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?