So here is an order of events as I’m trying to figure out why I’m having hesitation problems in my idling.

Replaced all 4 oxygen sensors
them fuel pump went bad , so replaced fuel pump and fuel filter. When I road tested it, ran great. No hesitation, fast off the line.
Next day, I got in car and started driving it and started getting a hesitation at idle like a vacuum leak. Then I would get restricted performance popping up occasionally. I ran the codes and it said possible EVAP problem so I retraced all my vacuum lines and electrical from the gas tank. Electrical good and I also did a smoke test and no vacuum leaks. I had also replaced the coolant sensor and the MAF sensor.
my code PO453 went away but PO174 and PO171 popped up. I did a fuel pressure test off the stradler valve and it’s 35 psi at idle. When I blocked the vacuum line that goes to the pressure regulator, it jumped up to 41. When I shut off the engine, the pressure would go from 35 to 40.
i’m thinking it’s the fuel pressure regulator but open to ideas.
This is very frustrating because I haven’t had any problem since I’ve owned it since 2019.
Also I changed my fuel filter before the fuel pump went bad. I was thinking longshot that maybe some sediment got into the fuel filter after I changed the fuel pump.

 

wrong sub-forum sir
you probably want this one: https://www.jaguarforums.com/forum/xj-xj8-xjr-x308-27/

 

Noticed something else this afternoon, When I take off the vacuum line off the Fuel Pressure Regulator, the idle smooth's out. When I reattach it, the idle starts surging again, very slightly.

 

YOU ARE STILL in the wrong forum section!!!!!