Hi!
My MBP 15 was kind of unusable. Running very hot and the fans constantly spinning at full throttle. And of course, that meant the CPU was throttled and the MBP became very slow. Plugging in my external monitor meant the fans spinning up after several seconds, so obviously the dedicated GPU was also affected.
A quick look at what iStat menus is telling us:
Ok, so the CPU is near melting… I don’t believe the GPU temperature shown there (Radeon)…any GPU usage immediately puts the fans in turbo mode. Just opening Lightroom or any other program that switches form the iGPU to the discrete GPU makes the MBP sound like a jet.
So, what are the possibilities:
- runaway processes gobbling up the CPU: no. Not a software problem.
- restricted air flow: always a prime suspect. Computers suck in air and with it come lint, crap and dust
- heatsink problem: the problem is massive…I will take a look at this
After opening the MBP:
OK, some crap in the fans. Easily removed. But not that much too…let’s remove the heatsink (the “bridge” between the two fans):
OK…this is the problem!!! Apple, shame on you and learn how to apply thermal paste!!!
You see that there are areas where the copper is completely clean where it should contact the CPU/GPU. So, a layer of air was between the copper of the sink and the CPU/GPU. Obviously, that will lead to overheating. A common misconception is that thermal paste is an excellent heat conductor. It isn’t. Metal is an excellent conductor. And air is one of the worst. Thermal paste was one job: prevent any air between the cooler and the CPU/GPU. Any gap between them is a problem. So, a thin layer of thermal paste closes the gap and displaces the air. There is a lot of thermal paste around (!) the CPU/GPU, where is serves no purpose at all and too little where it’s needed.
So, step one: clean the crap away.
Then, apply a tiny amount of good thermal paste:
Just a small drop on each one (remember, it’s not a great conductor, it just needs to displace the air and close the gap). Set the sink on the CPU/GPU and wiggle it a tiny bit to set the paste between the copper and the dies. Then put everything back together. Of course, I cleaned everything before. 
Let’s see:
Success, substantially lower temperatures!!!
Now, I am writing this post, exported the photos from Lightroom, have an external monitor attached…a situation which 20mins ago meant fans going crazy and now they are just idling along. Silence! 
While 11° might not seem much, they make the difference between the CPU throttling and running at full speed. I did a test putting with a heavy load (export ~100 images from Lightroom) and the MBP is running way faster. No dreaded “kernel_task” slowing the system almost to a standstill due to heat problems.