We all know the only thing an XJR6 cannot pass is a gas station.
However, mine is drinking excessive amounts of fuel. Spark plugs look sooty (NOT due to oil consumption).
Wallet is looking anorexic.

Suggestions on possible causes?

Thanks

 

The engine regulation ( mixture or short term fuel trim ) must go into closed loop by seeing the engine coolant temp reach 165 degrees C abouts

Try changing the very cheap ECT sensor that can be calibrated against a chart on page 78 of the 801s doc

801S TITLE

If you ever change the thermostat , never install the large O - ring even if it comes with one , cracks the casting ears

 

Can you read the LTFT from the OBD2?

It will help diagnose your issue...

When is the last time you changed the oxygen sensors?