[DIYguy]

Slight update, see text-

I searched this forum (and others too) repeatedly for a solution to a sticky throttle problem but found nothing helpful.
I don't know if sharing this will help anyone else, but hopefully it might.

Over the course of the year plus that I have owned my '17 the throttle had been getting progressively "stickier" for lack of a better term.
While there is no physical connection from the pedal to the throttle plate, the way it responded mimicked a classic sticking cable or throttle plate perfectly.
Push the pedal slowly, nothing happens, and as you push a little more all of the sudden the engine snaps up to a power level.
I had suspected, checked, cleaned, reset, re-taught and otherwise tweaked a number of things and nothing helped.
So after finally discovering witness marks in the throttle bore (shiny spots, scratches and a very tiny ridge) using a 10x jeweler's eye loupe, I did the following-

Using 220 grit paper I carefully sanded out the marks and ridge.
This was followed by 800 grit, 1500 grit, then buffing compound.
Then I gently rounded off the knife-sharp edges of the disk where it is closer to the bore as the throttle is being closed and polished some manufacturing burrs.
I did not round the edges farther from the bore as it closes, if that makes sense.
In this process I did not re-shape the bore or disk noticeably, perhaps 0.001"-0.002" at most anywhere.
I then carefully cleaned any grit out using carb cleaner and re-installed it.

It was much better immediately, but the problem was not completely gone.
So I repeated the process which finally worked.
After many hundreds of commuter miles and probably 50-70 start/warmup/drive cycles it is still holding up.
No codes, no odd behavior and no pseudo-stickiness either.

I do not have "before" photos of the bore and the witness markings, sorry.
But I have added a couple of "after" photos because I happened to have it off the car today for another reason anyway (23OCT19).
The witness marks were scratches along the axis of the bore toward the top and bottom of the bore as shown in the photos.
They are totally gone now with no re-occurrences.

If you feel that you are having a problem like this, before taking it to the dealer or buying a new throttle body why not give this a try?
If you see any odd marks, scratches, scrapes or wear ridges in your throttle bore, even if it takes a jeweler's loupe to see them, the plate is contacting it for some reason or another.
Try taking small corrective steps until the problem is gone.

There was another slight benefit that is worth mentioning.
The car feels a bit more responsive in general.
Nothing that would likely register on a dyno or anything of that sort, but it is definitely more fun to drive now.
Perhaps this is just because it was sticking before and was limping a little, I don't know.
But polishing the throttle bore and plate may be something worth considering doing even if your throttle plate is not sticking.

I do hope this helps someone else.
Thanks for reading.