99 XJ8 with 88K miles.
I've had a restricted performance yellow light and message briefly come on when the car is warm and under power for about a year now. Never lasts longer than a few seconds. Never triggers an engine code. I keep waiting for the problem to get worse so I can get more clues. Just doesn't seem to want to happen. What would you guys do with this? Dont feel like replacing parts blindly in hopes I find it.

 

If the car crank but doesn't start it may be the fuel pump check the fuel pressure.

 

Sounds like a vacuum leak, after-all, she's 20+ yrs old, there's bound to be dried lines....hows the performance?

 

It could also be a coil which is beginning to fail. In my case, driving with a scantool connected allowed me to search pending codes, which were only present during the failure. Of course you may not wish to be a distracted driver so it's best to have a passenger looking at the data. At first I got a cyl. 6 code, then after replacing that coil, I got a cyl. 8 code. I am waiting to replace them all when funding permits. You may wish to check fuel trims as well; this could also be a fuel-related concern, in which case codes may prove unreliable.