I acquired an early 2008 Mac Pro on Thursday last week and it is working ok. When I got it I noticed it was VERY dusty inside so I got my air compressor and blew it out. After that I took it apart and replaced the thermal paste under the cpus and the Northbridge. After that I have installed Mac OS Mojave and a few programs. I installed hardware monitor and noticed very quickly that for some reason the cpu heatsinks are running VERY cool but the cpu cores are not. I then installed smc fan control and ramped up the fans to 1000 rpm with no change. Also another odd quirk is the Northbridge core and chip temps are frozen at 63c and 38c never changing. I have replaced thermal paste many times and have never seen this happen so to rule out me putting to much paste I took it apart and checked my work and it had a great suction on the cpus so I am clueless why they are so hot yet the heatsinks are so cool. Even being a computer guy I can't figure this one out so I'm reaching out to y'all 
