I had this problem after starting up a defender thats been sitting for a few months, 19j 2.5 td engine. Last time it ran it was fine. Very little smoke from exhaust on cold start which clears up so nothing abnormal there. Oil looks fine and coolant is full and clean but there is a lot of smoke from the oil filler cap. Is this a head gasket or piston rings? I would think a head gasket would mean more smoke from the exhaust and some coolant loss.