Ever since ive owned my 1997 f350 with a 7.3L its had a knock in the engine. Ive been trying to figure out what it was, theres no real loss of power but i get terrible fuel economy. Im pritty sure its a injector now from the reasearch ive done. but im not sure which injector it is or how to change it out. how do i figure out which injector it is and can i change just the one or do i have to change out all of them. id like to go the cheapest way about this. thanks a lot.