I replaced my idle control module and “valve” to cure idle surging to 1200 RPM. I bought them off this site and I think they were for a 500SEC. The valve was aftermarket, not MB. I have an idle of about 600 RPM on the car’s tachometer. My idle is good but I have replaced the ignition control module (cured dying) and the fuel injectors which made the biggest difference in idle quality. But at certain temperatures the idle surges 50 or 100 rpm. If the car runs for 30 min and is up to temp it seems to run great but after an hour on the road it seems to surge but just a little bit. My mechanic thinks it runs very well. He also said the US 93 octane is still too low for the Euro car. One of the MB dealer troubleshooters said that these cars never idled very well. For best results change plugs, wires and fuel injectors. Vacuum leaks are sometimes the problem. These are all my amateur opinions. Good luck.

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Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si