I had a similar issue with over heating when I did my coolant flush. Like Sal mentioned, on the upper radiator hose where it goes into the engine there is a hex screw. For future reference (I found this out the hard way) you need to take that hex screw out and fill it with fluid until it comes pouring out. If you dont do that the coolant wont circulate properly and do exactly what your XK did. I did the rear heat/ac delete so I dont have to worry about ripping my lines off!