CPU usage almost always %100

Feb 19, 2018
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Hello, so my brother's PC had an GTX 1070 and i5 6600K with a non Z series mobo and it was stuck at 3.6 ghz max and naturally while playing games CPU usage was always %100 and GPU was around %35-%60 usage and it was causing some low fps, stuttering issues.

So yesterday we finally managed to upgrade it to a Z270 motherboard and and I overclocked it to 4.3 ghz with 1.260 v first and went up until 4.5 ghz 1.3v

The problem is even though GPU usage and in game FPS greatly increased CPU is still at %100 usage and it gets hotter than I expected during stress tests and gaming

70-85 degrees usually even at 1.25 volts and 4.3 Ghz

Stress test was successful at 4.5ghz but CPU went even more hot so i didnt go further

Any ideas on what might be wrong i will give PC specs at bottom and sorry for my English and formatting skills

Specs:
Gigabyte Z270-HD3 motherboard
i5 6600K
GTX 1070
8x2 Gskill 2133Mhz rams
CPU cooler: Coolermaster hyper 212 led
Case: Coolermaster K380
Note: There is only one frontal fan in case

Edit:

I have already run scans and it seems to be clear.

And I was thinking about thermal paste being applied badly but the issue is %100 cpu usage so
I think heating is a result of that but not sure
 
Feb 19, 2018
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I forgot to mention I already ran virus scans etc. and nothing found
Also there is no application using cpu without my knowledge
So oddly whatever it is games or any other cpu extensive program cpu goes to %100 usage even when it shouldn't


 
Feb 19, 2018
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I did some research on benchmarks for same specs on internet and while my gpu usage is at %70 and cpu %95+
other people with same specs use %95+ gpu and lower than %90 cpu and some examples like that some being closer to other people and some a lot worse compared to others

also my gpu never goes to %100 usage due to cpu goes %100 first no matter which game