Well that’s interesting! Autozone suggested it was the intake manifold. Hoping that’s not it. It may have also been some loose connections between the MAF and the intake tubing to the throttle body. I have a code scanner en route so ideally I’ll see it pop up before I get the RP again. I did not find any cracks in any hoses as I dis and re assembled.

 

Forget the intake manifold. Check the plastic air ducts on the intake system. They are old and any crack will cause these two codes. Look real close if you have any flexible sections with corrugations. It's very common to crack in-between the corrugations. Very hard to see! Might check the air box for cracks and make sure the lid is on right and is sealing.
If you can monitor fuel trims and spray some carb cleaner around you should be able to find it. But since it has not come back it might be a very small leak at this point?
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As for cracks in those corrugated vacuum hoses; the hose that runs from the left front valve cover, under the supercharger on my former XJR, and is then connected into a port under the throttle body, had been chaffing for some time and wore a hole in the hose. I found it by blowing compressed air into the hose at the valve cover end while plugging the hose where is attaches under the throttle body. Could hear the hiss..... ended up just running a rubber line from the valve cover fitting to the fitting under the throttle body. Your leak could be anywhere that these plastic hoses are connected. Check the little o-rings on the tubes themselves;. broken or missing is a problem. Also check the o-ring on the dip stick and the oil filler cap too. I spent a great deal of time looking for leaks on one of my cars until I finally realized it was the oil cap o-ring. Bought a new cap and finally ended the problem...

 

Good points!!
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An update on the "lean" codes - to the extent it was not the altitude (a comment I understand but correlation-wise I have a hard time buying, though I know it's likely true), it may have been one of two things - either a.) the MAF itself, which appears to be an OEM part, that I have since cleaned, or b.) the connection between throttle body and intake box. We had previously changed the air filter and I believe when we re-secured those two parts together (box, intake piping), there was a small gap. When I cleaned the throttle body and MAF, I made sure that connection point was lock-tight and also spent a little time gingerly but more firmly securing the collar around that junction. The code scanner I ordered arrived over the weekend - it did not show any hiding P0171 or P0174 codes, and nothing has lit up since on the dash. I have gone through about twenty drive cycles or more since that time, so I am cautiously optimistic. The code scanner I purchased is apparently junk despite the price, and did not give me a reading on the throttle figures. If anyone has any recommendations for a code scanner that can clear codes (just in case) and will read the throttle figures, let me know. I manually inspected the throttle cable and it seemed relatively taught but I would like to get that little item out of the way. I got discouraged when the code scanner didn't work and abandoned my plans to do the brake switch. The "Engine Failsafe Mode" message has remained off so long as the cruise control is not in use, but I will come back to that with the throttle cable in due time.

Yesterday evening we dropped the Jag off at the detailing shop. PDR guy found 10 very minor dents which he will fix for a few hundred, and then we are doing a three stage paint correction and a ceramic. The gent who owns the shop I use seemed very optimistic about what he will be able to get from it, and I will post photos once I have her back. Should be about a week. I also asked him to put a little patch of PPF on the body near the gas tank. Unless I am missing a secret technique I cannot for the life of me figure out how to put fuel in this car without the fuel pump handle resting on the body, itself. That's driving me crazy so I will have some film put on that limited area to prevent any paint scratching or flaking from the pump handle. Thanks as always for the recommendations on the codes - if it develops further I will begin down the long and winding rabbit hole to sorting that, in time.

 

I'm happy with my Foxwell NT530. About $160 loaded with one manufacturer vehicle specific info. Can add multiple other manufacturers for about $50 each. All kinds of live data. I bought it specifically because it can ID air bag faults.

 

If lean codes persist, best way to find is a smoke machine. I have a thread on how to make one cheap. Before I made it I tried spraying starting fluid and listening with a stethoscope with no luck. Also spraying starting fluid could be hazardous to your eyebrows. Smoke machine immediately found it. Leak around an injector.

 

I cut a piece of leather into a vague flap, added a hole to go around the filler opening and protected it with waterproofed. Seems to work fine. Got the idea from my older 911.

 

Sounds like you fixed the problem with that many drive cycles under your belt.

Yes the stethoscope method does not work on modern fuel injected cars. They react so quickly you just can't tell any difference - even if watching the RPM with a digital meter. I have tried it multiple times and I have fallen back to watching the short term fuel trims as the quickest and most accurate way to find vacuum leaks.
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Well, my assessment of my success appears to have been great exaggerated. I got her back from the detailer (looks absolutely spanking brand new, pictures later), and have been driving a bit since. Today, I was putting around town, and got the CEL again. Stopped by the local autozone, same codes. I replaced the battery, which my mechanic suggested I give a go. The old model was only a 2021, but I gave it a shot. Of course, no codes upon restart. Took it for an extended drive this evening just to make sure all good. While on the highway, it felt as though it was, in fact, running somewhat lean. Sort of a feeling that could’ve been mistaken for a lot of wind buffeting. RPMs were constant, but it was a strange feel underfoot. I got off the highway fortunately about a mile from home. Restricted performance came up as I slowed on the off ramp. Attempted to use an old (read: cheap) reader, and every light on the dash went off. Promptly disconnected it, and then only had restricted performance and CEL. The restricted performance cleared after circling the block, then came back on as I parked. Sigh.

Have a new code reader (the Foxwell) arriving Thursday. Going to see what that says, and then I suppose from there it is 1.) build a smoke machine, 2.) progress to O2 sensors, 3.) possibly do a fuel filter, or 4.) when my mechanic services the supercharger he will let me know what else lies beneath. Of note, the spark plugs probably haven’t been changed in about a decade (though only about 20,000 miles of use). That’s on the agenda when I get her to the shop, as well. Fingers crossed. While the 0171 and 0174 appear to be potentially costly they at least (for now, I’m praying) don’t appear to be catastrophic. Will update when the code reader is in, Thursday.

 

An update - code scanner arrived. The information that it was useful for - my throttle cable was at about 80-85%, so I adjusted that. The information it was not useful for (partly because I cannot understand it for the life of me, and yes, I've looked at just about every thread on here for tips and it's all just greek to me) is the fuel trims. Following a little bit of some other threads on here I did perform a hard re-set after checking the connections to the throttle body, the air filter box, in the intakes, and even got around to cleaning out the part load breather hose which was terrifying to remove. I was able to do that without breaking anything.

Presently there are no codes, but it feels like it is occasionally chugging a bit at about 60-70 mph. Hard to describe, especially because with the re-set the check engine light, restricted performance, and underlying substantive codes are not re-appearing. So I have no idea what I'm actually dealing with. Part of me thinks I should just buy a new MAF but at $400 from anywhere, that seems like a crazy spend until I know what the actual issue is. Need a professional on this one, given my abilities.

The LTFT seems to sit at 14.8 when the car is just sitting. It varies between 3-10 when driving. STFT varies but hovers +/- 4 mostly, when driving.

Going to let her sit mostly except to warm up the engine going around the block for now. Once it's sorted it should be time to go into hibernation. Should be a lovely, fully sorted car for next summer, but it looks quite lovely.

 

I'm 100% certain that you have a vacuum leak. A smoke machine (as suggested by others) is the best solution. You'll drive yourself crazy fiddling with a scanner, resetting the CEL, learning about fuel trims. You have a vacuum leak, and the smoker will find it.

Amazon sells them, or you can build one as I think someone suggested above, or you might find an independent mechanic that will charge you a few bucks to use theirs.

Just a quick thought before you buy a smoker: on the RH side of the supercharger gear housing, towards the back of it, there is a small vacuum port that has a small rubber hose attached to it. That hose slides off easily. Perhaps someone, sometime, accidentally knocked that hose off while doing other work or cleaning the engine. Take a quick look there first, it might be your problem. If that's not the problem, proceed with the smoker!

Edit: beautiful car, by the way!

 

Purchased the smoker. Will report back!

And thank you for the compliment! Here she is post paint-restore:

 

Great to follow along as you get to know her. Keep the updates coming

 

So, I ran the smoker today. First and foremost, need a new hose breather from the engine to the intake. Already had ordered one of those as a likely suspect.

SECOND, is this the hose?! See photos. First is of the hose, second is the general location. American driver’s side. Where the hell does this attach? It was blasting smoke into the engine bay, so presumably it’s supposed to be affixed to something. Those were the large leaks I found. Lots of smoke so I’ll have to start there and keep narrowing.

Anyhow, definitely several air leaks. Smoke machine was the move. Thanks so much, all!

 

Only hose that looks that way is the fuel pressure vacuum hose although it's intended location is pretty far from there. Anyway, locate the blower lid with the v8 logo. Find the large allen coolant port. Just behind it is a vacuum port. That line should go to the fuel pressure regulator...check and see of it isnt

 

So - it is connected there (“there” being the vacuum port at the supercharger). It is not connected to anything, on the other end. Tough to see anything that far in but I assume not being connected to the regulator would be a major issue with fuel trims? Now to find the actual port to which this connects.

 

Yes, that is problematic. It would cause fuel pressure to be high at idle and part throttle but dangerously low in boost.

The fuel pressure regulator is near the back of the passenger side fuel rail under the intake tube. Will be close to throttle body but can be replaced without having to take anything but the tube off

 

Separately, I went to move the car out of the garage this AM- kicked me a bit when it shifted into drive. Parked it and got the “Transmission Fault.” Scanner shows P0705, which after scouring the forum, looks to be the dual linear switch. Everything else underhand the shifter looked solid. Jagbits has one for sale for $625.00. I sourced one across the pond for $80. Not a part SNG sells, as they’re no longer produced.

Am not sure the hose at issue above is actually coming from the supercharger. Wiggled the hose at the SC and didn’t see any movement on the loose/unconnected hose. Further exploration next weekend, when I should have the breather hose.

I’m looking very forward to getting her sorted - starting to sense that as much as I enjoy the driving experience already, there is so much more potential to tap into. Onward!

 

Reporting back - see photos below.

Two items -

1. When I try to wiggle or get slack back with the (possibly) subject hose at the supercharger (or by contrast, take the loose end to see if I can reduce any play at the supercharger), I do not feel movement at one end relative to the other. This line appears to feed under the SC so maybe it is otherwise secured and can’t get play at either end. I would feel more comfortable if I could find the origination point of the unsecured hose, but it disappears into the engine on its end much like the possible-other-side disappears from the supercharger to underneath it. The hose, for whatever it is worth, is the same diameter and has a white stripe running down it.

2. I located a T-Junction (pictured) that had no hose on it. I am assuming that’s where this plugs, so I’ve done so. I have not started the car with this “mended.” Wanted to get an opinion on whether that’s the right place to put it. Otherwise, I have a T junction with no hose and this hose needs to go elsewhere.