The idle speed control valves have been off to renew the connecting pipes and bushes but I did not take apart the valve itself.

See: https://www.jaguarforums.com/forum/x...gasket-284048/

Now the engine starts right up and idles around 1200rpm. As it warms up the revs slowly lowers to around 850/900. It runs not very smooth, an occasional misfire can be heard. When applying throttle when cold it will rev to around 2000 rpm and then some sort of correction cuts in and lowers the revs to below 2000 and then the sequence starts over again.
When warmed up this cutting in will take place at 1200rpm so it self adjusts to go no faster than 1200rpm. Also when blipping the throttle the engine shortly hesitates. I used an Autel MD808pro and it says no faults are stored/present.
In a different post about the temp sensor on the LH thermostat housing a link was posted with the relevant temp and resistence values but that link does not open.
https://www.jaguarforums.com/forum/x...-codes-255760/

What can be the cause of all this?

 

A member on this site had the same issue. The throttle shafts/butterfy shaft was only loosely connected to the rest of the throttle linkage. You’d rev the engine, it would dump in fuel, but no extra air because it wasn’t a tight connection. There’s lots of pinch bolts between the throttle spindle and the actual throttle bodies… check them out. Confirm you are getting air.

when you’ve confirmed you are getting air, we can move on to confirming you are getting fuel. On that topic I could imagine throttle position sensor issues or MAP sensor… lots of electrical possibilities.

 

Finally the verdict is out. After much measuring of all the values that go into the ecu of the engine management it was concluded that the ecu itself was faulty. No water ingress, no moist, nothing visible. The ecu was replaced and engine runs like a dream. A bit high on idle though (1100 rpm).