My 1999 Ram 1500, with 5.9L and 51k miles was running just fine. It bought a new air filter for it, and while I was at the parts store, I thought I may as well get some spark plugs for it, seeing as the original spark plugs were still installed. Get home, pop in the air filter, and change the old spark plugs with a new copper core plug. Crank it up, truck will barely idle, and is knocking horribly. I shut it off immediately. Pull out the plugs, double check the gap, all was well. Put the old plugs back in, crank the truck, but the same issue. Truck was running fine on the old plugs before I pulled them, but now it will barely idle with them as well....same symptoms as the new plugs. What the heck am I missing.....any ideas on what could have happened? Only thing I did was air filter and plugs. Im stumped.