Dell 7810 precision Tower single CPU 685 W GPU Upgrade

Hi,
I have a Dell 7810 Tower with a single CPU (32 GB RAM) and the standard 685W power supply. I currently have a Quadro M2000 card on it. I added a second GPU (a TitanX) in slot 4, the PCI-e 3.0 16x (225W + 75W) slot but it doesn't show up in my list of devices, lspci. (I am running ubuntu 14.04). I am externally powering my TitanX, so it cannot be a power supply issue.

My question is, why does the second PCIe slot not work? Is it disabled by default? Do I need a second CPU for slot 4 to work? I check the forums and could not find any reasonable answer. And the system wiring diagram does not mention that I need a second process for slot 4 to work.

Any help would be appreciated.