Hi and thanks for using 2CarPros.
Since the vehicle is under warranty, I would demand they find and fix the issue. If the dealer won't help, then I suggest contacting the manufacturer's zone office for help.
Now, I will assume there is no check engine light staying on with the engine running, otherwise they would have found something. So, I am going to start simple. The first thing I would check is for an engine vacuum leak. Here is a link that shows how it is done:
Let me know if that helps or if you have other questions.
Tuesday, January 15th, 2019 AT 6:52 PM