Theoretically an inline six should idle butter smooth, the the requirement to maintain a stoichiometric mixture at idle for proper operation of the emission control system causes some slight idle roughness that cannot be cured.

MB service literature states that lambda equipped 103s will not idle as smoothly as non lambda verisons that run a richer idle mixture. Best idle is usually achieved at at A/F ratio of about 12.5:1, which is 20 percent richer than stoich.

Duke