IIS App Pool High CPU usage despite no requests

This really looks like some code stuck in an infinite loop.

A request comes in, IIS starts serving it, something (probably a bug) triggers this behavior, a worker thread enters an infinite loop and pegs a CPU to 100%, and then it just stays this way until the app pool is recycled.

Even if no new requests come in, the CPU remains in use because the stuck thread never actually terminates.

Sometimes a new request triggers this behavior again, and then you get two stuck CPUs (or three, or four...).

Recycling the app pool of course terminates all worker threads, thus the problem gets solved... until it happens again.

Had the exact same issue with Sharepoint 2013 and IIS 8 on 2012... We never troubleshot, but instead downgraded to SP2013 on 2008 R2 and all was well.

You can try using the Debug Diagnostic tool to track down what is causing the problem. It usually is for troubleshooting crashes and memory leaks, but could help find which component is causing the issue.