I have a 2009 Jaguar XF Sv8 with 110,000 miles on it. I recently got a check engine light for an inefficient catalytic converter. When starting the car after its been sitting for awhile it knocks 3-5 times then runs smooth. It has a rough idle as well, bouncing between 500-750 rpm. No issues when in gear and no noticeable loss in power. Really not sure what the issue could be. Id appreciate any help! Thanks!

 

Nice to see more 4.2 supercharged popping up on this forum, too bad that all these cars are engineered to be scrapped after 10 years.

One or both of your cats are busted. This engine is vacuum leak city and if there is unmetered air getting in for too long, the fuel trim compensation will kill the cats eventually. Don't replace any cats until you find all the vacuum leaks unless you want to replace them over again, the most effective method is a smoke test. You want fuel trim as close to 0% as it will go, and test again after plugging every leak location. If the import specialist you are talking to doesn't know what fuel trim is, run away.

If the misfires continue after vacuum is tight, try cleaning the MAFs. If the engine runs smooth but still races up and down at idle, reseat the MAF plugs one at a time and it may fix it.

 

as litteredwithfaults says, the fact that its got a rough idle means the ECU doesn't know how much air is really in the engine and putting in the wrong amount of fuel. Most likely is a vacuum leak somewhere (which is common with this engine when it gets old), the hoses get brittle and don't seal properly. Could also be faulty MAF or MAP sensor. Do you notice if the rough idle is worse on a cold start than a warm one?

 

Littered with faults is most appropriate!