Thanks Bob! Just dropped it off and shared Kirby’s thoughts on the subject w/ my ever patient listening Jag mechanic… apologizing in advance of course! 
But as I told him, sometimes it’s the silly stuff that the experts aren’t considering.
I probably should not say anything here, as I know little to nothing about the 1992 set up, but on the 6.0L V12, none of what Kirby speculates is happening with the ignition control is described in the manuals. Ignition retard is only specified for hot air and on request by the transmission (which is clearly not the case for the 5.3L with the TH400). The idle switch probably also moves timing but the document it doesn’t say how.
I’ve asked before the 40 second timer functions and when does the ECU knows to switch from open to closed loop upon startup from cold and these threads should be in the archives.
Several items there need to be checked. That “Idle Switch” needs to be checked. Whatever that “Ignition Retard Request” from the transmission is needs to be checked. As I suggested before, the “Intake Air Temperature Sensor” needs to be checked. And this page doesn’t show the bad fuel jumper, but it needs to be found and checked.
One thing is certain: There is something wrong with this car. 8 MPG is not even close to a properly operating Jaguar V12. Finding a fault that would cut fuel economy in half yet the owner would still claim that it seems to be running well is the challenge.
Bob, yes in fact on all 6 of our vehicles I reset the trip odometer [an old habit] w/ every new load of fuel, in part out of curiosity, and in part because the last quarter of a tank [as indicated by the fuel gauge] seems to differ by brand and I’ve been known to push things to the last quarter of a gallon 
Mileage has been around the 10 - 11mpg range but this last tank was easily in the 8mpg range. Thanks for the feedback. ps It was cruising at 80mph yesterday down I-83 on it’s way to the shop… seemed happy, guzzeling away!
Kirby, printing all of these observations out, thank you! As I replied to Bob, it pulled quickly and easily up to 80mph on its way back to the shop yesterday, this going down I-83. Stay tuned!
Actually to date they’ve been weekend top down cruises in the rural horse country… very small roads where 30mph is the norm interspersed w/ a few larger two lane roads where 50mph works but until yesterday I’ve never had it out on the interstate where I had a chance to open it up a bit. Might just need a good run which is what it’s getting next Thursday (Montreal trip). Repeating myself some here but again, I’ve had the Jag since November of 2017 and only had time to put 1,500 miles on it.
Any chance you can get us a vacuum reading?
Bob, thanks again for the info… I’ve already made one stop past my guys shop on the way in this morning and left the previous suggestions with him. Fortunately he’s not a prima-donna and enjoys ideas narrowing down the possibilities. In fact, if any of you need parts he has a couple of XJS’s on his lot that could either use some tlc or parting out. I believe both are the V12 but pre-1992 (headlights/taillights give it away). Kirby, I’ll ask him about the vacuum reading… turns out his doctor called and urged him to hustle to the ER to have his “vacuum” checked today so my car will be there at least another day.
Oh, one other suggestion: He might check the status of the vacuum line from the crossover pipe to the ECU in the trunk. Leaks are bad, but it’s also been known to get clogged.
Important to note that we are dealing with two ECUs here… the Marelli ignition ECU in the right side footwell (US cars) and the Lucas FI ECU in the trunk. I think all ECU references in the pages I posted are to the Marelli ECU.
Thank you both… I’m loving this site! Kirby, I’m not smelling any gas but I’ll pass that thought on to him. Jim Kinkaid is my mechanic and he’s been working on Jags for some 40 yrs now… you may know him? Bob, I’ll relay as well, thanks!
Leaks in that line suck air in rather than gas fumes out. But it interferes with the vacuum signal which in turn can make the engine run rich.
BTW, a look at a spark plug might be helpful. Some of them are a PITA to get to, but a couple of them are pretty easy. You could just sample one or two of the easy ones and tell us what they look like. You might even post a photo here.
!. The LT1 is incredibly reliable able of many trouble free miles. OTH, one can fuss with it in the quest for more power and get it!!!
I like mine a lot. Pure luck lead me to it!!!
- Decades ago I read a lot on “H” and “X” pipes. Each can add scavenging. A good thing. Supposedly the latter better than the former. but, location is every thing. How, an interesting path.
Each does effect tone.
When I had the pipes done on my car, I discussed the cross over issue. He recommended the H. No real advantage of the X on my “stock” engine. Prettier and more costly.
3. Back in the day, guys split the manifolds on straight six engines. Isolate each three or have a balance. A hole in the plate that isolated them. Much difference in the noise
Back rap eliminated in most cases by the “hole”.
- When installing dual pipes on a flat head Ford V8, the “heat riser” in the block functioned as a cross over. Mellow tone…
Easily blocked. A penny on each side fits just right!!! Cold driving not as good. but the pipes took on a sharper note… My last project on headers, straight pipes and blocked cross over had a sharp tone. Delighted me… Even impressed cynical son!!
Carl
Thanks Carl, interesting stuff! Apparently no different than designing a musical instrument… clarinet to sax; trumpet to trombone or … alas my mechanic had a medical emergency and so my XJS sat in his garage the better part of a week, he calling me the morning that I was well on my way to Montreal… in the Benz. No reliability concerns w/ the Benz so peace of mind prevailed and it was a great 5 day circuit up through the Adirondacks to Montreal and back.