Raman, you got the codes read, what were the codes? This will give us a fair amount of info as to what your problem may be. Just to say that the car is running rich can be a number of things. Getting a code like P0305 would be very helpful as that would tell me you have a problem with cylinder #5 and mixing that with the rich code would tell me that it is either a stuck fuel injector or a coil that is bad (most likely suspects). So, if you can, post the codes here and we can take things from there. Right now, there are too many possibilities as to what the problem could be.

 

 

Raman, P0174 is an indication that you have a vacuum leak on the Bank 2 side of the engine. If you open the hood/bonnet of the car and look at the engine, it will be the cylinders on your right side. As to what will cause this issue, the most likely thing is the intake gaskets. This is where blowing a little bit of smoke into the engine bay when the motor is cold and the A/C is turned off will allow you to find the vacuum leak. You want the fan to not be running on the car (hence why the motor should be cold and the A/C turned off) and then you blow the smoke into the engine bay in a limited area. Watch the smoke to see where the smoke is slowly wandering through the engine bay and then makes a sudden change in direction and speeds up. This will point to where the vacuum leak is.

By chance, what year is your XJ?

 

It's a 2011 XJL 5.0L, per his Personal Profile.