I have an 02 xj8 4.0 and it seems a little down on power, i tested fuel pressure and with regulator connected it is 31 psi and disconnected 42 psi, already changed fuel filter, is this pressure acceptable?
I doubt fuel pressure would have anything to do with “a little down on power.” Have you checked for fault codes?
Yes, there are no codes however it only acts up when warmed up and goes into failsafe engine mode.
This may be of help:What To Do To Get Your Jaguar Out of Failsafe Engine Mode - Cerrones European
Have you checked the brake pedal switch? Does the cruise control work? Check also the throttle position sensor.