EVGA RTX 2080 super black, which is not yet 2 weeks old, gets very hot. It gets around 45-50 degrees at idle. In the game tests made on the internet, their GPU is reaching maximum 75 degrees at full load, but in my tests, it reaches 85 degrees . When I took it to the technical support, they told me that there was no exhaust fan in the case and they made me buy an exhaust fan, but this only helped to lower the temperature of the processor. I asked my computer advisor and he told me the GPU's thermal paste may be missing and there might be a problem with the bios settings. Nevertheless, I think the problem is my case. There is one glass in front of the 3 sides each except the backside. The passage of the air that can flow into the case is very constricted. I will consider your suggestions and I will try to answer your questions as much as my knowledge. Components are listed below.
 
Processor: Intel i7 10700K
Motherboard: MSI MPG Z490 GAMING CARBON
Case: COUGAR PURITAS MIDT ATX CASE
PSU: COUGAR CGR-STX-700W
Liquid Cooling: THERMALTAKE TH240 240mm
Exhaust Fan: CORSAIR iCUE SP120
 
Note: I couldn't find the COUGAR brand in amazon or any international shopping sites. I hope you can find it's pics in google. 

 
I'm at the point where I can't wait to get rid of mine and move to the 3080 when it comes out, I want that day so badly.
 
Anyways,  after purchasing a p400a case to try and cope with the heat from my 2080 Super,  I started a thread of epic proportions, well in numbers of posts anyway lol.  Kevin, GTXjack and many others patiently helped me try almost everything imaginable to get my heat under control.    If you have the patience to wade through it all and find any tidbits that can help, here it is:
 
https://forums.evga.com/Now-my-2080-fans-are-going-nuts-m3019239.aspx#3019267

Sorry for delayed reply. I have tried to take off side panel but nothing changed. Also I have ran the GPU at 300Mhz(if it was that what you meant with "downclocking"). My room temp is around 25-30C, too.
 
Edit: I have been using MSI Afterburner software to track the values. And I set up a fan curve which the fans are starting to rev around 0-40C at %20. That is how I keep stable the temp. And it stays at 40C all the time.

Yes, but I need to get at least 10C drop at full load, so I still don't know what the problem is. Since it is brand new, 6 months of 1 year from now, it will exceed it's safety temp because of fouling eventually. 

Firstly, I don't use AC, so I can't change the ambient temp unless I could test it during day and night. Here, the ambient can reach around 30-35C at day, 20-25C at night. Secondly, I cannot afford for another card model.

keremtanis61
Firstly, I don't use AC, so I can't change the ambient temp unless I could test it during day and night. Here, the ambient can reach around 30-35C at day, 20-25C at night. Secondly, I cannot afford for another card model.

The thing I did that had the best effect on lowering my temps was to use a Kraken G12 and a H55 AIO using silent wings 3 in a push/pull config, the ones that are rated for the highest static pressure in a push pull config.    The tubes are really short though so mounted at the rear is probably the best with that one, or if your case has a side or floor vent perhaps? 

keremtanis61
 
The liquid cooler for the CPU is located right next to the motherboard. In addition to this, it transfers the air inside the case to the radiator. Does this reduce the static pressure inside the case? For this reason, can it disturb the air flow inside the case? There is no air outlet chamber on the side or bottom of the case. If I install the liquid cooler on the top of the case, and its position is to transfer the air out, will there be a change?
 
Edit: Here is inside of the case https: //img ur.com/be5thFT get off the blanks

Intaking cooler air through the rads is always recommended but depending on the case, you might have to set it up as exhaust. 

The thing about your case and many other similarly designed cases is a lot of folks are having thermal issues with those 2 internal side fans that flow behind the MB instead of being in the front.  Never been a fan of that design while it looks nice, it's becoming a headache for the few that I've seen so far.  The air flow design of this case imo is not optimum. For example, if someone has it set as exhaust as you do now, those fans are sucking in any cooler air there might be brought in from other parts of the case leaving other places with AIOs starved of air.  If they're set to intake, you're circulating the rear warm exhausted air, back into the case resulting in a warm air loop. I would take that case design and shoot it behind the bar.
 
Try lowering the speed of the side exhaust fans while ramping up any intake fan in the case so it can make its way to any AIOs that you have.  If you're looking to install an AIO for the GPU, I'd install it to the side fans.  You'll have no choice but to deal with negative pressure since you aren't forcing enough air into the case but that's all you have to work with otherwise your top AIO as intake, will be intaked by the side fan AIO while some of that flow will route its way to the rear fan.  Again, just make sure there's enough intake air flow coming from the bottom if possible.