Weird GPU temperature behaviour on iMac (late 2014) with macOS 10.15

[the-tml]

Lately (maybe over the last year or so) the machine has started spontaneously entering sleep mode now and then during use. This is during non-gaming use, even while just web surfing, but more easily reproduced when doing things like viewing Google Maps in 3D. (Plus whatever macOS might be doing in the background, like analysing photos and whatnot.) But simply running purely CPU-bound stuff in the Terminal, like compiling LibreOffice (all four cores (eight threads) running at 100% for over an hour), causes no problem.

Now recently I have started becoming interested in flight simulation again. I bought X-Plane and started playing with it a bit. And yes, while doing that the machine goes to sleep spontaneously much more often. So I started investigating, built the open-source XRG program, which showed that when I use X-Plane the "GPU Die" temperature goes rather quickly to a whopping 110 ℃, and sure, macOS then puts the machine to sleep to avoid overheating. But oh well, it might be that the GPU thermal paste has deteriorated over time, or some such issue, and the machine should be taken in for service. That is as such not surprising for a six years old machine.

Hower, and now comes the odd thing: Even for long (tens of minutes) after I quit X-Plane, with nothing intentionally going on that would use the GPU, still its temperature was in the 95–100 ℃ range.

But, and now it gets really weird: When I then started a LibreOffice build (i.e. using the CPU fully), the GPU temperature dropped to below 80 ℃. Huh? Does macOS do something in the background that taxes the GPU a lot when the machine is otherwise idle?

 

[wilberforce]

Have you tried using a utility like Macs Fan Control to force the fan to run at full blast (2700 rpm), and does that make any difference to GPU temp?
You can see what processes are using GPU in Activity Monitor ("% GPU" column in CPU tab).
Apologies if I am suggesting things you already know.

 

[the-tml]

The fan did run at full blast when the GPU temperature was over 100 ℃. The weird thing was that the temperature stayed around 100 ℃ for long even after I quit the 3D game in question. (And then suddenly, luckily, dropped down to a normal around 75 ℃ temperature for no obvious reason.) But yeah, I didn't think of checking what Activity Monitor perhaps could have shown, will do that next time I see the same thing happening, thanks for the hint.

 

[theadder]

I had a very similar issue with my 2014 27” Retina iMac.

My thread was here.

Blow compressed air in through the intake on the bottom edge of the display — this fixed it for me.

 

[the-tml]

I had a very similar issue with my 2014 27” Retina iMac.
Blow compressed air in through the intake on the bottom edge of the display — this fixed it for me.

Thanks, that indeed sounds like exactly the same issue. I also don't really feel confident enough to open the machine, after all the problem happens relatively infrequently. Plus it is high time to replace this old workhorse anyway...

Once I have a replacement, I might take this old one to service, so that I can then sell it without feeling like cheating the buyer by selling a machine with a known problem. Assuming service wouldn't cost more than what I can reasonably expect somebody to pay for it of course.

I did already blow compressed air into the vents, and some dust did come out of the other bottom holes while doing this, but I fear that the large amount of concentrated dust is deeper inside the machine. The GPU temperature still rises occasionally to around 100 ℃ (212 ℉), but not high enough to make it go to sleep.

 

[theadder]

Yes, I wasn't comfortable opening it. By the time I tackled the issue, my machine was going to sleep extremely frequently. I found that it happened often in games; Twitter video also triggered it very easily - I think there was something about certain video codecs stressing the GPU.

I didn't check when I first got the machine, but I think that it is normal for the GPU to get to 100F. Now that the problem is under control, I can keep using it until the replacement Apple silicon iMac. Hopefully that system will run at lower temperatures. I'll probably sell this machine to Apple through the trade in program.

 

[the-tml]

I didn't check when I first got the machine, but I think that it is normal for the GPU to get to 100F.

Oh, I meant degrees Celsius... 100 ℃ is 212 ℉. (And that is as such still within the operating limits of chips; but the lower the better of course.)

 

[theadder]

It should have said Celsius - here it is playing a Twitter video: