Strange IAC behaviour?

[benno25]

Hi,

Looking at logs from my vehicle, I've noticed the IAC position showing it spiking consistantly as shown in the picture attached.

Does anyone know what causes this?

Cheers
Benno

[VodeAn]

Can I have more information about the setup, stock TB and intake? When did the issue arise? Miles on the vehicle? Mods, big freaking potato cam/stock cam?

Could just be a bad IAC, although it doesn't look to be sticking open or shut.

You could try the dealer fix... REPLACE IAC, REPLACE TPS, REPLACE PCM, TELL CUSTOMER NO CEL = NO ISSUE, MAKE SURE A DIFFERENT TECH WORKS ON THE VEHICLE EVERY TIME.

[benno25]

Car has had an LS3 conversion done and has the following mods.

Texas Speed cam 235/239 111LSA
PRC 260cc aftermarket casting LS3 heads
Harrop Hurricane ITB intake

Car has done about 130,000km, not sure what that is in miles?

It has a cable operated throttle and has a reasonably new IAC motor. I've looked back through a couple of logs and it only seems to do it when the IAC is all the way open.
I think I still have the old IAC motor somewhere, so might put that back in and see if the problem persists.

Cheers

[LSxpwrdZ]

Log commanded vs actual IAC and that'll tell you if it's the PCM commanding it or if it's the actual stepper motor freaking out.

[SSUte01]

Because the timing between the spikes is so consistent, I am going to suggest it's either a logging issue - I've seen the log tool miss a PID address from time to time and may need a reset, re-poll of PIDs or if it is real, because the IAC is fully closed at 0, it may be a wiring issue or potentally in a super low resolution range of the IAC steps versus comanded IAC. Like above, commanded IAC would be good to see.

[04blackgmc]

What year is it? I had some REAL weird IAC issues with a SD 2BAR procharged 2000 Camaro a few weeks ago. It would keep dying and i would watch the IAC go crazy and timing dip to about half of what was commanded. The car had a mail order tune and after trying every single thing i could think of i went ahead and wrote a 2bar SD 2002 OS into it.... FIXED. Was a first for me.

[DSteck]

Because the timing between the spikes is so consistent, I am going to suggest it's either a logging issue - I've seen the log tool miss a PID address from time to time and may need a reset, re-poll of PIDs or if it is real, because the IAC is fully closed at 0, it may be a wiring issue or potentally in a super low resolution range of the IAC steps versus comanded IAC. Like above, commanded IAC would be good to see.

He might have an issue with the throttle cracker setup. When you command none and ride the rpm/speed breakpoint in the table, it can cause it to "ficker".

[benno25]

He might have an issue with the throttle cracker setup. When you command none and ride the rpm/speed breakpoint in the table, it can cause it to "ficker".

I do have the throttle cracker and follower zero'd.
I find with the ITB's driveability and returning to idle etc is perfect without it.

Cheers
Benno

[SSUte01]

He might have an issue with the throttle cracker setup. When you command none and ride the rpm/speed breakpoint in the table, it can cause it to "ficker".

Indeed, zero can cause anomalies between OSs, that's a good point I didn't consider. I had an issue many years back, I posted about it here circa 2007 that zeroing cracker on my old OS caused the IAC to zero and an annoying dip. Putting a small value like 0.01 g/sec was enough to cure it. It bugged me for a while.

[JamesLinder]

Damned the IAC on LS motors equipped with throttle cables; dealing with these are more difficult than tuning the entire motor. What works for my 2000 Camaro is to pull the two PCM fuses in the fuse blocks under the hood to reset the TPS to zero and erase all idle trims learned in the PCM. I find this to be necessary sometimes after uploading a revised tune into the PCM even when the throttle body butterfly adjustment screw has not been touched at all. Not sure if this is a Gen3 PCM specific issue or not, however if you are not careful you will end up trying to tune idle when TPS is not at zero just because you uploaded a revised tune. %TPS is something I always have displaying on my DashDAQ unit just because of this problem. One of course must avoid increasing the IAC in park settings too high or this will cause car to drive itself with throttle closed. I found even with my highly modified 408 stroker that the stock IAC park air settings were sufficient. If you have your TB butterfly screw adjusted to give the motor most of the air it needs during idle, then you will have plenty of IAC left for the extra air needed for cold starts, and your base running airflow idle air values will not need to be increased as much. There is a tendancy for inexperienced tuners to increase these entries too much. Again, it is all fruitless if your TPS is not at zero percent when adjusting idle settings.