I have a golf 140 tdi, when i start it on a morning it seems to fire slowly, the revs rise to about 800rpm slower than normal and then a cloud of white/blue smoke comes from the back. Once up and running its fine, no smoke, no loss of performance and no fault codes. Any ideas what would cause this, i'm suspicious the smoke is due to the slow firing. For the rest of the day itll fire up quick and smoothly.