Hi, my 2004 xj8 n/a recently developed a rough idle. It would start shortly after driving. It is very pronounced at a stop, but it also gets bad when driving up an incline. What's really got me stumped is that the CEL came on this morning, and the idle smoothed out. The codes were random cylinder misfires and something about excessive misfires affecting emissions. Why would it smooth out when the CEL comes on? I'm getting no restricted performance warning. Thank you!

 

One of your sensors may be bad and the vehicle is now running with default settings since it has determined the fault. There should be other codes - possibly Jag specific.

 

Only get RP when absolutely necessary. You can have lots of bad stuff before that (such as running rich & burning out cats).