my 190 was doing the same thing, but in my case it was the timing chain, replaced it, it was 3cm longer then it should have been and really loose. It almost always tapped on low revs and on a cold start. It's was one heck of a job to replace it, took two days. Now engine runs very quiet, but I gotta get a new thermostat and housing along with a tensioner bearing. Housing cracked causing a huge coolant leak, and bearing started whinning. All will be take care of after my exam tomorrow night he he.