Started my 2001 XJ8 today and the check engine light was on. Threw the code readed on it and I'm getting,
P0300, random cylinder misfire
P0301 cylinder 1 misfire
P0302 cylinder 2 misfire
P0303, cylinder3 misfire
P0304, cylinder4 misfire
P0305, cylinder 5 misfire
P0308, cylinder 8 misfire
P1316, injector circut/ idm codes detected

Where do I start? Should I change plugs? Coil pack?

 

Is the engine actually misfiring that badly? Check the battery voltage for12.5 or more, better to have it load tested. Also delete the codes and see if they repeat.

 

I would second the battery check, do a hard reset and check for codes again. You gotta narrow it down a little more. And yes, is it really misfiring on 7 cylinders ?

 

I cleared the codes three weeks ago but then I was only getting,
P1316, injector circut/ idm codes detected and misfire on cylinder 7

 

On a lot of the jags P1316 is about severe misfires doing real catalyst damage. If it's that then fix those misfires ASAP.