Car was running fine. I stopped at light, when I started driving car bucked, but then ran fine. The next day after driving for a few miles I turned off car. Everything was fine until I started the car. There was a very hard bucking, I immediately turned it off. White smoke was coming out of exhaust. I had car towed to mechanic. They changed the valve cover gasket. This did not fix the problem. When car was started a lot of smoke still came out of exhaust. We gave up on mechanic and drove 5 miles home. Again, when started there was a lot of smoke. After 5 minutes it stopped smoking. We now have it parked until we can get a diagnosis. Does anyone have any ideas what is wrong? I love my car! It is a 2008 with 114,000 miles.