Thought I'd throw this to fellow members as I'm somewhat stumped! I have just completed a total top end refurb due to head gasket failure on my 2000 XKR. In summary I've replaced all gaskets, seals, timing gear, knock sensors, all hoses, had the supercharger rebuilt, replaced heads (due to a machine shop ruining my originals!), installed custom stainless 400 cell cats, new 02 sensors (all 4), this has been a thorough and complete refurb!
That being said, other than a compromised head gasket on A bank, cyl. 3, all systems were functioning with no CEL.or DTC's. Upon reassembly, the engine fired immediately, and runs quietly with no leaks. As I allowed the engine to idle and warm up, I got a restricted performance and CEL. My iCarsoft LR V1.0 reads a P0300, and P1316, showing misfires on cyls 1, 4, 5, and 8.
I've swapped coils several times, installed new NGK IFR5N10s, oil and coolant are at perfect levels and circulating properly.Compression test shows 145 to 150 across all 8 cylinders, leakdown is less than 4%! Plugs in all cylinders are firing, but plug readings show cyls 1, 4, 5, and 8 look to be lean.
My question is what commonality would those 4 cylinders have, as 1 and 4 are both front bank A and B, 5 and 8 are both rear A and B? There are no misfires on all 4 A and B center cylinders! Any suggestions on what to look for next are more than welcome!
Thanks,
BK

Beautiful car!!

Misfires are very often caused by vacuum leaks. That'd be the first place I'd start.

Following that, low fuel pressure will also cause lean conditions and misfires across several cylinders.