Last year I purchased a used car Honda civic prosmatec 2006 but suddenly I noticed that it a lot of carbon soot is settling in exhaust tail pipe. My car mileage is also very poor i.e, 8 Km per liter or maybe less. Got its engine tuned from a local workshop, spark plugs changed with new ones, throttle body cleaned, tappets clearances readjusted & air filter changed but no improvement. I also inspected the old spark plugs and these were also found covered with carbon and only the center electrode was found partially red brown. No oil was observed on spark plugs. Then used STP injector cleaner twice in my fuel tank but again no improvement. Due to this high carbon soot I noticed that my engine oil is also getting black very very early i.e, after driving only 300 Km. Fuel filter not yet changed. Can anybody suggest me the real root cause of this problem. Khawaja Jodat Chemical Engr. Wapda town, Lahore. 03458437344