I recently replaced the cylinder head gasket for a mate's 300tdi (head needed some filling and skimming). A few days later it started to billow white smoke especially at low revs and on overrun. The vehicle's owner thinks that it could be dislodged valve stem seals. I really wouldn't like to remove the head again unnecessarily. Is his theory a logical one? Is there enough space between the stem seals and the spring cap to allow the seals to get dislodged?