I recently had to replace my V6 4.2 motor. With the new motor had a very rough idle and bad gas mileage. I also have a pretty noisy driver's side cat. I replaced upper intake gaskets and isolator bolts, and cleaned EGR ports. This drastically improved drivability, however I'm still not getting the best gas mileage as possible and I get an occasional sputter/rough idle when stopped at a light. I'm wondering if a faulty cat can cause the sputtering, bad gas mileage. When I say faulty, I mean rattling. I'm not getting any heat under my feet and don't hear any whistling and no bad smell. Pretty sure it isn't clogged. I just hear a metal on metal rattle. I also have not changed plugs and wires yet.
Am I on the right track?