500SL really rough idle
 

I have a 1992 500SL 112,500 miles. All of a sudden my car has a bone rattling very rough idle when the engine is cold. It evens out somewhat after the engine has warmed up but still idles unevenly. It seems to run fine when not idling. Any thoughts? Thanks
JR

This seems to be a common problem among MBs, especially CIS cars. There can be a number of things from rusty fuel lines, to injectors to vacuum problems and the list goes on. Go to the Search button on this site (little blue buttons above) and type in rough idle and I bet you will get tons of answers!!! Good luck.

Rough Idle? Check for stored diagnostic codes.
 

The CIS injection control unit stores malfunctions in the system. Several discussions on this website describe how to retrieve the codes. They are taken from pin 3 of the diagnostic connector. The interpretation of the codes are as follows:

1 No faults in system.
2 Throttle valve switch (full throttle contact).
3 Coolant temp sensor.
4 Airflow sensor potentiometer.
5 Oxygen sensor.
7 TNA (RPM) signal.
8 Altitude pressure signal from EZL Ignition control unit.
9 Current to Electro-hydraulic actuator.
10 Throttle valve switch (idle contact).
11 Air injection system.
12 Absolute pressure valves from ELZ ignition control unit.
13 Intake air temp signal.
14 Road speed signal at CIS-E control unit.
16 EGR (Y27)
17 Oxygen sensor signal.
18 Current to idle speed air valve.
22 Oxygen sensor heating voltage implausible.
23 Short to positive in purge switchover switch over valve circuit (Y58/1).
25 Short to positive in start valve circuit (Y8).
26 Short to positive in shift point retard circuit (Y3/2).
27 Data exchange fault between CIS-E control unit and EZL ignition control unit.
28 Loose contact in coolant temp sensor circuit (B3/2).
29 Difference in coolant temps between CIS-Eunit and EZL unit.
31 Loose contact in intake air temp sensor circuit (B17/2).
34 Faulty coolant temp sensor signal from EZL unit.