MacPro early-2008 (8X), power led on, fans spin, no chime, no boot.

Hello fellow repairers,

I have a MacPro 3.0 early 2008 with 8 GB of ram and Dual 3.0 Ghz Xeon, Original GPU (don’t know the model, Nvidia without a fan).
It was running very well under linux (Xubuntu) 24/7, mainly for foldingathome project, so heavy load (it’s like mining on CPU but for science).
During the week-end it stopped working, and when i tried to access it, the fans were spinning at 100% speed but completely cold, screen was black, mouse and keyboard were not working, power led was steady on. So, I long pressed the power button and tried to reboot, power led come up steady, fans spin (low speed), HDD spin, 4 red leds flash once on each ram riser. Nothing more, no chime and screen black, the computer keep running like this forever.

So here is what I have tried so far :

• DIAG button: LED “ Trickle'“ Yellow, LED “PWR-ON” Green.
  So PSU is good.
• All Ram combination possible, one in each, two in each, three in each, …
  No changes at all, even without the rams the computer act the same, especially the power led don’t blink and all riser leds flash once at startup.
• RTC reset, Removing the cell 20 sec, put back in.
  Computer restart itself when power cord is plugged back.
• SMC-Rst button.
  Shutdown the computer if ON and no restart. if pressed when Computer OFF, CPUA and CPUB OverTemp flash red once. Shouldn’t block the booting process. But could be the core of my problem
• GPU on all PCIe is unresponsive. Tried an other GPU (Nvidia quadro FX1700), fan spin at full speed but nothing on the output. Tried without the GPU and same behaviour than with it. I will try the GPU in an other computer.
• Disconnect ambiant board : Fans spins at full speed
• Disconnect heatsink temp sensor: Fans spins at full speed
• Power on the USB ports (mouse light up during two second then off).
• Didn’t touch the CPUs for the moment.

I’m not a newbie in laptop diagnostics (even at motherboard and microsoldering level), but never tried a desktop one and especially an apple device. So if you have any advice on things I can try, just tell me. Also if someone have a link for the motherboard schematics, diagrams or repair manual it would be super usefull.
Thanks in advance for your time.