Are you sure you didn't sneak in and steal my iMac?!
I've got exactly the same machine except I only have 2GB RAM!
Of course, the most obvious thing to check is that the air intake is clear of any dust, cat hair, paper, etc. Might as well open the RAM door and make sure no one left any sponges in there after the "operation!" But that should also assure you that there is no obvious fur balls in there.
Actually, if you compare your reported temps to mine, you'll see mine are much hotter, by at least 15° and as much as 40°. Yet my fans are barely working. The only thing you didn't mention was your ambient air temp. Mine hovers around 78. Are you, perhaps near a warm air duct? A window with direct sunlight? Sitting
way too close to the machine?!!
Another possibility is that one of us has a batch of bad sensors. Their probably one of the cheapest parts of the machine. Or the fan's speed controller(s).
OK, I held two folded sheets of bond paper over the entire bottom grill for more than 10 minutes. The ambient air temp peaked at 98.6, the cores got to 179.6 and 181.4. The CPU fan was solidly above 1400 but never over 1715. The HD Fan finally got to 1430. But I still never heard them, even though I could hear the drive being used during a TimeMachine update. My head was within 10 inches of the screen during this whole time. Now my wrists are sore from holding my hands upside down and applying upward pressure to keep the grill covered!
The main thing I can deduce from the above is that the ambient air temp sensor must be right where one of my fingers was under the grill! But I think can also say that the fan sensors and controllers are working.
Maybe your fan motors don't actually have any fans on them!
I just hope you are happy, the HD Fan now seems stuck around 1430+/- 5! The CPU is down to 1410 and the cpus show 174.2°. Ambient is back to 78.8. I'm about to retire to the bed, we'll see how things are tomorrow. The ambient air
will be much lower in a few hours as the thermostat is set to go down at 10pm. Good night!