Jaguar XJ8 issues (possibly gearbox)- please help

That’s the unit on a ford auto box
Worth checking I’m sure your Jaguar Must have something like this ?

I hope it's a simple fix. It'll be interesting to see what it is. Hopefully, it's not too pricey

have you done a flush and change of the gearbox fluid and filter? Also check cam position sensors and i guess the idel control is part of the drive by wire on these so check the throttle bodies are not all cacked up and all vacuum lines are in good shape

The throttle body is about as clean as clean can get, I've made sure its not dirty as one of the first attempts to solve the issue when it first started. Also of note is that I cleaned the connection between the TPS as I know this can be a failure point. Yesterday, I checked the TPS using OBD live data but did not notice anything unusual. No spikes, or flat spots. However I did notice that at wide open throttle it was only showing about 80% open and so i tightened the throttle cable and went for a drive. It hasn't cured the stall but I do think the car is much more willing to kickdown/downshift now. I think the throttle readings might have been throwing the shifts off.

Regarding the gearbox oil. I want to change it but I also have heard that doing a complete flush can damage it. I know you have to drop the sump to remove the filter- could I just remove the sump plug, let as much drain out as possible (probably about half the capacity), refill, drive it for a week, do it again, and keep doing that until the ATF that comes out is clean?

Here is the latest update.

I've been really thinking about this and my opinion is that there may be numerous issues here. I'm wondering whether or not the stuttering and stalling is a separate issue to the limp mode and P1797 fault code.

Today, I drove the car about 30 miles. It was on the motorway so to be honest, I didn't notice any issues as I wasn't slowing down or speeding up enough to cause the gearbox to do much of anything. I was just in 5th gear the whole time. As I reached my destination, I plugged my scanner in to see what the live data was doing and was interested to see that almost exactly as I did that, the dash lit up like a christmas tree with the usual errors of ABS, Traction and Stability control error, and gearbox fault. OBD data revealed that the car was once again reading very high fuel trims (+25) on one bank, the other bank hovering around 0. Code P1797 was confirmed. It's something to do with the CAN.

It was at this point that I thought to check the MAF sensor. I found online the acceptable values at idle and fast idle and my MAF sensor readings were in-line with those. Just out of curiosity, I decided then to unplug the MAF and see what happens. Immediately, the car's idle dropped and it nearly cut out but it then corrected itself. Within probably 5 seconds, it was idling smoothly, no misfires, and no fluctuation to the RPM. I decided to give it a run around the block with the MAF unplugged just to see how it drove. The fuel cut off over 3k rpm caused by restricted performance mode meant I couldn't test the kick down but my initial impression was that the car felt much quicker than it has recently. Pulling away, it felt more responsive and while moving, it was quicker to downshift. It also didn't hold onto gears for too long when shifting up. Most importantly of all, I took it for a roughly 20 minute drive and found that it did not stutter or try to stall at all when coming to stops. I was braced each time I came to a stop but sure enough, every time it was rock solid and didn't give me any issues.

I've been mulling the information over in my head and the hypothesis I have come to, is that the stuttering/stalling is most likely caused by a vacuum leak. With everything plugged in, my understanding is that the engine is dynamically altering it's map based on the MAF readings, but if there is a vacuum leak allowing un-metered air in after the MAF sensor then the air/fuel ratio cannot be right, because more air is entering the engine than the MAF can detect. I'm thinking that unplugging the MAF has forced the engine to run on a default map which is not dependent on MAF readings, so any leaks would not have such a drastic affect on the engine's performance. The reason I suspect a vacuum leak is that the MAF sensor is confirmed working as per the OBD live data, and the fuel trims are telling me that there is a lean condition in one bank. Does that logic make sense, or am I wide of the mark here?

Something else of note is that I had my battery checked today and whilst the voltage is 12.7, the CCA were showing in the 500 range (the battery when new was rated for 800CCA). The battery does not struggle to start the car whatsoever, so perhaps someone with more knowledge could chime in on whether or not the drop of in cranking amps could be causing these electrical gremlins. It does make me think that the battery has seen better days and the drop in CCA might not be a culprit by itself but more of a sign that the battery is internally worn out.

I changed the maf sensor on the Mondeo mk2 I have
No difference, then I changed the switch sensor last week as my above pic
The car is now running like a dream autobox is working perfectly
And the response on kickdown is now very responsive
I had changed crank sensor, gearbox sensor
Full flush and new fluid before this

I can only relate this to trouble I had with my wifes E70 BMW X5 auto. Previous owner had fitted a cheaper battery and coded it to the vehicle. Unfortunetly the battery was wrong for the car. The battery died shortly after I purchased the car, I replaced like for like not knowing it was the wrong type (less CCA). Not long into the ownership I was getting injector faults and gearbox harsh changes with gearbox codes.
I investigated further which led to a bigger battery of the correct type. Result gearbox and injector issues went away.
Not saying that yours is the same but..........

I fitted the new purge valve. It was not a difficult job, the valve mounts to the firewall with 2 x 10mm bolts. It has one small vacuum line and 2 larger ones which are clipped on. The most difficult part was removing those clips, but I was able to do it with some perseverence. Total time spent was about 30 minutes. Over the last three days, I have been working and not needed to drive- so I stuck the battery on charge for the whole time. It showed as only half charged on the charger when I started.

The new purge valve has solved the hiss noise. I cannot discern any obvious hissing noises now. I drove the car this morning and initially thought "wow, it's fixed!". The car felt fine, it changed gears well, it didn't stutter or stall, it generally felt how it should. I parked it up at a petrol station to fill up and was pulling out of the garage and slowed down to a junction, and it stuttered. Sped up again, slowed, and again, nearly stalled. The rest of the journey it was doing the same old stuff- short shifting to 5th very rapidly, being very slow to downshift (I have to mash the throttle for it to shift down), and nearly cutting out every time I slowed. The only thing I did notice was that I had turned the heater on as I left the petrol station, so maybe the extra load was the culprit. Either way, very annoying as I had gotten my hopes up! I checked the fuel trims at the end of my journey and noted that the fuel trim on bank 2, which was +25, is now +19. It seems to me as though there is another issue at play with regards to the fuel trims. I listened to each fuel injector with a piece of aquarium tubing- all were ticking away heartily but one of them had a "wooosh" noise whereas the others didn't. I wonder if an o-ring has perished and that's causing an air leak and messing the fuel trims up?

Your dedication and patience are impressive! Hope this does the trick!

Nick