GPU running at 8x instead of 16x when Gen5 NVME SSD is installed in M.2_1 slot (Asus Z890 Hero)

[RavenMaster]

I just recently built a new PC with the Intel Core Ultra 9 285K and an Asus ROG Maximus Z890 Hero motherboard.

I have a Samsung 9100  Pro (gen5) NVME drive installed in the top slot (M.2_1) directly below the CPU and a Samsung 990 Pro (gen4) NVME drive installed in the bottom most M.2 slot (M.2_6).

I installed them in those slots intentionally so that my RTX 5080 would run at full 16x speeds since the top M.2_1 slot is supposed to have 4x PCI-E gen5 lanes dedicated to it, which are not shared by top GPU slot. And the 990 Pro in slot M.2_6 is supposed to be drawing 4x gen4 lanes from the PCH.

So why is my GPU getting gimped down to 8x? Is this a BIOS bug? I'm currently on BIOS version 2201 (latest) and my drivers are all up to date.

:UPDATE:

I've tried removing all the M.2 NVME drives completely so that only the GPU is taking PCIE lanes from the CPU. I've done a fresh install of Windows 11 on a SATA SSD and the GPU is still running at 8x. 

I then put the GPU into another PC just to rule out any chance of it being faulty. The GPU ran at full 16x PCIE 5.0 in the other PC.

It would seem the Asus ROG Maximus Z890 Hero motherboard is faulty - most likely the top GPU socket. I will return it to the store I bought it from.

[n0pslide]

How did u establish the x8 speed? GPU will reduce the link speed for power savings when idle. So go to BIOS and disable everything ASPM. While u at it, set the pcie slot bifurcation to PCIE X16 Mode.

Edit: Setting the bifurcation to x16 will just disable whatever devices might be cannibalizing the link speed if there are any without the need to pop em out.

[RavenMaster]

I established it was running at x8 in GPUZ and in the Nvidia control panel under System Information. It also was stuck at x8 in the BIOS and was greyed out. I even tried what you suggested but it was still stuck in x8 mode. Even with no NVME's installed at all with just a SATA SSD, it was still stuck in x8 mode. 

I tried putting my RTX 5080 in my other PC which has a X870E Apex + 9800X3D in it and my GPU ran at x16 no problem in that system.

Since testing, I have returned the Z890 Hero along with the Core Ultra 9 285K and exchanged them for another X870E Apex + 9800X3D. Intel has screwed the pooch 3 generations in a row now. Maybe they'll get their act together again with Nova Lake-S next year. 

[n0pslide]

Wait, so does the issue persist or what's the situation? I'm confused

P.S. Intel's been sh!tting itself since the 13/14-gen stability fiasko so I dipped a while ago too. Repping the 9950X3D gang now

[RavenMaster]

The issue did persist after testing so I sent the Z890 Hero motherboard back for a refund and exchanged it for a X870E Apex. Also returned the Intel 285K and swapped that for an AMD Ryzen 7 9800X3D. The graphics card now runs at x16 and the PCIE gen5 NVME now runs at x4 (both of them full speed). I've put my gen4 NVME into the DIMM.2 card. All is good now after switching boards and CPU's.

[n0pslide]

I agree - ditching Intel was hella solid troubleshooting (troubleRMAing) ✔️