Hi Guys. I've owned a 99 XJR for about 18 months now. I have put about 17K on it for a total of 85K. For the last few months it has been running a little rough at random times, like it will be idling rough at a stop and seem to be less responsive and then the next day it will be back to smooth idling and strong running. I have replaced/rebuilt a lot of parts including timing chains, hoses, belts, pulleys, water pump, intercooler pump, MAF sensor cleaning, new air filter, supercharger rebuild and new seals as well as a new fuel filter. I also used some Techron fuel system cleaner and change the oil and filter religiously every 4K with full synthetic. When it idles rough it is the same whether it is in gear or in neutral. It isn't throwing any codes.

Just curious if there is an obvious issue like the EGR valve or Supercharger bypass actuator that may be the culprit. I also suspect the throttle body and have replaced the vacuum lines and tanks from when the cruise control was acting up, no further problems there. Another note is there seems to be a little soot in the exhaust pipe when it runs rough, chalky not oily (the car exhibits no symptoms of blow by and has good compression.). This seems to be a sign of running a little rich at times.

If anybody has experienced similar symptoms I appreciate any feedback, thank you!

 

I would suspect the throttle body. have you cleaned the connections?