I have a custom built gaming PC from one of those online build shops (iBuyPower) and this is the second one I've owned. I have never had a real problem with either PC. I have had this PC for just over 2 years.

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Last night, I noticed one of my fans was making a loud buzzing noise. So I opened it up, and cleaned out the dust from the case, and the radiator for the liquid cooler, and blasted all the fans with a shot of air. I powered up the PC and checked that all fans are running. Then I closed it up and hooked up everything, and I noticed I'm getting high temps on my CPU using Core Temp. So I opened it up again and cleaned up, then applied more thermal paste (I work on VFDs for a living, I'm familiar with how to apply thermal paste). I got a good even coating across the whole processor.

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I reassembled everything and put it all back together, but I still have moderate temps under no load. Before I was idling at 90+ degrees C. Now I'm idling at 75-85 degrees C. The cooler pump sounds like it is circulating, and I didn't see any leaks in the system. What could be causing this issue?

If the cooler pump works like any other liquid pump the impeller might be broken, but the electric motor is still spinning it up.

I dunno how you'd check the flow rate, but if the pump doesn't circulate the fluid you're not going to get decent cooling.

Makes me wonder if the buzzing sound I heard was the pump /impeller coupling slipping, and then it finally broke about the time I cleaned everything out.

It's possible. But unless the pump housing or pipes have some sort of clear section to view the liquid moving, how the hell are you supposed to diagnose pump problems? They're obviously not built to be taken apart, because then they'd be a massive leak risk if the seals gave out.

I know. But process of elimination already had me looking here. The CPU is not under any load (1-2%), The radiator isn't clogged, the fans are working, and the thermal coupling to the heat-sink plate is good. This really leaves that coolant isn't circulating, which was my first thought before posting, but I was looking for something I may have been missing. I'm not any kind of expert, but electronics is what I do professionally. But I also am willing to learn if I'm missing something or making a mistake.

Never told us about your setup. Assuming you got an AIO? I keep hearing more and more negative feedback
about AIOs from the usual tech youtubers. I'm also thinking you dont have clear tubing, as that would tell you
if the pump is pumping. What should your temps be at?

The first thing you should check is whether the liquid cooler fans are working and installed in such way air is pulled in (pull configuration) or pushed out (push configuration) or pulled/pushed (pull/push configuration). Also if you got other fans installed check which direction they are installed and airflow isn't blocked or impeded so hot air is pulled in and can leave the system in whatever direction.

To determine if your LCP is functioning and coolant is circulating, connect the pump cable directly to a fan header on your motherboard, power-up the system and enter your BIOS. If the reading of the CPU-Fan 1 or 2 speed names can be different says 0/RPM your LCP is malfunctioning and needs replacement. In case it's working yet still showing relatively high temperatures you should try to increase the flow rate.

Since I don't know your BIOS version, I can't provide you with specific steps where to look, or what additional settings exist you might want to take a closer look at or change.

Does the pump pump ?

I would grab a cheap cpu air cooler from somewhere nearby and check what happens with the cpu temps if your running with the cpu air cooler instead of the aio.

lol you wouldnt smell burning anything if your hard drives were at 99c they would just be dead by that point.

As Zimerius said, trying an air cooler is probably the safest bet. ID-Cooling SE-224-XT is an excellent cheapish cooler, but of course, check your case clearance first.

For monitoring software, give HWiNFO a try. Supposedly it is the most accurate, according to Gamers Nexus. Open Hardware Monitor is not developed anymore. Try its fork Libre Hardware Monitor. I use it mainly for the fans control function.

Yes, I have an AIO cooler. No, it doesn't have clear tubing.
My temps should generally be at 40-45 degrees C idling. 90-100 Degrees C is risking damage to the equipment. I will try something like that. I don't think the pump is pumping. I think it is spinning, but the impeller is broken for some reason.

Good point XD
Thanks!

So I bought a new AIO cooler, it will be here Saturday. It was as cheap as any air coolers on sale ($80 on sale) so I will install this next weekend and see what happens. Really glad I have my Steam Deck.

Cooler came in early, and I installed it with a little headache because power adapters aren't the same... But I managed to figure it out. Cooler was definitely my problem. I now have idle temps in the 20-30 degree C range, which is MUUUUUUCH betterer and makes me feel comfortable using the thing again.