The gas engines are very sensitive to vac leaks. I replaced ALL seals/o-rings/hoses under the air cleaner on my engine, but made the mistake of buying cheap air hoses instead of original parts, so while my idle is lower/better, its still a bit rough from some minor leaks of those hoses. I'll sadly be re-replacing some of those this winter to cure it once and for all. 
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-diesel is not just a fuel, its a way of life- 
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)
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