kernel_task using 300% cpu

Please refer to If kernel_task is using a large percentage of your Mac CPU ... including the links within it.

If you are unable to correlate it with anything in particular (which appears to be the case, based upon your description), then an SMC Reset is justified. That should fix it.

If it doesn't, and it continues to appear without justification, a hardware problem is likely to be the cause. Given its intermittent nature though, diagnosis can be difficult. Consider using Apple Diagnostics / Apple Hardware Test, which will readily identify an inoperative exhaust fan (for example): How to use Apple Diagnostics on your Mac - Apple Support. Other hardware-specific causes will require more extensive testing that isn't generally available outside an Apple Store or AASP.

Try using "Safe Mode" long enough to determine whether the problem persists or not: Use safe mode to isolate issues with your Mac. If kernel_task is its normal quiet self in Safe Mode then you can probably find a software-related cause.

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A runaway kernel_task can be due to a number of factors, such as:

• a failed or disconnected temperature sensor
• cooling system inefficiency (an inoperative exhaust fan on Macs so equipped, possibly clogged with dust, including the heat sink)
• a worn out, missing, or non-Apple (user-installed) battery on portable Macs
• similar hardware-specific causes including internal hardware additions or modifications, external USB and other connected devices, or the driver software required to use them.

EtreCheck is a simple little app to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac.

http://www.etresoft.com/etrecheck

Pastebin is a good place to paste the whole report...

https://pastebin.com/

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck.

So I was having a similar problem and I think it might have to do with the encoding of newer software updates that Apple has been putting out. I am not a computer engineer but I thought about a "fix" someone posted on a forum where they said if they unplugged their Macbook pro from the charger it would cause the Macbook kernel to stop eating all of the memory on the computer when viewed in the Activity Monitor. Then I thought about the lesser amounts of voltage coming in when your not charging the Macbook so I downloaded an app called Volta which is for UnderVolting Macbooks which makes the process very simple. You just need to click on it on the top menu bar and adjust the voltage negative until it shows a warning sign that it is to low which could cause your macbook to shutdown from not receiving enough power. Undervolting does. not have any negative effects such as Overclocking so dont worry. This app has a free 7 day trial and the 8 dollars for the license key after is really affordable since it Fixed my problem along with another free app called Mac Fan control which allows you to turn up the fans on the macbook manually . I care more about functionality than whether or not I can hear the fans. Apple seems to be obsessed with keeping fans quiet which is dumb when the reaction is your computer coming to a halt. All I do know is a CPU and GPU that are cool are going to run a lot better. Best of luck and I hope this post helps some other people that want a more permanent fix than just unplugging your battery since with older models of Macbook's that would require dropping like 70$ for a new battery due to decreased life over time. Not to mention Apple is recalling all batteries for Mid 2015 to 2017 Macbooks. I have a 2014 so I am out of luck on that end haha.

Good work, nice report. :)