CPU overheating issues

I'm running an Intel Core i5 at 3.2GHz with a single core.
My initial issue was just a noisy fan, and even then, temperatures were hovering between 55°C and 71°C while I was just browsing the web on Windows 7.
I decided to pull the cooler off to clear out a thick layer of dust, but ever since I put it back together, things have gone south. The fan is whisper-quiet now, but the temperature has spiked to 99°C, and the whole system feels sluggish. I’ve tried letting it sit and cool down for a few minutes before booting up, but the CPU temp refuses to drop below 90°C.

I am somewhat concerned about this sudden slowdown. I know I need to pick up some thermal paste to help regulate the heat, but I can't help wondering if I've already caused permanent damage to the CPU. Would the computer even function at all if the processor were actually damaged? 🤷

Open up the case and confirm whether the fan is actually spinning. If it isn't, check if the fan cable is properly seated into the motherboard. Assuming everything there is fine, then the heatsink likely isn't making proper contact with the processor. Give it a little nudge by hand; it needs to be perfectly, firmly secured.

If everything looks correct, just pull the heatsink off and swap out the thermal paste. Clean off whatever old residue is left using a cloth—do this for both the processor and the heatsink—then apply a very thin layer of new paste over the entire surface of the CPU. Reassemble everything, plug the cooler back into the motherboard pin, and it should be fine.

http://www.youtube.com/watch?v=X-wBqii4xXw

The cooler is performing beautifully—it’s quiet, and the RPM readings look perfectly normal.
Does this mean there could still be an issue if the cooler isn't seated quite perfectly? To be honest, I had a bit of a struggle during installation; I just couldn't get the holes to line up properly, and I actually ended up bending one of those plastic tabs meant to mimic a screw. I eventually managed to get everything to sit flush, but I can't say for certain if it's aligned down to the millimeter 😕.

Pull the motherboard out of the case and take a look at how everything settled on the underside. And make sure you swap out that thermal paste.

I suspect those four pins didn't quite seat properly for you. When they finally drop into place, you should hear a distinct click. Before you even attempt the installation, rotate those dials at the top to the left. Once that's done, align all four gently over the holes and press them down one by one until they click. After everything is seated, just rotate the dials back to the right.

The heatsink has to sit perfectly flush, otherwise you’re going to deal with overheating issues like this. I guess you should just pull it off and reseat it—there are plenty of tutorials on YouTube. If you ended up snapping one of those plastic clips, though, you might just need a new heatsink.

I applied the thermal paste, and now I'm just browsing the web, and the temperature peaked at only 52°C—yesterday, before the cleaning, it was hitting nearly 70°C. The noise is almost non-existent; it's such a massive relief ☕.

On another note, I wanted to ask if there are any real pros or cons to running the case with the side panel off? It’s positioned to the side, so it isn't really visible. Would leaving it open help drop the temperatures even further? To be clear, this 52°C reading is with the case open.

Leave it open if you don't have anyone poking at the internals—kids, for instance, much like my own—and if the visual aspect doesn't bother you too much.🙂 Or, if you're working with a poorly designed case that lacks adequate ventilation, then opening it up becomes an absolute necessity.

It’s just gonna be some dust, maybe a pile of debris 😍 and probably a bit more noise, but I guess if nobody minds, we can keep it open