is it ok for the fuel filter to be black after say 12 months ? i have been told that it should not go black as this is a sign that the injector seals are worn ? i am asking this as i have had mine replaced over 18 months ago and am wondering if the job was done rite ? also cant remember if they used new stretch bolts on the injectors ? i did buy 03G198051A X 4 can any body tell me whats in this part number please ?