I fill the plastic reservoir to the full mark and about 300 miles later it is riding on the add line again. Well, I'm not going to invest in a new cylinder head for now because adding a little coolant once a month does the trick. The vehicle seems to run well enough as is (I do not see any signs of crankcase oil contamination). Just thought I would share my opinion on this all too common subject matter.

 

Coolant just doesn't evaporate - it is going somewhere. If it were me, I would be a little more alarmed and not driving until I found the source of the leak. Coolant mixed with oil = no bueno.

 

Assuming the worst catch it soon enough it is just a head.
Give it long enough and will end up trashing the lower end and now you are talking about a motor.
Probably be in your best interest to figure it out sooner rather then later.

 

Obviously the coolant is going somewhere. The reasonable thing to do would be put a pressure tester on it and look for leaks, including the heater core . If after that you're absolutely sure there are no leaks get an oil analysis to look for coolant in the oil.
Or you can wait and see if the engine blows up.

 

Both pressure testers I have tried will not close over the radiator cap and I don't understand the problem there (they just plain won't screw on).

 

Sadly...been there, done that. Heater core external leaks either smell like sweet antifreeze in the cabin or drips out the drain hose overboard and leaves a puddle. All other external leaks leave some kind of puddle. Buy a good UV flashlight on amazon...15 bucks. Antifreeze glows bright yellow under UV in the dark. Pull your dipstick, don't wipe it...and hold a match/ lighter under the oily end. Oil only does nothing...antifreeze or water spits and hisses

 

I already bypassed the heater core, so that avenue of coolant loss is now closed. I did find a small crack in the plastic coolant reservoir and a damp spot immediately underneath it. I was pleasantly surprised that Autozone sold the overflow tank and even had them in stock. I was pretty sure that was the mysterious, slow but sure, coolant leak (being the fact that I did not notice any decrease in performance). However, when the problem persisted even after replacing the cracked plastic tank, it's back to square one. Thanks for the above suggestions.

 

Unfortunately modern pressure tester caps don't fit the XJ radiator. I've encountered the same issue.