Is it normal for the iMac 5K's GPU to be at 90℃ with fan on constantly? I have found that the GPU die on my iMac is around 90℃ and the fan is kicking in all the time. Is this normal? If not, how should I fix it?

I have iMac 27″ Retina 5K (late 2014) with AMD Radeon R9 M295X 4096 MB, and I have Dell UltraSharp U2715H 27-inch LED-Lit monitor connected to the iMac.
Powering that many pixels will be a stretch. You're powering a 5K display internally to your iMac and a 2560×1440 external display. Having your GPU at 90℃ is standard for powering that many pixels.

If you don't wish for the GPU to be that hot, disconnect the external display. It's still well within the thermal limits of the GPU, so I don't think it's something to worry about particularly. Apple always seem to run components hot for the sake of keeping the fans quiet, but you can override the fan speed to cool the GPU further using an app like iStat Menus.

There's a nice discussion about that GPU in the 5K iMac over at MacRumors forum:


AMD Radeon R9 M295X Core Clock Throttling, Heat, and Performance (page 34)