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I didn't realize (or didn't read) that you saw marks on the inside of the tb itself.
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Yes, that was the "tell-tale" for the issue.
I feel a bit embarassed that I did not look closer when I saw the tiny ridge earlier (which had prompted the earlier shimming of the hard close stop).
Regardless, both are gone now.
If you look at the complex shape of the bore, it is obvious that GM (or whoever produced it) went to a lot of trouble to design and manufacture it.
So I am pretty sure it is only a matter of cost that they did not finish out the bore better, particularly in the entry throat area.
Diminishing returns and that sort of thing.
All the more for me to play with.
The other thing I discovered in this process is that there are two little perforated air dams just inside the opening to the manifold on my car, top and bottom.
I am curious if anyone might know why they are there.
I am reluctant to trim them out until I know why they are present, though I
was tempted to do so.
My guess would be something to do with off-idle airflow balance when the AFM function cuts out the middle cylinders.
Any thoughts?