Idles high after being in gear.

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BigRedOne

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No, it's the other way around, 0 is closed, 140 is at the open "parked" position.


What PCM service number do you have? I'll look it up for you.
My Original ecu was a 1227747 I'm looking at the old one now and its factory PROM is ANTY right on the case and on the prom I removed. The replacement from GM is a 88999146

As far as open or closed its at 145 with all the air coming in and 0 with the pintle blocking all the air. The service manual I read for a 5.7 confused me by saying manually screw the pintle into the closed position and when you do the relearn it will full open to 0 and then it is set. So I don't know if open is closed or closed is open or what(damn engineers) lol...

But I do know this.
With the scan tool I have been using you can not turn of the 10k resister and I get a very different results that are almost like its fixed. So.. With the scan tool plugged in it idles in park at 1000rpm as it gets up to there at 750rpms I can see that my IAC counts are about 38 (little high I know it should be 30) at the 1000rpms it settles and the count is about 56 counts. But get this. When I put it into gear the rpms drop to around 625 and it slow increases to 750rpms and stays there.(good right?) The IAC counts are at full tilt 145!! Now I go back to park and it rpms race up to 2000rpms and hang there for about a minute then they start to come down and settle at ...... "1000RPMS!!!!" WHAT THE HELL IS GOING ON???? Why does having the scanner plugged in have to do with it doing this??? without the scanner and it is back to perfect idle in park 750rpms and it idle up to 750 when put in to gear then back to park 1500-1700rpms and NEVER COMES DOWN BELOW 1400RPMS No matter how long you wait. And yes I did the same test several times back and forth with and without the scanner with the same results both ways. Is it creating a ground? what does the 10k resister have to do with it? Can I put a resister in somewhere lol.. Starting to feel more like a bad (shorted) chip.
I just don't get it. Spent hours on this today. Oh and put a new temp sensor and egr valve in just to make sure that was covered. The temp sensor seems to be a little cooler. Fully warmed up the old one read 185 and the new one reads 179 so I don't know if that helps or hurts.
 
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BigRedOne

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Has anyone checked that the ECU is receiving / reporting the correct vehicle speed?

My 1995 S10 won't idle-down when in shifted into neutral if the vehicle's in motion; only once the vehicle is stopped (or nearly so) will the ECU return the engine to base idle. So, at least in my case, the vehicle speed bears on the target engine RPM and so IAC.
I thought the same thing and in park the speed sensor in the scan tool is showing 0 in park and in gear as well as back to park.
Good thought thou. Keep digging and maybe one of us will have that ah ha moment and all will be clear.
 

BigRedOne

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I have been on other sites thru this adventure and several people have stated that the PROM on the 454 wasn't unusual to go bad and cause erratic idle issues without throwing a code. A few had stated they just got junk yard ones and their problems went away. Me I'm not so into used computer stuff. Never lucky enough to not be more of a problem. but that being said the problems non the less went away. So my issue is. A replacement new prom isn't available as far as I can see. Second party isn't for me (trust issues) so aftermarket may be my only hope. The problem there is they only really make performance upgrades and I need a towing tune. Don't need my timing table wrong causing heating issues while loaded. So I see that Jet performance makes what the call a six pack deal. It has a towing tune as well as an economy tune and 2 performance tunes. Is anyone familiar with Jet performance? I heard of them years ago but have never heard good, bad or otherwise as to if their products are good. If they have a towing tune thats good and a eco tune that works that would be great but if its junk it will be $252.00 down the drain especially if this doesn't fix the underlying issue. Thoughts?
 

1998_K1500_Sub

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Calling All Members:

Is it truly a “MEMCAL” on that ECU… the MEMCALs I’ve seen had a ROM and then some adjunct circuitry (on a small printed circuit board w/in the MEMCAL assy) that did various functions depending on the vehicle:

- knock sensor signal filtering (I think)

- limp home (if ECU malfunction) functions for fuel injection control (I think)

- others?

- WHAT functions might this 454’s MEMCAL’s adjunct circuitry support that could cause this idle behavior?


Are the 1989 service manuals on the server? They might detail the MEMCAL functions.
 
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Sabinoerc

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My Original ecu was a 1227747 I'm looking at the old one now and its factory PROM is ANTY right on the case and on the prom I removed. The replacement from GM is a 88999146

As far as open or closed its at 145 with all the air coming in and 0 with the pintle blocking all the air. The service manual I read for a 5.7 confused me by saying manually screw the pintle into the closed position and when you do the relearn it will full open to 0 and then it is set. So I don't know if open is closed or closed is open or what(damn engineers) lol...

But I do know this.
With the scan tool I have been using you can not turn of the 10k resister and I get a very different results that are almost like its fixed. So.. With the scan tool plugged in it idles in park at 1000rpm as it gets up to there at 750rpms I can see that my IAC counts are about 38 (little high I know it should be 30) at the 1000rpms it settles and the count is about 56 counts. But get this. When I put it into gear the rpms drop to around 625 and it slow increases to 750rpms and stays there.(good right?) The IAC counts are at full tilt 145!! Now I go back to park and it rpms race up to 2000rpms and hang there for about a minute then they start to come down and settle at ...... "1000RPMS!!!!" WHAT THE HELL IS GOING ON???? Why does having the scanner plugged in have to do with it doing this??? without the scanner and it is back to perfect idle in park 750rpms and it idle up to 750 when put in to gear then back to park 1500-1700rpms and NEVER COMES DOWN BELOW 1400RPMS No matter how long you wait. And yes I did the same test several times back and forth with and without the scanner with the same results both ways. Is it creating a ground? what does the 10k resister have to do with it? Can I put a resister in somewhere lol.. Starting to feel more like a bad (shorted) chip.
I just don't get it. Spent hours on this today. Oh and put a new temp sensor and egr valve in just to make sure that was covered. The temp sensor seems to be a little cooler. Fully warmed up the old one read 185 and the new one reads 179 so I don't know if that helps or hurts.
Let me start by saying I just read thru thread and don’t have any great ideas. Also, the people responding here have much more experience in this than I do so would defer to them on any technical detail.

That said, it seems that at this point it’s fumbling around in the dark until you get a tool which can capture all the data, status, sensor inputs, ECM outputs when it’s idling correctly and when not. Time spent getting TunerPro capable will pay off in chasing this down. If you end up getting someone to tune for you, you’ll likely need it there also.

There is something different in some flag, state, sensor input between the initial idle and after in gear. Whatever it is is repeatable so doesn’t seem like noise causing random events. Firmware just acts on inputs and I think unlikely there is a bug in a stock system. So it’s likely commanding something differently in the two cases based on something its monitoring. There are all sorts of conditions and states the ECM acts on which you can never see by messing with stuff under the hood. You can also see what ECM is commanding to engine. For example, I think you see what the ECM is telling IAC to do in both conditions? It’s been a while since I’ve messed with TP so don’t remember. PlayTBI and others are much more capable here.

its a really frustrating problem and sorry I don’t have any better suggestions than to dig in deeper. Good luck!
 

1998_K1500_Sub

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Firmware just acts on inputs and I think unlikely there is a bug in a stock system. So it’s likely commanding something differently in the two cases based on something its monitoring.

Agreed.

I'm also curious about what other functions are on the MEMCAL's circuit board (assuming it's got the adjunct board, I need to find out). OP claims others have told him they've solved the problem using a MEMCAL swap. Like you, I doubt the firmware is incorrect, so if a MEMCAL swap fixes (reportedly) the problem then the issue lies with the MEMCAL's circuit board.

Back on the topic of firmware: I haven't monkeyed with the older ECUs in a while, but any system designed worth a $h!t would put a checksum in the firmware, such that if the ECU's microprocessor determines the checksum is invalid it goes into limp-home mode. Too, this problem is so predictable / repeatable it IMHO rules out a host of other possibilities.

This wil be a fun one to solve. I wish I was on-site with this truck and could log some data, but IL and FL aren't close.
 
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BigRedOne

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Agreed.

I'm also curious about what other functions are on the MEMCAL's circuit board (assuming it's got the adjunct board, I need to find out). OP claims others have told him they've solved the problem using a MEMCAL swap. Like you, I doubt the firmware is incorrect, so if a MEMCAL swap fixes (reportedly) the problem then the issue lies with the MEMCAL's circuit board.

Back on the topic of firmware: I haven't monkeyed with the older ECUs in a while, but any system designed worth a $h!t would put a checksum in the firmware, such that if the ECU's microprocessor determines the checksum is invalid it goes into limp-home mode. Too, this problem is so predictable / repeatable it IMHO rules out a host of others possibilities.

This wil be a fun one to solve. I wish I was on-site with this truck and could log some data, but IL and FL aren't close.
From what I understand MEMCAL and PROM's are physically different and this is a PROM if that helps anyone. The others that stated replacing the PROM fixed their issues stated that it was because of the chip itself shorting(grounding) to itself? Not necessarily a firmware corruption. Now I have read a few posts that are from early 2000's when junk yards had these trucks by the dozens available and they would grab several PROM's from vehicles cheap and many had different problems when installed. They just tried a few until one ran correct. That is why these posters came to the conclusion of it not being a rare thing to happen and why many of the trucks were probably junked in the first place. That being said I am not a PROM or MEMCAL expert by any means so I may sound stupid in my statement that maybe the chips themselves were just made out of cheap material at the time Causing possible grounding problems that don't throw a code and that's why so many problems? I don't know but I am EXTREMELY frustrated either way.
 

BigRedOne

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Let me start by saying I just read thru thread and don’t have any great ideas. Also, the people responding here have much more experience in this than I do so would defer to them on any technical detail.

That said, it seems that at this point it’s fumbling around in the dark until you get a tool which can capture all the data, status, sensor inputs, ECM outputs when it’s idling correctly and when not. Time spent getting TunerPro capable will pay off in chasing this down. If you end up getting someone to tune for you, you’ll likely need it there also.

There is something different in some flag, state, sensor input between the initial idle and after in gear. Whatever it is is repeatable so doesn’t seem like noise causing random events. Firmware just acts on inputs and I think unlikely there is a bug in a stock system. So it’s likely commanding something differently in the two cases based on something its monitoring. There are all sorts of conditions and states the ECM acts on which you can never see by messing with stuff under the hood. You can also see what ECM is commanding to engine. For example, I think you see what the ECM is telling IAC to do in both conditions? It’s been a while since I’ve messed with TP so don’t remember. PlayTBI and others are much more capable here.

its a really frustrating problem and sorry I don’t have any better suggestions than to dig in deeper. Good luck!
Ordered a ALDL cable with bluetooth so I can run ALDLAndroid and be able to see more data. Going to be over a week till I get it thou.
 
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