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27" iMac CPU temps under Minecraft
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sek929
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Apr 30, 2019, 01:44 PM
 
Decided to check out the latest Minecraft version as I'm waiting for a bulk shipment today, game runs liquid smooth but within minutes the fans on my Quad-Core i7 iMac spin to maximum and iStat tells me my CPU core temps are around 200-210ºF. Couple that with the smell of hot plastic I quit Minecraft and the machine was dead silent and cool within seconds.

Cursory googlin says that max temps for the i7 are around 210ºF, and that they'll shut down before melting themselves. Still, this is quite disconcerting, I haven't heard the fans ramp this high ever, even when gaming during the summer. My old iMac struggled to play Minecraft years ago, and even with the fans in jet mode I never saw temps over 180ºF. Is this something that should concern me?
     
reader50
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Apr 30, 2019, 02:55 PM
 
What year is your iMac, and when was the last time you opened it up to clean the air passageways? Does anyone smoke in your house?

I once prepped a free iMac for a relative. I could tell right away the last owner smoked.

     
sek929  (op)
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Apr 30, 2019, 07:19 PM
 
Recently opened it up to install an SSD, it was already quite clean inside but I blew out the fans anyways.
     
P
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Apr 30, 2019, 07:23 PM
 
Intel CPU therm al management is complex, but...

For desktop CPUs of 6th generation and back to the first Core I-something, the key temperature is called Tcase. Tcase is supposed to be the temperature of the top of the heatspreader, but in practice there is no sensor there - Intel generates it from other sensors and magic. If the CPU is 100% loaded, it will thermally throttle at 71.7C. Thermal throttle means that clockspeed will drop to keep the chip cool.

For mobile chips and newer desktops, the magic temperature is called Tjunction. This is the temperature of the chip itself, and in practice is the highest of a bunch of temperature sensors in the chip itself. When Tjunction hits 100C (or in a select few cases 105C), it will thermally throttle.

If Tjunction hits 125C, the CPU will shut down. This is only supposed to happen in case of “catastrophic cooling system failure”, basically that the heat sink detaches.

It sounds like your iMac is in thermal throttle (212F=100C), so now to figure out why. Cleaning the vents is step one. Also check that the fans are spinning at all.
The new Mac Pro has up to 30 MB of cache inside the processor itself. That's more than the HD in my first Mac. Somehow I'm still running out of space.
     
reader50
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Apr 30, 2019, 08:29 PM
 
High temps + fans running + clean case = not good. Did you blow through the heat sink radiators? In my experience, dust collects in the fans themselves, and the radiators the fans blow through. Blowing from either end just shuttles the dust between those two locations - you have to pull the fans to get a clean shot through the radiators.

The fact that it cools down immediately suggests little thermal inertia. Heat sink grease has probably dried out.
     
sek929  (op)
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May 1, 2019, 04:52 PM
 
Oh great, good thing these iMacs are such a breeze to open up...
     
   
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