2003 Jaguar XJ8; 120,000 miles
I did some more fuel pressure testing.
42psi at idle with vacuum hose off of the fpr. 32psi at idle with the vacuum hose on. The tester was leaky, so I think the 42 is actually a 44 and the 32 is actually a 34.
I believe the reference readings are 44 and 38. Bad regulator?

Also, when I tried to depressurize the fuel system, I pulled the fuel relay, but the car was still able to run?
I pulled the fuel pump fuse instead and the car would stall, but still have enough gas to restart over and over. I was unable to fully depressurize the fuel system. Any thoughts on why this would be? Other cars, I pull the relay and the car starts, sputters, and then fails to start again.

To the main concern, I have included videos of the car while it's idling poorly. You can't feel the vibrations in the video, but you can see how the RPMs dip every time I give the throttle a blip. It's also hard to see but the lights dim every time the RPMs drop.
https://drive.google.com/file/d/1mpi...ew?usp=sharing
https://drive.google.com/file/d/1H4Q...ew?usp=sharing
Any ideas what would cause this?

 

Are there any DTCs pending?

 

no dtcs

 

In my experience once the fuel relay is pulled and the car stalls but may restart briefly is what usually happens. Those fuel pressure numbers are also pretty much correct as well. I'd not expect it's the fuel pressure but perhaps something else. On thing that has plagued me and I now pay attention to is the fuel trims that will be shown using a OBD plugin reader. These cars are very sensitive to vacuum leaks. High numbers are a sign of this. Leaks can come from the plastic line from the left front valve cover host or an intake manifold gasket, or throttle body not being tightened enough. Even the -0ring on the dip stick will l cause a vacuum leak. I had a bad o-ring gasket on the oil cap and spend an inordinate amount of time fixing anything that might cause the problem. Also, it could need plugs or perhaps a coil is weak. Had a weak coil that caused stumbling until I finally found and replaced it.
Just some suggestions; by no means is an exhaustive list.....

 

How old is your battery and have you measured V? off and running...Min 12.5V standing.