2004 Dodge 2500 2 wd, started to loose power on the interstate, when check engine light came on. Limped to Auto Zone and had code read. Showed bad batteries. I replaced them with new ones. Still has problems when accelerating. I have to push peddle slowly or it will not go faster. I am changing the fuel filter tomorrow. Do I fill with diesel before I replace the filter cap or will it fill when I start up?