For some reason this summer i was having the same problem, except i know there wasn't anything wrong in the engine bay because i parked it and checked shortly after... Still haven't found the problem so hopefully you find out what's up!
i think it was leftover coolant from the flush sitting on the manifolds. i ran it in the driveway for a while reved it up to 3k for a few minutes, and nothing.
The first drive was fairly short <2 miles