Question PC will not power on when 8 pin is plugged into GPU ?

[Keishi]

I'm building my wife a "new" pc using some of my old parts, all of which I know work. TheGPU I'm using is my old GTX 980 ti, which has a 6-pin and 8-pin power connector. Problem is when I plug in the pcie power 8pin into the GPU the PC won't boot. If I only plug in the 6pin the PC will boot just fine.

I have tried just the 8pin without the 6pin and still will not boot. All parts I know work and the PSU is brand new. I've tried resetting CMOS, reseating the gpu and the other PCI slot. Could it be something related to bios or chipset since I'm using a preexisting ssd with windows already installed on it? Any ideas?

PC:
Asus ROG b350-f Mobo
Ryzen 7 1700
32gb Corsair DDR4
GTX 980 ti
Corsair RM750 PSU

 

[--SID--]

You use the original PCI-e cables that came with the PSU?

Disconnect all storage and remove 3 of the 4 RAM sticks and try to boot to BIOS.

The 980 Ti is tested positive in another system?

 

[Keishi]

You use the original PCI-e cables that came with the PSU?

Disconnect all storage and remove 3 of the 4 RAM sticks and try to boot to BIOS.

The 980 Ti is tested positive in another system?

Yup using the pcie cables that came with the PSU. I've tried removing piece by piece including ram, that's how I figured out the exact plug that was causing the issue on the GPU.
Haven't tried the GPU on another system yet. That's my next step.