alright it might be a bad egt sensor "exhaust gas temperature" i have a bad one right now and i get **** mileage and the black carbon on the exhaust tip is usually from a misfire or it might be from running rich. unfortunatley the egt sensor now only comes in a set including both of them and it will cost you about 500 dollars. good luck

 

you would have a code for misfires and a bad egt usually.