what power supply, do you have a voltmeter?
the system would not be running with only 8v on the 12v. if the system is running the numbers are most likely wrong.
when you enter the BIOS/UEFI what are the voltages displayed there if your BIOS is so equipped? you can test the output with a voltmeter. https://www.lifewire.com/how-to-manually-test-a-power-supply-with-a-multimeter-2626158