I drove the Jaguar in the Swiss Alps for a long distance down the valley. I blocked the transmission in the 2nd Gear to protect the brakes. The number of revolutions ranged between 1000 and 2000 revolutions per minute. When I got downstairs, I turned off the engine. Five minutes later, I left the engine going again. An ugly white cloud of exhaust fumes came out of the exhaust tailpipes. It smelled weird. After a few seconds the haunting was over. The engine always ran flawlessly. What can be the cause?

Valve seals comes to mind first.

Jack

Waater/condensation in the exhaust .it was steam not smoke

Unless it had a sweet smell, in which case it was coolant.

A bit of oil has accumulated in the combustion chambers due to vacuum effect of a long descent using the engine to slow the car, then at the first acceleration it was burned away, maybe with some condensation too, only for few seconds. If the engine has no oil consumption, there's nothing to worry about. I have a V8 4x4 that I use in the mountains, it does the same, the engine is ok. Never tried a long descent with the Jag, I bet it's the same.

Yep, this gets my vote.