Luigi,
When I purchased my 88XJ years ago, it would occasionally blow a puff of white smoke out the exhaust on hot days. The head gasket would rise ever so slightly to release coolant into the combustion chamber and then reseal itself. Solution? I pulled the head, had it pressure-tested/resurfaced/reinstalled properly (i.e.used aluminium paint on block/head/gasket-an old hotrodder's tip) and no more problems.
When I bought my 00XJ years ago, I was constantly loosing coolant. I pulled the head, had it pressure-tested/resurfaced/properly reinstalled(i.e. used aluminium paint on block/head/gasket- an old hotrodder's tip) and had no more problems. Remember, when retorquing the head, one head bolt(IIRC upfront/driver's side) is torqued to a lower torque than the rest. Double-check a manual to make sure which head bolt gets the lower torque.
Best regards,
CJR