Question CPU temperatures instantly at 100C after boot up, out of troubleshooting options

May 23, 2020
I've noticed performance issues in my gaming PC that have gotten progressively worse over time. A few weeks ago I checked the temperature and it was way too high, so I started trying to fix that. After exhausting a lot of options, nothing has improved, so I'm here to try and find help.

Things I've done:
  1. My PC has 2 fans, one under the PSU and one touching the radiator for the water cooling system. The radiator fan was dead so I replaced that. This fan is a temporary replacement I found in the house, with a new one being shipped, but I don't see any reasons why it should be causing problems.
  2. I replaced the thermal paste that came with my PC. It arrived this morning and I took all the proper steps to cleaning off the old stuff and putting the new in (cleaning alcohol, pea sized drop, etc.)
  3. I made sure that the water pump was working by hooking it up outside of the PC, and it also shows that it's running in bios
  4. The first three are the notable fixes, I've tried various other things found on the internet, mainly composed of different system scans
  5. Edit: I also made sure that the water pump was screwed down tightly onto the cpu, so that they should be touching
My computer is high end, I have a 1080ti and an i7 4790K, and it has always run fine in the past, so the fact that the CPU temperatures instantly go to max on start up is very odd. When I went into bios to double check the temperatures, my system temp was only 35C, if that's anything.

I'll seriously appreciate any help, I don't know what to do at this point!
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On point #3: just because bios shows that the pump is running, doesn't mean it's actually 'working'. If there is a clog, for example, the pump may be running, but the water isn't flowing.
Did you feel the cpu block, tubing, and radiator? The heat should be spread out, instead of being concentrated in one area, namely the cpu block.
You also could've temporary dislodged whatever was causing the clog when it was being run outside of the PC.