My 2000 XJ8 VDP is working like a dream until this last week. It started to run rough on the freeway, went into Restricted Performance and engine light came on. Limped home (short distance) pulled out the code reader. Gave me codes telling me missfire rate exceeds emissions (1316), Cyl 6 missfire (0306), and Generic Ignition coil. I swapped coil packs between 2-3 to 2-2. When I got done, cleared codes and it ran fine. Thought it was maybe a little oil in the usual location (spark plug right below the filler).

Only did short drives that day. Next day, on the way to work, started again, only in 2-2. Ok, Coil is bad. Got a couple coils from a local Jag "gently used" place, also bought new plugs, since I was going to be there.
Replaced all the plugs on that bank, made sure all the springs in the coil packs were stretched a little more, cleaned the coil pack rubbers, and reinstalled it all.

I took it for a test drive up/down the freeway and felt I had the issue home free. Pulled into the local gas station to fill up, and it started missing AGAIN!!! Now the codes read: 0308, 1316, 0358, and 0460 (new fault). So initial indications are Cyl 8 fault, but something is happening when the engine warms up, and causing a bad signal. what should I check as the root of this fault. I'm missing my Cat already!

 

Well, lets see whats common... You had one misfire, then changed all the plugs. So did you keep the coils matched to the cylinder locations? If not, who knows which coil is now on number B4? Have you done any thing to wet the top of the engine? If not, I would get another coil and go after B4, unless you see something obviously loose or misconnected there. The other one sure does not seem to match the misfire problem, so I would reset and see how it goes.

 

Reset codes, replaced B4 Cyl Coil pack and error went away. It looked like a roving issue, and replaced 2 coil packs before it cleared. I kept the colis matched to the cyl's, so it was getting frustrating.