Fans going crazy, CPU 100% idle

[.Neo]

I'm going insane over the behavior of my 27-inch, medio 2010 iMac. The CPU fan continues to spin up even though the iMac doesn't do a thing. I've reinstalled OS X Lion multiple times, I reinstalled OS X Mountain Lion several times. I did PRAM-resets more times I can count, same goes for SMC resets. Today I even disconnected the iMac, laid it down on my bed and cleared all the vents with the vacuum cleaner on the weakest setting. The other day a friend of mine walked in and immediately asked why my iMac was making so much noise.

The first picture shows the CPU Fan right after startup. The second less than two hours later. I haven't done a thing with the iMac except play music and browse the web a bit.

How on Earth is it possible the fans spin up, sometimes over 4000 rpm, when the CPU sits next to idle most of the time? The worst part is they won't power down once the iMac cooled off. There are no optical drives in the SuperDrive either.

The measurements are in Celsius. The room temperature averaging at 20,5 degrees throughout my apartment. It's only 14 degrees here in Amsterdam.

[+LogicalApex MVC]

assuming your temps are in Celsius it is spinning up because the chip is hot. Is there anything to justify the chip being hot? Dust on the HSF, high ambient temp, etc?

[.Neo]

I can't really say for sure how much dust has accumulated inside the iMac itself, but judging by how little there was directly underneath the grill I seriously doubt it's a lot. I tried looking inside with a flashlight but everything seemed clear.

[f0rk_b0mb]

Did you recently install an SSD?

http://forums.macrumors.com/showthread.php?t=1235234

[+jamesyfx Subscriber²]

If I leave my iMac on for long periods of time the fans will gradually get faster and faster until they are 100% on, and they will not go back to normal speeds unless I shut down and power back on. Rebooting just keeps them running at full tilt.

It's strange. It's like the driver/sensor controlling it has failed to do it's job properly. :/

[Farchord]

Yeah if it's THAT hot at idle, you have a problem on your hands.... either your system has absolutely 0 airflow or the heatsink aint applied correctly, if so I'd take the computer and bring it to a store.

[Elliott]

Did you recently install an SSD?

http://forums.macrum...d.php?t=1235234

That actually affects the hard drive fan, not the CPU fan (at least it did in my Late 2010 iMac). Also, it happens a lot sooner than 2 hours after boot. :p

Maybe your GPU is being taxed by something? The CPU fan cools both, so...

[.Neo]

Yeah if it's THAT hot at idle, you have a problem on your hands.... either your system has absolutely 0 airflow or the heatsink aint applied correctly, if so I'd take the computer and bring it to a store.

It's an iMac. It's normal for things to run a little bit hotter.

Did you recently install an SSD?

Everything is stock. No modifications whatsoever have been made to the iMac after I unpacked it. Also, the SSD issue wouldn't affect the CPU fan, rather the HDD fan.

Maybe your GPU is being taxed by something? The CPU fan cools both, so...

Okay, this is something I can work with. Is there any way of checking what the GPU load is?

If I leave my iMac on for long periods of time the fans will gradually get faster and faster until they are 100% on, and they will not go back to normal speeds unless I shut down and power back on. Rebooting just keeps them running at full tilt.

It's strange. It's like the driver/sensor controlling it has failed to do it's job properly. :/

That's EXACTLY what ****es me off. The fans won't slow down once things are cooling off.

[.Neo]

PS The thing is I vaguely remember my fans spinning up faster as early as during the OS X Lion Developer Previews. When starting up using Mac OS X Snow Leopard the issue was gone.

[.Neo]

UPDATE:

Okay this is incredibly interesting. I'm now 100% sure this is indeed a software issue, rather than a hardware related one. For whatever reason OS X Mountain Lion is telling my iMac to increase CPU fan speed in order to cool off. But apparently this is a one-way street. As time progresses the fan speed just continues to go up until it hits a maximum of about 4150 rpm. Like jamesyfx noticed they it will continue running at its maximum until I do a full shutdown. So for whatever reason OS X Mountain Lion won't tell my iMac to reduce fan speeds once it cooled off.

After a lot of Googling I noticed other people having the same issue with OS X Mountain Lion. Not just iMacs but MacBook Pros too. One person came with the suggestion to temporarily download and install iStat Menus 2, set Fan speed control to "Medium" (it's on of the default "rules") and then return to "Default". In literally an instant my iMac's CPU fan would drop from its maximum of ?4150 to a quiet 1940 and then continued to drop.

Check out the below screen shot and note the uptime: Over 6 hours and a regular CPU fan speed of 1885 (it's still dropping as I type).

So what's going on here people?

[+jamesyfx Subscriber²]

It does a similar thing on my system, CPU fan speed drops down to a steady 1190rpm when I set to default.

Seems like it kicks the default behaviour back in once you set it as such.

My iMac is a completely different model to yours, so the issue must be software related.. and I think using this utility has proven that.

[.Neo]

Yeah that's exactly what seems to happen. iStat basically slaps OS X on the wrist telling it do proper fan control. After a while CPU fans would indeed drop to an average of 1190 rpm on my model as well. Here I was thinking my CPU/GPU fan was constantly triggered because it couldn't handle the new OS X Lion/Mountain Lion graphics effects. But that's not it either.

Anyway, it seems like you can uninstall iStats Menu from your system until you do a SMC reset.

[LaP]

I'm not familiar with Mac since i never owned one but why does OSX has to control the fan ?

I would never ever trust any OS to control the fan speed of my computer. Right now i have a fan controller but if i would not have one i would let my Motherboard control the fan speed or at worst install my MB software (or a 3rd party alternative like speedfan) to control them.

[.Neo]

Honestly I have no idea if OS X is fully responsible, but I do know the OS can trigger different fan speeds.

[LaP]

Here's an interesting blog post about the issue. You probably have already read it since it was in the first 10 results of a google search but i still post it just in case.

http://gigaom.com/ap...lion-heres-why/

[.Neo]

Here's an interesting blog post about the issue. You probably have already read it since it was in the first 10 results of a google search but i still post it just in case.

http://gigaom.com/ap...lion-heres-why/

Spotlight reindexing doesn't take days or weeks.

[Pedro3]

Did you try to install Lion or Snow Leopard? Just to have sure that the problem is in ML

[Hum]

Maybe this helps:

http://www.hddfancontrol.com/

OR:

Symptoms

After plugging in the iMac and turning it on, the fans (blowers) may be noisy and appear to run at full speed. In this event, shutting the iMac off and turning it on again wouldn't fix the issue.

Resolution

If you press the iMac power button while you are inserting the power cord, the iMac will enter a mode in which the fans (blowers) run at full speed. In order to correct the issue, reset the SMC and then start the computer by pressing the power button after the power cord has been fully inserted.

The iMac should now start up and operate as expected.

http://support.apple.com/kb/TS1433

AND:

First download and install smcFanControl 2.2.2. Set it up to provide (still quiet) minimum fan speeds of 2200 rpm for the HD and 2800 rpm for the CPU, leave the Optical Drive fan at 600 rpm to minimize dust accumulation (or just tweak it up if/when you will be using it much) ....

http://hints.macworl...110606033134359

[Shadrack]

UPDATE:

Okay this is incredibly interesting. I'm now 100% sure this is indeed a software issue, rather than a hardware related one. For whatever reason OS X Mountain Lion is telling my iMac to increase CPU fan speed in order to cool off. But apparently this is a one-way street. As time progresses the fan speed just continues to go up until it hits a maximum of about 4150 rpm. Like jamesyfx noticed they it will continue running at its maximum until I do a full shutdown. So for whatever reason OS X Mountain Lion won't tell my iMac to reduce fan speeds once it cooled off.

After a lot of Googling I noticed other people having the same issue with OS X Mountain Lion. Not just iMacs but MacBook Pros too. One person came with the suggestion to temporarily download and install iStat Menus 2, set Fan speed control to "Medium" (it's on of the default "rules") and then return to "Default". In literally an instant my iMac's CPU fan would drop from its maximum of ?4150 to a quiet 1940 and then continued to drop.

Check out the below screen shot and note the uptime: Over 6 hours and a regular CPU fan speed of 1885 (it's still dropping as I type).

So what's going on here people?

I'm confused. Did that software package end up solving the problem for you or is it still an issue?

[.Neo]

I'm confused. Did that software package end up solving the problem for you or is it still an issue?

It solved the problem until the next restart.

[FightAndLive]

Off hand I would say the temp gauges are wrong, without touching the cpu or the motherboard touch the bottom of the heat sink(while the computer is on) and see if its hot. That feels like the CPU is over heating(which would cause the 100% cpu usage).

[.Neo]

Off hand I would say the temp gauges are wrong, without touching the cpu or the motherboard touch the bottom of the heat sink(while the computer is on) and see if its hot. That feels like the CPU is over heating(which would cause the 100% cpu usage).

I have no idea what you're talking about or why you think there is 100% CPU usage. :/

[iUserX]

Buy a new iMac :p

[GABLAD]

Hi there, old thread, but hopefully someone is still around.

I have an iMac 27 late-2009, 3.06 Ghz, 8gb RAM, running OS 10.8.5, and all the hardware is original (no HD swap...). So, it has a similar problem: it takes about an hour for the CPU fan to gradually speed up from the initial approx 1200 RPM to around 4000 RPM. Then, all I have to do is to put it to sleep for enough time to let the fans stop spinning (that's not even 5 seconds) and then wake it up to see the CPU fan at normal speeds. And here it goes for another hour of gradual increase... See the screenshot of the iStats Menu, the graph shows the increase in the last hour, and the three previous increase/sleep patterns... The other two fans are just fine. All of that time, the iMac is at idle, nothing running at all. My iMac isn't my main computer, so i kinda left the problem as it was, spending an hour or so every couple of months trying to solve the issue, but after these 5 years of fruitless efforts, I can't believe I haven't solve it yet! Even though I get answers a lot from forums, here's my first post in one of them (ever!), in hope it will help.

 

Here are the solutions I tried already: smcFanControl, HDD Fan Control, iStats Fan Rules... Nothing seems to be able to override the fan speeds, nothing at all! I vacuumed the air input and output... And of course, I did the SMC reset. I try to remember all the other things I tried to do, but it gets confusing after those years of diverse problem solving. I did many upgrades (and even downgrades) too but I doubt it has anything to do.

 

Even though I never opened the iMac's case (I DID open the bottom latch to replace RAM memory at some point, comes to my mind... but that's not the back case) I wonder if it has anything to do with some hardware that would just be misplaced at the factory of because of a shock while moving the computer around... 

 

Anyone has any idea? Anything I could try?

 

Screenshot:

https://www.dropbox.com/s/vfvkr1zordrbznb/Screen%20Shot%202015-01-14%20at%2011.59.57%20PM.png?dl=0

[+Warwagon MVC]