Fuel mileage drop. Consistant 21mpg dropped to 17mpg in 30 day time. Have no codes listed.Have replace fuel filter, air cleanr, spark plug&wires, coil, cdc, mass air flow and run numerous injector cleaners through system. Injectors sound like running correct. still 17mpg. No fuel leaks found any where.runs smooth and briskly. checked brakes for drag, tire psi ok.passed emission inspection same #'s as yr. before.Could an injector be bad and is any way to check short of removal? Any other possible areas to test?