car has poor gas mileage and soot coming from tail pipe when starting

car has sat for two and a half years. my husband replaced the pcv valve gave it a oil change changed the oil pressure switch because low pressure plus it was leaking pressure at normal now.car had four codes P0455 and P0422 which went away after replacing poor gas cap. codes P0171and P0601 went away after replacing 02 sensor downstream bank 1 driverside along with changing the spark plugs which the passenger side were normal but the driverside had soot on them and looked burnt. car had numerous problems hydrolic pressure hose on power steering replaced along with the rear bearings and seals which locked up wheels along with brakes replaced as i said car sat. our problem is there are no more codes but gas mileage only went from 4.5 mpg to 6.6 plus it still has soot from tail pipe when starting and suggestions we are clueless fuel filter will be also replaced soon