high temps Very high idle temps with AIO and 9700K

[MickDevv]

CPU: Intel I7-9700K

MB: Gigabyte Z390 M

PSU: Corsair RM750

AIOs: Corsair H100i, Arctic liquid freezer 240mm

Case: Corsair Carbide series 175R

 

I recently bought a second hand Corsair H100i from eBay. I installed it in my case in the place of my Hyper 212X (radiator always above the pump). Sometimes I got boot failure detected and was sent to the BIOS, and when the PC booted I was either greeted by idles of 70-90 degrees, or mid-40s, but when it was idling at the mid-40s as soon as I would use more than about 40% of my CPU it would shoot straight up to 100C with no overclock and an undervolt of -75mV. the BIOS only detected the fans that were connected to the pump block, never the pump itself. It also never showed up in iCue. The logo on the AIO sometimes turns red while running, but not always. I could see the fans turning and hear the pump working, and one of the tubes ran hot and the other cold. I thought there was something wrong with the AIO, so bought an Arctic Freezer 240 new and tried that. Same result, though neither tube was running hot even after a couple of hours of idle. I initially thought it wasn't getting enough power from the CPU fan header, so I plugged it into the SATA power using an adapter so it would get more juice and run at full speed. No better. I reinstalled both AIOs several times, to see if I was doing it wrong. Every time I installed the pump block, I put a pea-sized amount of thermal paste on the CPU and didn't spread it, letting the mounting pressure do so for me. Every time I removed the pump block the paste was spread over most of the CPU so I'm pretty sure the contact is fine. When I installed the block I also made sure to slowly tighten the screws in a criss-cross pattern. I have re-installed my 212X with the same thermal paste and my idles are sitting in the mid 30s, so it's not the paste either. I have absolutely no idea what's wrong with it.

 

(Please excuse terrible cable management lol)

Any ideas anyone?

 

[III-Argon-III]

To me it sounds like the AIO block had poor contact with the CPU. If the AIO was properly installed it would take a while for the temps to rise (a few seconds) even with a broken pump. However you're saying that the temps rise instantly.

Would it be possible for you to include a picture of the thermal paste after you've installed and removed your AIO?

[Heliian]

Do you have your pump 3 pin connected to cpu fan or sysfan? 

[MickDevv]

I tried it through the CPU fan header first and thought that it maybe wasn't getting enough power so I bought a SATA power connector to give it some extra juice. With the Arctic liquid freezer when it was connected to the CPU fan header I couldn't find any way of monitoring or setting the pump speed. It detected the fans that were connected to the pump, but not the pump itself though I could still hear the pump running so I knew it was running. The H100i RPM was visible through HWinfo but not in iCue for some reason even though I'd connected its usb cable.

Also here is the pic of the thermal paste after being removed from the CPU (I've reinstalled my air cooler for now because I need my PC for Uni)