For some time now I have heard that you can assess your engine combustion condition by carefully exams your engine spark plugs after a long highway trip or verify exhaust tail pipe for light tan color. I drove over 1400 km last week (all highway) and my exhaust tail pipe is totally black. I decided to check the spark plugs condition specifically the color of the electrode. All spark plugs have light tan in color from the center of the electrode to the top of the ground pin. I don't understand why my exhaust tail pipe is black ? The car is running fine. I lost a bit of engine oil but I think I have minor leak somewhere along the valve cover seal since my spark plug is clean.
Any ideas?