I know that the subject of the expansion cap hard to come out was discussed before, however my situation is a little different:
When forced out, the only thing that comes out is the outside cover of the threaded inside core which remains stuck inside thus unsafe to be pried out with the screwdriver.
As I found that my coolant disappeared in sudden large quantity (see pic 1), I had to force my expansion tank cap. Unfortunately only the outside covering cap came out, the inside core remaining stuck in the expansion tank neck (see pic 2). I plan to go back to NY soon and therefore, to fill the mysteriously emptying tank. What do you advise me to do: if I buy a new cap, what is the best to choose not to get it split like this one which came from Advance Auto Parts?
Thank you.

the disappeared coolant requiring filling up, I popped up the expansion tank cap

the only part that came from the expansion tank cap was the outside covering of the inside threaded core

 

When I was faced with the problem, I would drill a series of small holes in the center of the unit so that I could insert a flat metal bar or maybe a couple of holes that i could insert 2 screwdrivers. Then gently unscrew it from the tank. If you are careful you should be able to unscrew it.
Once it is out clean the interior of the threads, and I would apply a high temp "never seize" product before installing a new cap. Unfortunately it isn't unusual for the cap to separate. I have had to do it on my XJ-8

 

https://www.jagbits.com/product/MJA4440BA-OE.html

https://www.sngbarratt.com/us/#!/Eng...7-86f732df8a6a

 

Thank you very much for the addresses.

 

I could force it with the side of the screwdriver. Thanks for the tip with the "never seize": is it available at any auto parts store?

 

Yes it is.