About a year and a half ago I was having problems with no heat in my 2000 XJ. I flushed the entire cooling system and then did the heater core separately both ways. The top heater hose would get hot and the bottom one stayed much cooler at idle. Running the engine at higher rpms for a while would get both hoses hot, but the top one was always hotter. During this time I also replaced the thermostat.
I figured it was a bad heater core since coolant did not appear to be flowing through it. I had the heater core replaced. The heat seemed to work better. This was in the fall of 2010. In the summer of 2011 the radiator got a hole in it so I had to replace it.
It started to overheat a few days ago. My daughter was driving it and when it started overheating and she parked it. Since I've replaced everything else for various reasons (heater core, radiator, hoses, thermostat and coolant) I figured the only thing left was the water pump so I decided to pull it today.
As others on the forum have reported, there was almost nothing left of the impeller. I've replaced many water pumps over the past 30 years or so, but I don't remember seeing anything like this. It's been a while since I've replaced one, and in every case prior, they were replaced due to leaking, so it might be common. Not sure.
Here is a picture of the impeller...
After replacing it, everything is back to normal with my XJ.
To anyone having overheating, heating or cooling issues, make sure you check out the water pump even if it seems to be turning normally.