I have 2003 Nissan Frontier w/ 4cyl. Auto. The engine shudders under light acceleration at low rpm. If I press the pedal on down or let off completely it seems to smooth out. I am getting a code for " cylinder # 1 misfire". I replaced plugs, wires, cap and rotor with no noticeable improvement. My mileage has went from 26mpg to 19 and I noticed the tail pipe is sooty black. Any hints would be appreciated. Thanks.