High idle 24v V6

Have an 02 Sable LS with about 111k miles. This is used car I bought for my son. Plugs needed changing so pulled top intake to get at rear, put in new o rings.
Idle is now too high, took into shop mechanic did smoke test for vacuum leeks none found. Determined IAC valve (idle air control valve) need changing . Replaces IAC but test drive for me indicates no change.
So now I am paying for close to 400 for labor and parts and the car still has same high idle problem.
Will not drop below 1k, take foot off gas and just keeps going.
At this point not sure on how to proceed.
Told shop not happy with outcome said they would look into it further.
So looking for ideas.
The mechanic thinks I need to check the lower intake for possible vacuum leaks even though he did a smoke test and found none.
I only pulled the upper intake which is plastic and replaced the Ole rings but never pulled the lower.
Kind of hesitant to take it back there even though I think they're probably perform some Diagnostics without charging me any additional fees.
It's been my experience in the past that I can usually figure these things out on my own.
So if anyone has any experience with this type of problem perhaps I can proceed not taking it back to the shop.

Thanks,

Jay