Good afternoon guys, interesting problem with my 94 V12 XJS. Ps. do not look at Temp gauge, due to angle of the camera it looks incorrectly.
Couple issues:

1. With Hot engine (Mostly) - idle is not stable (As seen in the movie) - it kind of stays on ±750 rpm but suddenly goes down to 500 or lower, tries to stall but then comes back to ±750. No vacuum leaks (All hoses and connections new), most of the sensors are new, throttle gaps are 0,05, new throttle bushings and AAV was recently refurbished (Maybe incorrectly?), all new fuses and switches.
2. FF44 is always there.. new 02 sensors, cleaned all Earths, new additional earth attached to AAV as suggested in bulletin, no exhaust leaks as well.
3. With HOT engine it's not possible to restart/start the car once more. Only after 2nd,3rd crank it starts up. While with cold (in the morning) it starts absolutely fine from half crank. (New battery, AC working);

Unstable Engine video

Overwise Car drives and runs smoothly and really strong.

Any help would be more than welcome.

Gracias,

 

Fluctuating oil pressure at idle is unusual. I can't determine if the two are synchronous from the part of the clip that shows both gauges. Do you see a correlation there between the engine speed changes and the oil pressure fluctuations?

 

I would suspect your coolant temp sensor, very inexpensive part and often worth replacing just because.

Here is a link on how to test it:

Testing the Coolant Temp Sensor

 

My experience with 6ltr cars is zero.

One only here where I call home, and I have never seen the bonnet open.

BUT

On the 5.3 cars the EFI Power Resistor Pack, located RHF corner on the engine bay on those cars, has a multi pin plug inserted from the bottom. The crud, along with old age, creeps in, and HOT starts are a mongrel.

2 hex screws hold it to the inner guard panel. Remove them, twist it around for access, unplug that Multi Pin, and clean the plug and the socket. I use Lemon Juice (the real stuff), and rinse off with WD40 or similar.

That simple process has revived many a V12 for me over the long years with them.
Another FREE thing.

Clean the face of the 2 Crank Sensors. Front one and Rear one. They are a magnet after all, and get very grubby, and that causes all sorts of havoc. Cleaning that sensor on my X300 (only has one) restored sweet running.

 

Regarding the FF44 issue, what brand of O2 sensor did you use?
In my case, I had the same issue, the sensor was replaced, but it didn't go away until it was replaced with a Bosch one. Apparently the other brand didn't respond quickly enough for the computer's liking....