So I'm 10 days into jag ownership. Driving down the motorway, decide to press the sport button and floor the throttle as another car has gone past. A HUGE clunk and car car carries on. Now the accelerator feels loose, it's not changing gear, I'm very worried. I drive another hour home in the same gear. Stop the car, use the manual gears. All good. Put it in drive, kind of seems ok, changing gears anyway. I'm not sure if its ok or I'm imagining fault. Quick 10 min drive and now I'm writing this. Is this a huge transmission failure??? Seems fine in the manual 2,3,4. Do you think I've lost a gear or had temporary limp mode??? Any options welcome. I didn't want problems this soon

 

I can relate to the "HUGE clunk" as my XK8 did that on throttle caused shifts from 5th to 4th gears. By 'throttle caused' I mean acceleration by foot as you describe or by speed control as in automatic downshifting to maintain speed going up a hill. Since I live in the mountains that occurred often.The transmission shifted fine other than that particular shift point. Turned out to be a defective transmission control module that was not releasing the torque converter lockup on downshifts. I learned to drive around the problem until I could find a replacement TCM at something less than the dealer parts counter price of $2005.

 

since I wrote the first post the gearbox now seems fine. I drove almost 500 miles in 2 days following that without a problem. I'm just a little worried this is the start of something big. Hopefully not, i'll provide updates if anything further occurs with this issue.

 

Check the infamous transmission electrical harness connector, located right next to the righthand side catalytic converter at the bottom front right of the transmission. They are notorious for leaking, and you can encounter a problem like you described if you fluid is low. If this is happening, it can also wick fluid up the wiring harness into the TCM case, which can make it act really goofy. This is what happened to mine, having similar problems to what you described. I replaced the connector and took off the TCM to drain it, and it was full of transmission oil. All my problems cleared right up. It was very obvius that it was leaking when I looked at it, but it never left any drips on the garage floor, so you may not notice it easily. That'd be the first place I'd check.

Edit: Ah, nevermind, thought you had an XJR, none of what i said applies to an XJ8 and it's ZF transmission. Disregard.