1st Gen R53 Cooper S Mini overheated 255-260F NEED HELP

My mini overheated on the way up one of the hills in branson in a lot of traffic. Temps got to 260 before I killed it. A T fitting below the reservoir had broke, dumping majority of my coolant. Changed it and then checked fluids out topped up the water and ran the car and watched the coolant temps go to 198 then to 190 and bounce up and down as should but then I drained the coolant again since it was really just water and added pentofrost nf in there. When I open my expansion tank I don't see water squirting in from the side hose like it was when coolant temps were regulating properly.

I always thought that coolant cycled into that tank while the cars thermostat was opened or closed. See I just did a bunch of stuff to my cooling system. I added a new csf radiator, silicone hoses, heat exchanger, heater core, aluminum expansion tank, new oem expansion tank cap.

Could my water pump be at its end? Lately I have been slightly running hot at idle like 221 then down to 205 and back up. But the low speed fan doesn't come on til 221. It gets better as I begin to take off. I am trying to figure out if a bad thermostat could cause my coolant to not flow back into my expansion tank at that top inlet from the side of the tank. And if my water pump is not dead. Just seems weird the cars coolant was flowing perfect when I had first fixed the T and once the car hit 195 the thermostat opened it got to 198 then dropped to 190. Then I drained all the water and added that pentofrost mixed 50/50 with distilled water and topped it off with the same bleeding procedure I used before. I opened the tank cap and the upper hose bleeder screw. Once it got to 195 the upper hose began to fill and the tank was never empty. But never did I see coolant consistently spraying into it. Just little squirt here and there like it had air in it.
I don't know I am puzzled and need second opinions on what to try before I throw parts at it and lose sleep over it. Thank you in advance!

 

Also forgot to add that my oil level was great right under 5 quarts by a hair with no coolant contamination at all. Did have coolant in there before but that was before I swapped my cracked heater core. And since have no longer had coolant in my oil. The car also starts and runs fine. Sounds the same. For the last 3 months or so I have had a slight soft tap in the motor which I couldn't isolate. It's almost like a lifter tap but honestly not nearly as loud as I've heard before. You actually have to stand by either of the car doors to hear it. You can't hear it from in front of or above/below the motor. Might be my dsc. But that probably isn't my water pump so it's another issue unrelated but just in case I wanted to include the detail. Also the only thing I half butted on was the dang thermostat went with an eBay one. I can get the part brand for you if necessary but I believe it was an oriellys brand. I do have a spare same brand as the one I got from eBay thermostat which is a 165 I was planning to use for summer since I race. But read that it is just too low for our cars to run right.

 

And the upper radiator hose would never fill up until the motor warmed up which pelican parts claims it should fill up that hose as you add coolant.

 

Drove it around town and she was fine. I am going to pull the sc and do the water pump for fun. I did finally find the second bleed screw. I popped it and water came out so I topped up and took her for a spin. She regulated from 193 to 197 while moving and up to 203 at a short window of idle at a light. So I picked up royal purple water wetter. Going to pour 2 oz in and get me through the week til next weekend when I have all my parts and can pull the sc. Figured it's probably time for a sc oil change and a new water pump while I'm at it. The spare thermostat I have is 180F and made by motorad. Same brand as my 195. I will also install the bmw T fitting since the one I put in is just an oriellys fitting.

 

I am at 264k miles. Previous own said they had put a rebuilt sc on so mine now has about 80k.

 

Well the fan comes on at 221 F degrees which after some research is right. I don't think the fan plays a roll in controlling temps until then.

 

Thanks for the reply mcs02. I did just install a gates belt tensioner. I thought the idler pulley was built into the tensioner? Idk but I replaced that whole unit. A new belt would be a good idea. I will do the crank pulley after I do the sc work because I wanna go to a 2% od. So you all know that excuse we give ourselves to let us buy car parts? Well I jumped the gun and bought an ns1 cam. I figured a good way to punish my mini for acting up was to give her more power haha. I want to try to do it by locking the motor up with the brake and gear and not have to use a cam gear lock tool. All I ordered was the cam. Do I need anything else? Should I replace like the cam sensor o ring or something like that? How about the cam bolt, is it reusable? Also I just ordered a new water pump oem and 2 o rings. Already have supercharger oil. Does anyone know where I can get the other gaskets I'll need for it?

 

I have the cam lock timing tool so I can lock the gear in place and only remove the rocker arms and rails then cam journals. It's pretty straight forward job I looked under my valve cover before. Any recommendations on assembly lube? I want to be able to get it from o'reilly's.