuring instruments from the vehicle and install a new plug on the mixture adjustment screw.