IAC not functioning correctly

[Chewy1576]

I feel like I have the 383 running fairly well on the 80 # injectors with the EGR disabled. The next issue I want to tackle is the idle speed. I am 90% sure that there is not a vacuum leak. This is the second new IAC I have put in, along with new connector. I have adjusted the minimum air screw to idle the engine at 600 RPM with the IAC closed. The ECM should be trying to adjust the idle to 750 RPM at operating temp. No matter what I do, I can't get the IAC counts below 48 and RPM below 950 with 0% throttle. I have attached the bin, ads, and xdf I have been using. Do you guys see anything that would keep the idle speed and IAC counts that high?

[mmigacz]

If you do a datalog at idle, does it show its in idle mode? For example

8684

[Chewy1576]

Yes it will go in and out of idle mode based on vehicle speed and TPS. I have looked at the IAC vs Coolant tables (open loop) and the Coolant vs RPM table (closed loop) and it seems to follow the open loop table, but once it switches over to closed loop, it will hang at 48.

[mmigacz]

Can you post a datalog of the engine starting cold? Keep the datalog running until the engine is up to operating temperature (idling the whole time).

[Chewy1576]

I will put a log together a little later. Yesterday when I was replacing the MAP sensor and TPS connectors (The shop that did the motor swap broke all of the tabs off and one of the MAP terminals was destroyed) I noticed that the ground straps on the back of the block, firewall, and frame are pretty corroded. Going to replace those before I fire it up so I'm not snaking my hands around hot exhaust pipes.

[Chewy1576]

Here is a log from cold to hot after I replaced the ground straps. I also switched to the LD adx.

[mmigacz]

Weird!

I noticed some strange trends.

If you compare the actual IAC steps being put out on the data log, its always 38 steps more than what the Idle speed vs coolant is commanding. This 38 step trend stays consistant at temperatures less than 55 degrees C. For example, at 37 degrees C the table is commanding 65 steps. The actual steps being put out is 103 (38 +65).

When the temperature reaches 55C the step increase changes from 38 to 28. The Idle speed vs coolant is commanding 20 steps, but the actual steps are 48 (20 +28).

Here would be an interesting test. Change some values in the Idle speed vs coolant. See the image below.

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Post another data log with the same procedure as the last datalog. It would be interesting to see how the steps change with these values.
I'm also curious if the idle speed vs coolant temp table will start its logic if the RPM is allowed to drop below 700 RPM.

[Chewy1576]

Thanks for noticing that! I made your suggested changes to the Position vs Coolant table and I will get a log in a little while.

I have been looking through the parameters and is there some offset somewhere for A/C or low system voltage that would add IAC counts to keep the RPM up to support running the A/C compressor or normalize voltage output if the alternator is getting weak? I have read where this can be adjusted on '7747 ECM's, but not an '8747 yet.

EDIT: I just found this guy. There isn't really a description in the xdf but is says something about adding counts. It was set to 25 counts. I will zero it out and see what happens. The A/C doesn't work and I'm considering deleting it anyway.

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[Chewy1576]

With those changes, the idle speed is now 750 RPM and the IAC counts are 15-20. The first part of the log is kind of rough i'm assuming because I disconnected the entire ECM to change the chip and it had to get caught up again. I was kind of worried when the idle surged up to 1300 for a little bit, but then the IAC counts started dropping and settled in the low 30's. I thought I would try increasing the minimum air with the screw and the counts dropped down even further, telling me that the ECM will now also control idle with the IAC like it should. Thanks for your help trying to track those parameters down! I'm still new to this process but I'm learning quickly! :rockon:

[mmigacz]

Good news!

Do you know what change helped:

zeroing the A/C steps?
changing the Idle speed vs coolant values?

[Chewy1576]

Honestly, I made both changes on the same chip, so it could have been either or both together. Not a very good move diagnostic-wise, but it did fix the problem.

Now on to fix the surging deceleration...

Thanks again for your help! If I could buy you a beer I would!

[mmigacz]

One thing after looking at your datalog.

The strange value (10 steps) I had you put in at 80C did have an expected affect when 80C was reached. Make sure to change that value to 0.

[Chewy1576]

Right at about 80 degrees C is when I turned in the minimum air adjustment screw to see if the ECM would reduce the IAC counts to control the idle on it's own. I thought that the 10 there in your suggested changes was a typo so I set it to the same value as the others. Here is what the table is at now:

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