92mm DBC Throttle Body, IAC Passages Corrected, but Idle Steps Still Inconsistent

After running a half-tank of fuel through this thing just idling, and carefully methodically collecting crazy amounts of data, graphing in excel....i'm stumped!

First issue I solved (tip for anyone else with an aftermarket throttle body!) is to check and if needed, open up the IAC air passageways to and from the seat area (careful to NOT modify the seat itself since that is where all the controlling "valve" action takes place). At first, once the IAC steps reached around 120, no matter how much higher they went, flow was choked and no more idle contribution was actually being given beyond that. After ensuring all passageways were at least the same cross sectional area as the seat, it can now use full range of the IAC steps (although from my testing, I find that after about 200 steps, the airflow gains per step seem to be smaller than what the factory IAC area table shows).

As a preface to the issue that has me baffled, I did a quick check of how sensitive the engine was to IAC steps. Using the DVT tools, with timing locked and ensuring measured Lambda was consistent and cooling fan constant on, for a given TB set screw position I varied the IAC counts and saw (averages from data):
1 step = 625RPM, 7.25g/s airflow
45 steps = 1080RPM, 11.75g/s airflow
All steps in between fell in between as one would expect, so it seemed clear that the IAC effect is significant in this application. However, the issue at hand...

With DVT still keeping everything else consistent/locked, if I let the IAC control idle speed, one minute the engine can idle happy at 15 IAC steps, but if I shut the engine off and restart it, to idle at the same RPM and airflow now requires around 135-150 steps no matter how long after startup I wait?!? Keep in mind, I am keeping timing locked, cooling fan load constant, no steering or alternator loading changes, trans fluid at full temp and in park, and am ensuring ECT, IAT, and Lambda are extremely consistent. I surely thought a vacuum leak or sticking throttle had to be happening, but have double and triple checked and confirmed that's not the case. This phenomenon is very repeatable though; if while idling at 135 steps I gently open the throttle to 0.4 to 0.8% just enough that it doesn't kick out of idle mode, the IAC steps do come back down to around 15 and when I release the throttle and it goes back to 0.0% the IAC steps do NOT go back up, they stay down however the engine speed and airflow are still normal. I can physically go push the throttle arm closed to make sure its not sticking, and even blip the throttle multiple times, and it still is just fine at only 15 steps again. That seems impossible?!? Sure enough, if I shut the engine off, and restart it, when it levels out its back to 135+ steps again. This cycle between 15 and 135 steps is absolutely repeatable and has me baffled. While it is supposedly at 135+ steps, if I use DVT to command the steps lower and lower it does pull the rpm down and stalls the engine, so either there is some mystery airflow getting to the engine or its for some reason reporting 135+ steps when its really at something much less?

Anyone ever seen anything like this? Is the IAC motor maybe not finding its zero position correctly after shutdown/restart or something? I thought that the IAC system was designed to automatically re-zero. With that wild of IAC step range there is no way to keep the idle trims reasonable.

Thank you for taking the time to write up all of the results from what you have tried / found with regards to idle. Like most of us, I have played with / pulled my hair out messing around with idle / IAC circuit in general. I am going to throw something out there that maybe you have already tried. If so, then it will be something else to check of your list of possible culprits. B4308 Airflow Parked. The odd thing about this table is that it is scaled in terms of air flow (not IAC steps). I am wondering if this is still stock, then maybe it is why it jumps back p to 135+ steps???

Hmm, I have not done any testing with the airflow parked table. It had seemed on the surface that it only set the IAC position during engine shutdown or key on, engine off, but things aren't always as they seem on the surface. Since the strange IAC steps "reset" is happening after shutdown/restart I think I just might do some testing with that b4308 table!

One other thing I am going to check (which I should have done when I was porting the IAC passages) is to compare the actual IAC seat depth vs. the IAC mounting face to a stock LS1 DBC I have here.
*Edit* - checked the IAC seat geometry (depth and diameter) and it does match the stock LS1 TB, so that isn't a variable

SOLVED! Put an old grungy used LS1 IAC motor I had laying around on there and its dead consistent now! Apparently, the nice new IAC motor was reporting that it was increasing the steps for startup/restart airflow (to 135ish) but actually was still sitting at around 15 or so steps, wherever it was when it was shut off. It would still correctly move in and out to control idle but for some reason was not moving as it was supposed to during shutdown/restart. Still not sure how/why that could be but jeez, hopefully if someone else gets a bad new IAC and has similar symptoms they can find this thread! Wish it was still possible to rely on a new part being good.

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Originally Posted by turbo_bu

B4308 Airflow Parked. The odd thing about this table is that it is scaled in terms of air flow (not IAC steps).

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The name "Airflow Parked" is a bit of a misnomer. This table has nothing to do with the parked position of the IAC. I would recommend leaving the stock values in this table.

The new IAC is the identical part number to the old IAC...?

I am seeing a trend of poor quality control on new replacement parts, it costs the end user too much time/money, it costs the manufacturer nothing.