High IAC counts on cold start, but subsequent hot restart yields low IAC counts.
11-23-2016, 07:03 PM
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High IAC counts on cold start, but subsequent hot restart yields low IAC counts. So I'm trying to nail down the idle tune and have been following the Idle Tuning Guide. I have run into an interesting problem and am hoping someone can point me to the solution to this problem.
Setup:
Block: Stock LQ4 6.0L
Heads: Stock LS3
Cam: GMPP LS ASA Cam
Intake: L76 ported for 100mm TB
Throttle Body: BBK 100mm Cable Driven Throttle Body
Problem:
While the engine is cold, I start it up and idle to log the Base RAF for the idle. The Desired Airflow numbers are pegged at 40 g/s and 310-300 IAC counts until about 60C. Then when it warms up it settles at 20+ g/s and ~250 IAC counts. This would suggest that I need to open the throttle body blade stop and allow more air to the engine. Instead I did a restart without touching anything and the engine settled into an idle with ~9 g/s and 80 IAC.
So my question is what is causing this? Is there tables that controls cold start air requirements at temp and the decay rate?
Thanks in advance for any help.
Setup:
Block: Stock LQ4 6.0L
Heads: Stock LS3
Cam: GMPP LS ASA Cam
Intake: L76 ported for 100mm TB
Throttle Body: BBK 100mm Cable Driven Throttle Body
Problem:
While the engine is cold, I start it up and idle to log the Base RAF for the idle. The Desired Airflow numbers are pegged at 40 g/s and 310-300 IAC counts until about 60C. Then when it warms up it settles at 20+ g/s and ~250 IAC counts. This would suggest that I need to open the throttle body blade stop and allow more air to the engine. Instead I did a restart without touching anything and the engine settled into an idle with ~9 g/s and 80 IAC.
So my question is what is causing this? Is there tables that controls cold start air requirements at temp and the decay rate?
Thanks in advance for any help.
11-28-2016, 03:36 PM
#2
Gen 3 PCM?
It's the throttle body. They don't have enough IAC passage, so either cold or host restarts are typically problematic.
What I would do is disconnect the IAC valve, start the car, and let it come up to temp. Then open the blade until you the motor starts to idle high. Back off a very small amount 1/16 of a turn or so. Now you have your blade open enough to idle with the IAC completely open. Now, the IAC has control over the idle and can close down to bring the idle down. Now you can run the idle air config again to see if it helps.
You can also try to tune the IAC table a bit. Look at the IAC Steps vs Desired Idle Effective Area table under the Idle Airflow tab once you have adjusted the blade open enough. And look and see where it idles. Let's say from a cold start to warm it now idles at 80 counts. Look to see where the 80 counts occur. If it's around say 32-34 mm^2 look where 50-60 counts are. 4-5 cells to the left I would imagine. You can take the entire table and move it to the right 5 cells and now you're IAC passage is roughed in better than it was. You can do a lot more to tune it, but this is typically good enough. You should now idle around 60 counts hot and on restarts it may be a little lower. But it should not be idling at 310 (fully open). It will probably idle around 200 counts cold as that's where the stock IAC tuning cuts off (84 mm^2) in the table (Desired IAC Area Max).
Now you can re-run the idle airflow to get your base numbers set.
It's the throttle body. They don't have enough IAC passage, so either cold or host restarts are typically problematic.
What I would do is disconnect the IAC valve, start the car, and let it come up to temp. Then open the blade until you the motor starts to idle high. Back off a very small amount 1/16 of a turn or so. Now you have your blade open enough to idle with the IAC completely open. Now, the IAC has control over the idle and can close down to bring the idle down. Now you can run the idle air config again to see if it helps.
You can also try to tune the IAC table a bit. Look at the IAC Steps vs Desired Idle Effective Area table under the Idle Airflow tab once you have adjusted the blade open enough. And look and see where it idles. Let's say from a cold start to warm it now idles at 80 counts. Look to see where the 80 counts occur. If it's around say 32-34 mm^2 look where 50-60 counts are. 4-5 cells to the left I would imagine. You can take the entire table and move it to the right 5 cells and now you're IAC passage is roughed in better than it was. You can do a lot more to tune it, but this is typically good enough. You should now idle around 60 counts hot and on restarts it may be a little lower. But it should not be idling at 310 (fully open). It will probably idle around 200 counts cold as that's where the stock IAC tuning cuts off (84 mm^2) in the table (Desired IAC Area Max).
Now you can re-run the idle airflow to get your base numbers set.
Last edited by JakeFusion; 11-28-2016 at 03:41 PM.
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