GPU not detected - Diag LED 7 OFF No video

Recently I bought for 300 PLN a mac pro 8 core from 2008.

I borrowed from my friend GPU - Radeon HD 6850 from Sapphire

Computer powers on 3 of 4 diag led are on but the last one that should be on (GPU) is off. I boot from El Capitan HDD.

GPU spins. I used even ASUS GTX 285 and there was still the same problem

I also tried Quadro NVS 295 but it was as the only GPU that was causing RAM error.

I noticed that after some time goes asleep.

Thank you in advance for help!