Are there known chronic problems with the OM 642 BlueTec? I was considering a 2011 ML350 with 113K miles. It checked out fine except my mechanic "thought" he heard some noise on the cold start. Warned me that he had 2 customers recently with timing chain failures (at 140K) resulting in ruined engines. Is this a 1 in a 100,000 occurrence, or is this a lingering problem for BlueTec's approaching 150K mark? Does a timing chain failure always result in a blown engine?