With my wife's '91 190E 2.3, I had what I assumed were problems due to leaking valve stem seals - rough starting with misfiring and burned-oil smell, usually not so bad once underway and warmed up. Eventually, it got much worse - misfiring frequently and rough running. I pulled the plugs, expecting to find at least one completely oil-fouled. Instead, all were very clean and normal-appearing, except that #4 was completely missing the center electrode insulator. I suspect that the insulator was initially cracked deep inside, causing the misfires when cold, and would not have been obvious and thus hard to diagnose had I pulled the plugs at that point. BTW, engine runs very smoothly now with no misfires, no oil smell. A clue I missed was that oil consumption had always remained low.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7