The iMac CPU fan going ape **** after upgrade

I have the same problem about the fan going crazy. In my case is Apple Mail, that is running at 120%, I didn't think that was possible either, but that's what Activity monitor shows, so I have to force quit Mail for the fans to stop.

so far haven't found any solution yet.

Mines been doing the same thing, and only seems to stop when the mac is put to sleep, immediately restarted and the fan calms donw. I've tried the SMC reset, etc but made no differece at all. I installedSMC fan control but it hasn't made much difference although I noticed the temp drops to 0 when the fan goes ape which seems odd. My problems all started post upgrade and I've re-installed too. I'm in the process of backing up everything before giving up trying to fix it myself and heading off to an apple store. It's even come on when no apps are open and the activity monitor shows low cpu usage. I'm fed up with this, and wish i hadn't bothered upgrading at all, but it's heartening to know i'm not the only one. Apple need to sort this out centrally rather than each one of us popping by so they can resolve on a machine by machine basis.

On a four-core machine, an app can run at up to 400%. The percentages are all in terms of a single CPU core.

I take it Mail is nonfunctional for you? Can you try (in Terminal) typing "sample Mail 10 10" (without the quotes) and posting the result so that we can see what Mail is actually doing?

Edit: Just to clarify, it saves the result in a file in /tmp (and prints the filename). Send the contents of that file, not the text that says where the file is located. 🙂 To get to /tmp in finder, either press Command-Shift-G and then type "/tmp" in the resulting window or just type "open /tmp" in Terminal.