Idles high after being in gear.

BigRedOne said:

I understand that the PROM is read only but if the ecu is seeing the p/n switch change and shows that in the scanner that doesn't mean the PROM isn't faulty or hasn't been corrupted in a way that responds with incorrect data then the ecu will will respond incorrectly and no code will be set. also have read where they can get grounding issues within the PROM and that will give false data. Correcting me if this is wrong. Not many answers left at this point.

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If the eprom was corrupted, whatever is corrupted is constant/not changing. In your case, the ECM is doing two different things in response to some changes in state between the cold start and after being in gear. (At least that’s what I’m stuck on in at this point in this mystery). Regardless, whatever is changing is not in the prom and though I think it’s theoretically possible that some corruption in prom sends the code down some path it would not have otherwise gone in response to whatever is changing after being in gear … it seems that would be unlikely. but strange things happen, it will be interesting to see how this unfolds.

Sabinoerc said:

If the eprom was corrupted, whatever is corrupted is constant/not changing. In your case, the ECM is doing two different things in response to some changes in state between the cold start and after being in gear. (At least that’s what I’m stuck on in at this point in this mystery). Regardless, whatever is changing is not in the prom and though I think it’s theoretically possible that some corruption in prom sends the code down some path it would not have otherwise gone in response to whatever is changing after being in gear … it seems that would be unlikely. but strange things happen, it will be interesting to see how this unfolds.

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Agreed. But I am stuck on that once it goes into gear it acts like it never sees it go back to park. That is the constant. Even though you see it in the scanner going back and forth the ecu acts like it is not. Cycling the key just sets the ecu back to the beginning. Not sure if it's what is going on. I am still learning this era efi.

Got my ALDL Cable today. Going to load the programs tonight and look them over and hopefully tomorrow pull a data log that is correct and has better info.

BigRedOne said:

Got my ALDL Cable today. Going to load the programs tonight and look them over and hopefully tomorrow pull a data log that is correct and has better info.

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Hopefully you can collect data over time, in the form of an Excel spreadsheet that can be uploaded and shared.

1998_K1500_Sub said:

Hopefully, you can get something in the form of an Excel spreadsheet that can be uploaded and shared.

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Unfortunately this site doesn't support .xls(t) extensions unless you zip it. Click on "Attach files" to see what is supported.

OK, here's an Excel spreadsheet I just created, which I then stored in a "Compressed (zipped) Folder" (I'm using Windoz).

@BigRedOne, you may be asked to do something similar if we're to look through your collected data.

BigRedOne said:

Agreed. But I am stuck on that once it goes into gear it acts like it never sees it go back to park. That is the constant. Even though you see it in the scanner going back and forth the ecu acts like it is not. Cycling the key just sets the ecu back to the beginning. Not sure if it's what is going on. I am still learning this era efi.

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Building on this sub-thread started by @Sabinoerc with follow-up by @BigRedOne... Your comments are the reason why I'm stuck on a couple of hypothesis:

(1) I'm wondering if the IAC is physically tracking each and every "step" issued it by the ECU.

We know the IAC gets "buried" intentionally by the ECU (either at initial turn-on or at turn-off, I believe the latter) to reset its position. After that, during normal operation it's "step and pray" because the ECU doesn't have any feedback on the IAC's position. But...

@BigRedOne's original, and new replacement, IAC both exhibit the same behavior. He's also checked the wiring to/from the ECU - IAC and reports essentially low resistance.

So the only cause of the problem I can imagine (at this time) is the IAC drivers on the ECU aren't fully functional, or there's some resistance in the circuit that's "new", e.g., due to corrosion somewhere (PCB, connection, ?).

(2) I'm wondering if the "Minimum Idle Air Rate" and "Controlled Idle Speed" are proper, thus causing the IAC to be in its proper position during, e.g., engine warm, closed loop idle. I know we've discussed the service manual's instructions for these (they're uploaded into this thread) but it's not clear that we, collectively, knew for sure how to perform them according to the manual's procedures (see in particular pg. 4-44 the "open loop" requirement for setting the Minimum Idle Air Rate).

In any event, failing the above adjustments, the IAC control algorithm may I suspect find itself in unknown territory and react in ways unexpected, e.g., with high-idle as seen by @BigRedOne.

1998_K1500_Sub said:

So the only cause of the problem I can imagine (at this time) is the IAC drivers on the ECU aren't fully functional, or there's some resistance in the circuit that's "new", e.g., due to corrosion somewhere (PCB, connection, ?).

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I did replace the ecu and had no change in condition. Or are you suggesting the "due to corrosion" be in the PROM? (What is PCB? (Prom circuit board?)

BigRedOne said:

I did replace the ecu and had no change in condition. Or are you suggesting the "due to corrosion" be in the PROM? (What is PCB? (Prom circuit board?)

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No, I'm not suggesting a problem in the digital circuitry. In my scenario, as I tried to describe it, the problem might exist in any circuit (transistor or integrated circuit), wire, physcial connector or printed circuit board trace that carries current during IAC actuation and, therefore, might possibly impede proper operation of the IAC's stepper motor.

BUT... as you point out, you've replaced many potentially suspect parts and ohmed-out most of the other suspects... not all, but many of those which are typically problematic.

Damn this is an enigma.

BigRedOne said:

Agreed. But I am stuck on that once it goes into gear it acts like it never sees it go back to park. That is the constant. Even though you see it in the scanner going back and forth the ecu acts like it is not. Cycling the key just sets the ecu back to the beginning. Not sure if it's what is going on. I am still learning this era efi.

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I'm definitely still learning but have bumbled around in TP enough to know there are too many states/flags in the code to easily connect the dots between cause and effect in some cases. In the early ECMs like these, it seems a lot of stuff is open loop and ECM assumes if it reads it it's true or if it commands it it's done. EG. there is no sense on fuel pressure, ECM assumes it's good and if it's not - you get strange results as ECM is assuming it's good. I think this is what 98_K1500 is thinking about with respect to IAC - maybe IAC is not as ECM thinks for some reason.

When you get TP up you'll see what I mean with respect to all the flags and conditions which the ECM monitors or sets - look thru and see all the "idle" and "IAC" related stuff. In your case, perhaps something easy to check would be "Desired Idle RPM" (which I assume is the RPM the ECM is trying to control to?) and there is a flag on "Idle RPM High" (which I assume is whether the ECM thinks the idle is too high or not?) . I think you should be able to tell whether ECM thinks it's controlling to the correct RPM or not - and whether that "correct" RPM is different in the two cases. Attached is quick/simple trace as example - you can't see idle rpm high flag going high due to scale but it's going high and correlates with IAC changes. IAC desired and present overlap (also don't know how ECM determines "present" position on IAC).

Take what I say here to be perhaps ~50% correct I'm guessing at a lot of this when I get into TP. Good luck!