MacBook – Why are CPU fans running on the MacBook Pro 15″ full blast while the laptop has just booted up and waiting at the login screen

I suffered this today when doing a find / -whatever -exec this-or-that {} \;

Suddenly mtmfs got 100% CPU. It turns out that mtmfs is a special filesystem mounted on /Volumes/MobileBackups. If you run something that will access files indiscrimately, such as a find (something many "cleaner" programs do) mtmfs will use a lot of CPU when its files are being accessed.

You have two options:

1) As suggested, disable local Time Machine backups (sudo tmutil disablelocal)

2) Make sure to exclude /Volumes/MobileBackups (or even /Volumes, as it can be a p.i.t.a. to run a find over network file systems) from the search command.

That should solve the issue.

This model of MacBook Pro has a totally passive heat sink, so the two temperatures you mention are on the main die / logic board and not some measure of how hot your heat sink itself is running. The only way to correct this difference would be to have a new logic board installed (or get a new CPU mounted on your logic board).

As a technician, I'm more worried when the heat sink isn't closer to ambient than to the core when stressing the CPU with constant computing load. Then, it might be worth it to crack the case and re-apply the thermal paste between the metal cover (the CPU heatsink) on the logic board and the finned device (a.k.a. the Mac heat sink part) that gets the heat from the CPU heatsink to the air flowing out the back hinge area of your Mac.

If you had a faulty CPU you should be able to expose that in a geek bench test where it would throttle itself down under premature overheating. If you still had warranty left, I would open a support ticket (visit a genius bar or call AppleCare) to document this. They can then enter your concerns and offer a logic board swap if you are out of spec. I've had good luck getting concerns on the record and having Apple cover a repair if they had the chance to cover it under warranty, chose not to, and then the device failed within a few months after warranty.