My van starts fine and runs but blows a very thick white smoke and does not stop

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At first it would smoke for a few minutes and then stop and run fine. Now, it does not stop emitting a dense whitish gray fog and smokes a little after you turn the engine off. It was running okay when I took it for a tuneup and when I got it back it started making a rattling noise and then chugged and stopped. After letting it cool down, I got it home and started it up the next day, it ran fine but will not stop blowing white smoke and leaving a black residue on the ground by the tailpipe.