Many out there are experiencing bluetooth issues with iPhones running iOS11 and staying connected to your car ( experiencing dropouts).
I was exactly the same and the following is the only thing that worked for me.
Forget the device first – in bluetooth settings
Turn off bluetooth
Then go to settings – general – reset all settings
You will have to reenter a few preferences but my iPhone has worked a lot better and when I turned bluetooth back on it has synced and not dropped out with my bluetooth devices , including my car.