It is either the drip line on top or the injector is leaking where it screws together BUT I'll BET if you just bought it it was leaking and they fixed it or it is the valve cover leaking. Fuel CANNOT come out of the head past the copper washer on a 3500. Been there many times. It is leaking down the injector and coming back up. MAke sure it is diesel and make sure it is not coming from above. If one repairs a leak it will take an hour or two to 'boil' out the hole, now if you have compression coming out of the tube, thats a different story....