Thanks @NBCat, yes the fuel filter was also replaced. I don't see the ability to monitor fuel trim on the Torque app I use (android), but will research and see if there's a way I can...

 

Yes, long and short term fuel trims can be viewed on the Torque app.

 

My guess would be a vacuum leak, and there are lots of places to look for it.

 

If it's vacuum leaks, the fuel trims will show rather high numbers; generally the problem is on one back or the other but perhaps both. Many places to leak vacuum and these cars are very sensitive to vacuum problems.
Easy to check these things couple of things though; missing or loose O-ring on the dipstick and oil cap not sealing. After months of fooling with one of may cars, after replacing gaskets, o-rings, and hoses, I replaced the oil cap and ended the occasional DTC code. all the best....

 

Update - changed all O2 sensors and checked for vacuum leaks again, but still getting the misfires. Technician has diagnosed blocked or leaking injectors - he is suggesting rebuilt used to keep costs down. Is that generally a good option, or would people suggest its worth spending the extra for new OEMs if I can afford the extra? Could used ones likely cause issues in the future, or are a good reconditioned set practically as good as new?

Update: Actually, I have just seen the cost of new versus reconditiond... Definitely going for reconditioned!

 

Refurbished in Ebay $167
Genuine Denso Set Of 8 Fuel Injectors for Jaguar XK8 XJ8 Vanden Plas 4.0L V8 | eBay

 

My 4.2 is showing a p0206 code which is an injector code. Could you advise how the technician determined that the injector could be blocked or leaking as I have the same suspicion with mine but with the supercharger in place it does not seem possible to use a noid light to test if the injector wiring is at least working.

 

It was just through the process of elimination of all other possible causes of the misfires and rough idle I believe, as there was no injector code for me. Unfortunately, though, the reconditioned injectors haven't stopped the misfires or rough idle so will likely try a replacement ECU next...

 

have you solved the problem?

 

interesting info

 

Sadly not!

The refurbed injectors didn't help, and a week ago we swapped out the ECU but that didn't fix it either.

It drives perfectly until the misfire errors on all 8 cylinders appear. I can do 200+ miles without any errors some weeks, and other times I'll start the car, notice very rough idle, and get the errors straight away or within a couple of miles. It really does seem totally random. Almost has to be an intermittent (and hard to identify) vacuum leak - the shop has pretty much ruled out everything else at this point. The technician is kindly looking at it again this week so I will update you if there is any progress... If I wasn't in California I likely wouldn't worry but it won't clear the tests to pass a smog test whilst the errors are occurring so I need to eventually fix, or scrap :-(

 

Since you May scrap i would get copper ngk plug set from rock auto and maf sensor and new ebay coil set used denso are headache I run eaby for 2 years now works great
Also change the crank sensor if these didn't work then time to good bye