Actually, that is not a dumb or unreasonable question at all.
I was initially very doubtful myself because after all it is a round brass disk in a presumably round aluminum bore and the problem was worse when the car was completely warmed up.
The thermal coefficient of expansion for the brass disk should be a bit smaller than for the aluminum bore, so if it had clearance cold it should only have had more clearance when hot.
I did think of gear lash early on and there didn't seem to be much at all when gently pushing the plate back and forth at its non-powered position, which is slightly open.
Yes, there is some, but it is very tiny.
The throttle-by-wire plate on our Cruze has more than that and it doesn't stick.
For those reasons it took me a long time to even consider looking for what I eventually found.
I also thought that there could be a dead spot in the motor itself (it is just a DC brush motor after all) and considered removing it and bench testing it.
Nonetheless, when I saw the scratches and shiny spots on the bore there could no longer be any doubt that the plate was (for whatever reason) contacting the surface of the bore at some point in its travels.
These are referred to as "witness marks" in the machinist's trade (I am an engineer BTW).
Proof positive that two things are contacting one another.
If I had to guess at a probable cause for all of this I would also blame the gear train and some burrs on the edge of the disk that I polished away.
There is a hard machined stop for the travel under the plastic cover (a PITA to access because of the spot welded motor connection tabs).
I tried shimming that months ago to prevent the plate from contacting the tiny wear ridge I found, and it did help temporarily.
But it threw a code even though I had re-taught the throttle position using my scan tool.
So it was back to square one.
If taking into account the build ups of expansion coefficients and tolerances for the entire gear train, perhaps they just all add up wrong under certain conditions.
Or perhaps the slightly greater expansion of the aluminum body opens up some radial play on the shaft when it is really hot allowing the plate to move sideways, particularly because it is spring-loaded.
I had something like that happen on a TBI-equipped truck I used to have, and I installed Oilite bronze bushings which cured it (replacement throttle bodies for it were nosebleed expensive).
As I sort of implied in the original post, I was on the verge of buying another throttle body but thought I'd try this first.
Perhaps if I had done that this post might have never happened.
If nothing else it was fun to experiment with polishing and mildly porting the bore just to see what would happen.
So far I am quite pleased with the result and would do it again even to a brand new one.
That said, I
totally take your point and greatly appreciate that input.
Something to consider.
And better yet now there is a discussion to search for if someone has this issue in the future.
Before there was nothing.
Thanks!
