Alienware Aurora R4 problem

Hello everyone,

I hope someone can give an idea why my problem is happening because it's driving me crazy. I'm trying to fix a friend's Aurora R4. He gave it to me saying that it runs slow, it's very loud and shuts down by itself. I figured overheating, and that was the case. I read online a bit and a lot of people complained that the liquid cooling on this model has problem with the flow after some time and needs to be opened and cleaned inside. So that's what I did, i put thermal paste again, plug every cable at the right place and now the computer won't boot. When it was working, the Alienware logo would show, the bar under will start filling, and after that the PC will boot into Windows. Now, the first screen is the Alienware logo, but the bar under it is already full, the PC hangs like that for like 30 sec, the LED lights start and then the PC shuts down. At first I thought I broke the liquid cooling, so I got another one - Corsair H80i (read on the forums is better than the stock). result was the same. I unplugged the HDD, the video cards, nothing changes. I haven't reset BIOS, I'll do it today to see if changes anything, but I'm doubtful. I'm thinking it might be power supply as well, so thinking of grabbing one from work and trying it out, but if anyone has an idea what could be the problem, I'l lbe happy to hear any ideas. Thanks in advance.