Kia Amanti 6 cyl

Engine light was coming on and off. Took to KIA dealership and they could find nothing wrong. 4 days later happened again. I put Seafoam Additive in gas tank with full tank of gas. Drove fine rest of day and next day for about 12 miles , then engine light came on and car started misssing real bad and blowing white smoke. Made it home but when I tried to start it one hour later it would not start at all. had bad odor. Anyone have any ideas.