Of course the 3090 does produce much of heat. And if you don't have a full tower chassis, it's even worst. So one good solution is if you have an AIO or a custom loop liquid cooling that it doesn't provide extra warm air that it's combined with the 3090's and make things worst in terms of temperatures.
Even in my system that I am using an AIO and the MSI Suprim X RTX 3090, in the stock BIOS settings I can easily reach 90c with full stress. That's why I have made a specific settings that makes the CPU in full stress to reach about 80c. But the reason I chose a mid tower was because I didn't want a big case for my room environment at the moment and for aesthetics reasons too.
OMG THANK YOU FOR THIS. I didn't do all of that but just did TPU I because I am unsure about everything else. The idle temperature is now 40 C. Didn't know why it was sitting at 79+ C. Was so afraid I bought I dud 5950x.
In this case the results are surprising because the temps on the 5950X went up 10C just by adding the GPU into the loop. Even when the RTX 3090 FE is idle and drawing only >20W according to GPUz, I still see are large increase in temps.
That tells me that the restriction on the CPU portion of my loop is sufficiently high that I am diverting most of my flow to the GPU and affecting CPU temps. I am going to try setting up the loop in serial and see if my original results are restored.
Okay. Manifold is still in place, but now I go directly from the CPU to the GPU and the GPU back to the manifold and then out to the radiators.
Original temps on the CPU have been restored. Parallel CPU and GPU was definitely the cause of my 10C temp rise.
I think, at least in my case, the QD3 koolance fittings created enough additional restriction to force most of the flow down the GPU path. I didn't have a digital flow meter hooked up so I can't quantify how much went each direction.
But again, this doesn't really explain your problem. Now my system is in serial with an RTX 3090 in the loop just like yours. And my temps are far lower again.