I have recently had a few issues with the fans racing. The back of the case was frequently hot to the touch.
Having cut away the screen, on disassembly, the fan blades and processor cooling fins were thick with dust/dirt. One side was around 50% occluded and therefore not working.
I was tempted to renew the thermal paste, but because the processor is not attached to the board (it's held in place with the heatsink), I read that it's easy to bend the pins if you don't know what you are doing because the bolts are on one side of the board and the processor and heatsink are wobbling around on the reverse as you tighten things back up. I'm going to see how things go.
As you point out, at the bottom of the iMac casing there are air vents running the length of the case. I Hoovered these periodically, so they were nice and clean. However, inside the case, an inch above the vents is a black plastic strip full of little holes running the whole width of the computer. Those holes were 3/4 blocked with dirt.
Having sucked all the dirt out with a Hoover wand equipped with a silicone straw, everything's now a lot cooler. I've heard the fan running once this week. Used to be on all the time.
My 2019 iMac obviously isn't designed to be stripped down. It was a hassle and there is a real possibility of breaking the glass, even with the pizza cutter tool. Did a RAM upgrade at the same time and I'm not resealing the screen down until I'm certain that the problem is solved.
