Precision 7910 Owner's Manual/RAM chart online 

Sizes can be mixed as long as all other specs are the same, so I linked the RAM chart in case that helps.

Also try replacing the size 2032 battery if it's 5+years old.

Thanks for the info.  I'm not finding an error.  The sticks are same volts also and all ECC.

Do you have the latest BIOS that's for the 7910?

Note BIOS settings in case you made any changes and reset it using a new battery.  How to Perform a BIOS or CMOS Reset and Clear the NVRAM on Dell Computers 

claytonb just beat me to it.  : )>  Indeed, they all need to be ECC.

May be the 8GB sticks are bad or somehow incompatible?  What brand are they?

Tip:  KVR (Kingston Value RAM) can be hit or miss in Dell's.  Other Kingston's are ok.  Crucial is a top brand guaranteed to work in Dell's.

I’ve also tried filling all slots with 8gb sticks and it doesn’t work either. 

Yes. Dual cpu. 2 x E5-2686 v4. Everything works fine with 4GB sticks. 

Are they identical memory modules ECC ?

Are you running Dual CPU's

Ok but if I put a 4GB stick in Dimm 1 of each proc, it works fine. If I remove those and replace with a 8gb stick in the dimm1 slots, it doesn’t work. Is there a setting in bios somewhere to all modules greater than 4GB?

Micron. 4GB: 

MTA9ASF51272PZ-2G3B1QK

8gb: 

MTA9ASF1G72PZ-2G3A11G

Updated to A34. I think I’ll try resetting bios and clearing NVRAM to see what happens. 

Performed rtcrst, replaced cmos battery. Still no boot up with 8gb sticks. Put a 4GB stick in dimm1, 8gb in dimm2 (following logic from the chart in the manual), still nothing. This is a NK5PH. I have a 2TPVG coming. I’m not sure if that will make any difference. 

I'd blow the dust out of the slots on the mobo and use some isopropanol to clean the contacts.  Are the 8gb modules known good?  Or are they an ebay purchase? 

I think 2400 should work too, but am not 100% certain.  Manual specifies 2133, spec sheet (attached) mentions 2400.with dual CPU's.

Also helping another trying to get a 7910 going that was bought barebones that seems to be having RAM issues 

Yeah this is really weird. It takes 2400 in 4GB sticks, but hates 8gb sticks. I’m at a loss. I’m about to order a bunch of 2133 and call it a day. 

They are a eBay purchase, but the system is recognizing bad sticks by the constant hdd light. I found one so far. I have 8gb sticks in each of the channel 1 slots for both CPU’s. It powers up, no error codes, no constant lights. But the keyboard never initializes and I get no video. So I don’t know if it’s doing something internally that I can’t see.