iMac Mid 2011 27" black screen after cpu upgrade

Hi guys, I did upgrade from i5 2400 to i7 2600 for my 2011 IMAC 27" , they are both 95 TDP based on my research, however, after the installation, the screen went to black when I was trying to power it up, and after around 10 seconds, the fans were running on full speed without stop. And the screen kept black. I can heard the Chime sound every time when I started it up but the screen just kept completely black! Anyone knowing how could this still happens when the CPUs are at the same TDP?? And I have known that someone has successfully upgrade from 2400 to 2600 with no such problem what so ever. Really need some help here. p.s. I also added a SSD used OWC cables and upgraded to 16G RAM, but I don't think these upgrade could cause the above problem as I have replaced the original RAMs back on and also unplug the SSD which I added, but still not working......please help!!!