Over here we call this an idle "surging" problem...

...whereas, the idle speed seems to fluctuate in some cyclical fashion when stopped.

I have been trying to sove this problem with my W124 for years, and just learned to live with it hoping that one day I will find the solution.

But most here claim that a vacuum leak will most likely be the problem. Check all vacuum hoses, replace those that are brittle, and look for any that have come loose from the fittings.

Also a bad MAS (Mass air sensor) could contribute to the problem. This is the cylindrical unit that resides between the air cleaner housing and the main intake.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car