It could also be the DPF doing a regen, or an injector has some crap on it. Hard to tell without knowing a little more detail. The answer to the following questions will help:
1) Is your car an auto?
2) What year model?
3) Does the smoke start straight away or is there a small delay (example: until the engine warms up) before it starts?
4) Have you tried putting a bottle of injector cleaner in the tank?
Let's see if we can narrow things down a little. It's always difficult trying to diagnose problems in text!
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