I recently had the engine replaced on my 2003 Focus 1.8 TDCi after a bottom end failure. I recently got it back from the mechanic and when I start it after it has been sitting all night or day, lots of blue smoke comes out for ages after starting up. The car also runs very lumpy too. Eventually the smoke clears and the car runs great.

What could be the problem and is it an expensive fix as I am sick of spending money on this car?