Having driveability issues with vehicle- stumbling, rough acceleration, poor fuel mileage. R & R Throttle position sensor, cleaned Throttle body plate, replaced fuel filter. Pulled several plugs- noticed they had black residue on them- they were a 4 electrode Bosch high tech plug, not standard plug. Have heard that these exotic plugs like Splitfires, etc. Do not work well in all engines. Could this be causing the issues with this vehicle?