Injectors.

Usually when it's running you can see the leak (if it's big) from the general direction it's coming from. When an injector leaks, you can usually smell gas and if you're smelling a pretty big smell of gas you definitely are leaking. Mine has a leak cause I can smell gas with the hood down and walking past it. There is no easy way to tell if one is leaking BUT if one is leaking the others aren't far behind and if your XJ is throwing codes it must be a substantial leak. Mine is leaking and isn't throwing a code. Since the others are not far behind when you replace one faulty FI most likely one of the others is faulty or getting there so if you want to replace the FI you need to replace all of them at once. This is not a very inexpensive thing to fix. At Autozone the FI costs almost 50 dollars minimum EACH at least here in California and it isn't much cheaper at other Auto Parts stores. I would say Ebay or Amazon but I personally don't trust an anonymous vendor on an ecommerce website to give me the right part. Most vendors want it sold now and don't care if it's wrong for your car. If you don't know what part you need then the best way to go is an auto store but like I said it is EXPENSIVE. I chose not to replace my injector but then again it isn't throwing a code. So I have time to save. For you it'll probably be costly. When an injector leaks it's usually because the port is stuck open slightly and gas drips into the chamber then is expelled through the combustion exhaust. You aren't going to see very much harm BUT your MPG will vary slightly from it and your gas won't last as long as it normally would while the car is running. Hope this helps!