Weird Hanging Idle When In NEUTRAL And Moving
Weird Hanging Idle When In NEUTRAL And Moving I've attached a tune file and I'm hoping someone can help me with this issue. Vehicle is a 1994 GMC Sierra 1500, 2 wheel drive, that has been LS swapped with a 4.8L engine. The only mods from BONE stock is a CAI. Even has the factory cast iron exhaust manifolds-but later manifolds with no EGR. The truck starts, runs, and drives with no major issues. The problem is that if you are rolling down the road and you throw it in neutral, the engine spikes in RPM's and just hangs there. For example, I'm doing 30+/-MPH down the road and just bump the shift lever into neutral to coast-the engine idle will jump up to 2250RPM and just hang there. It will hold that RPM until you come to a complete stop, at which point I assume the IDLE pid comes into play and brings the idle back down. This really is a non-issue as how often do you throw your trans in neutral while coasting? But, at the same time, I feel that if someone else were to be driving and throw it in neutral it could be a safety issue. Not to mention it's odd trying to put it back into gear when the RPM's are hanging way up there.
If anyone can point me in the right direction, I would really appreciate it. My brain is telling me this is potentially a throttle cracker issue, but before I jack the tune up and make it even worse, I figured I would ask those that know. Any and all help would be greatly appreciated. TIA!
If anyone can point me in the right direction, I would really appreciate it. My brain is telling me this is potentially a throttle cracker issue, but before I jack the tune up and make it even worse, I figured I would ask those that know. Any and all help would be greatly appreciated. TIA!
Sorry, I should have posted that it is a 4L60e, and that it has NO NEUTRAL switch on the side of the transmission. Because the truck is a 1994 they did not come from the factory with a Neutral Safety Switch on the side of the transmission.. 1995 and later did. So this truck uses a column mounted neutral switch. For all I know the PCM does not even know that I have bumped the trans to neutral.. it may still show that the trans is in gear because I don't think any of the factory column mounted neutral safety switches are attached to the PCM to provide any signals. That was a great question G Atsma and I should have posted that info above.
Teching In
Joined: Jun 2023
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Resurecting this old thread as I am experiencing the same thing. The mine only occurs when the engine is still warming up. Once it's reached operating temp this does not happen.
Also a mosty stock 4.8L mated to a 4l60e. Also with no neutral or gear position indicator to the PCM.
I don't have, nor do I know much about HPtuners and I know nothing about throttle cracker setting.
Can the OP @Kawabuggy enlighten me on what exacty you changed and if the lack of neutral switch was related?
Thanks in advance.
Also a mosty stock 4.8L mated to a 4l60e. Also with no neutral or gear position indicator to the PCM.
I don't have, nor do I know much about HPtuners and I know nothing about throttle cracker setting.
Can the OP @Kawabuggy enlighten me on what exacty you changed and if the lack of neutral switch was related?
Thanks in advance.
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Prschuyler-if it ONLY happens when its warming up, there are a couple of things that could be at play here... BRAF (base running airflow) could be off in the lower temperature ranges. the OLFA table could be "off". Maybe the tune is commanding a higher idle in the lower temperature ranges then reduces the idle speed as temp comes up... So many things that can cause what you are describing. Can you post a copy of your tune, and maybe a log showing the anomaly happening? That would be the best/fastest way to help you. If you post a log you need to have CRACKER airflow as one of your channels, in addition to all of the other normal idle PID's...
What I did to fix my situation; Hp Tuners/engine/idle/Airflow/Throttle Cracker/Airflow Highlight the entire table, zero out the entire table, save it, burn it, test it.... I figured out the PCM was adding cracker airflow by logging Cracker airflow, and throttle follower airflow, and could easily see it on the log. Once I zeroed (I zeroed it AFTER the last tune i posted) it, all of these high idle in neutral/ run away idle issues when cold, went away.
Don't just randomly start changing things in your tune though.. You need to post a copy of your tune FIRST so that everyone here can review it for any obvious problems. Then the log will absolutely help drill down on the issue.
What I did to fix my situation; Hp Tuners/engine/idle/Airflow/Throttle Cracker/Airflow Highlight the entire table, zero out the entire table, save it, burn it, test it.... I figured out the PCM was adding cracker airflow by logging Cracker airflow, and throttle follower airflow, and could easily see it on the log. Once I zeroed (I zeroed it AFTER the last tune i posted) it, all of these high idle in neutral/ run away idle issues when cold, went away.
Don't just randomly start changing things in your tune though.. You need to post a copy of your tune FIRST so that everyone here can review it for any obvious problems. Then the log will absolutely help drill down on the issue.
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Joined: Jun 2023
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Prschuyler-if it ONLY happens when its warming up, there are a couple of things that could be at play here... BRAF (base running airflow) could be off in the lower temperature ranges. the OLFA table could be "off". Maybe the tune is commanding a higher idle in the lower temperature ranges then reduces the idle speed as temp comes up... So many things that can cause what you are describing. Can you post a copy of your tune, and maybe a log showing the anomaly happening? That would be the best/fastest way to help you. If you post a log you need to have CRACKER airflow as one of your channels, in addition to all of the other normal idle PID's...
What I did to fix my situation; Hp Tuners/engine/idle/Airflow/Throttle Cracker/Airflow Highlight the entire table, zero out the entire table, save it, burn it, test it.... I figured out the PCM was adding cracker airflow by logging Cracker airflow, and throttle follower airflow, and could easily see it on the log. Once I zeroed (I zeroed it AFTER the last tune i posted) it, all of these high idle in neutral/ run away idle issues when cold, went away.
Don't just randomly start changing things in your tune though.. You need to post a copy of your tune FIRST so that everyone here can review it for any obvious problems. Then the log will absolutely help drill down on the issue.
What I did to fix my situation; Hp Tuners/engine/idle/Airflow/Throttle Cracker/Airflow Highlight the entire table, zero out the entire table, save it, burn it, test it.... I figured out the PCM was adding cracker airflow by logging Cracker airflow, and throttle follower airflow, and could easily see it on the log. Once I zeroed (I zeroed it AFTER the last tune i posted) it, all of these high idle in neutral/ run away idle issues when cold, went away.
Don't just randomly start changing things in your tune though.. You need to post a copy of your tune FIRST so that everyone here can review it for any obvious problems. Then the log will absolutely help drill down on the issue.
Here's a link to the cold start up behavier which I think is also funky and likely related. https://youtube.com/shorts/8R8rWWrDJzg?feature=share
I think it's time I buy HPTuners
Yes, you will need HP Tuners if you want to fix this yourself. In the interim, if you will post what city and state you are in, you might find someone close to you who can READ your "stock" file... and then you can post it here for review. I cannot begin to tell you how many "stock" files I've seen that were NO WHERE near stock.... ha ha ha.. Yes the person believes they are stock.... but if one person has been into it.... then you might as well figure that 100 have been into it. Maybe it is stock... Stock files usually make for better starting points.
I watched your video above but nothing can be drawn from that video that would be useful. I hear the engine idle increase, then eventually settle down. But we need to know WHAT the IAC is doing, what the engine temps are, what the STIT is doing, what timing is doing... MANY things need to be logged and viewed in order to push you in a specific direction. Start with the simple things-have you scanned it with a good scanner to check codes lately? Even a basic scanner can help you figure out quite a bit. If it truly is a STOCK PCM tune file, and the engine is all stock-then a scanner/code reader may be all you need. Keep in mind you will need at least 100 miles of drive time with NO disconnection of battery, NO DEAD battery, and no code/information clearing by scanner, in order to get any useful code information. Also multiple key-starts in order for the PCM to log codes.
I watched your video above but nothing can be drawn from that video that would be useful. I hear the engine idle increase, then eventually settle down. But we need to know WHAT the IAC is doing, what the engine temps are, what the STIT is doing, what timing is doing... MANY things need to be logged and viewed in order to push you in a specific direction. Start with the simple things-have you scanned it with a good scanner to check codes lately? Even a basic scanner can help you figure out quite a bit. If it truly is a STOCK PCM tune file, and the engine is all stock-then a scanner/code reader may be all you need. Keep in mind you will need at least 100 miles of drive time with NO disconnection of battery, NO DEAD battery, and no code/information clearing by scanner, in order to get any useful code information. Also multiple key-starts in order for the PCM to log codes.
Teching In
Joined: Jun 2023
Posts: 9
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Yes, you will need HP Tuners if you want to fix this yourself. In the interim, if you will post what city and state you are in, you might find someone close to you who can READ your "stock" file... and then you can post it here for review. I cannot begin to tell you how many "stock" files I've seen that were NO WHERE near stock.... ha ha ha.. Yes the person believes they are stock.... but if one person has been into it.... then you might as well figure that 100 have been into it. Maybe it is stock... Stock files usually make for better starting points.
I watched your video above but nothing can be drawn from that video that would be useful. I hear the engine idle increase, then eventually settle down. But we need to know WHAT the IAC is doing, what the engine temps are, what the STIT is doing, what timing is doing... MANY things need to be logged and viewed in order to push you in a specific direction. Start with the simple things-have you scanned it with a good scanner to check codes lately? Even a basic scanner can help you figure out quite a bit. If it truly is a STOCK PCM tune file, and the engine is all stock-then a scanner/code reader may be all you need. Keep in mind you will need at least 100 miles of drive time with NO disconnection of battery, NO DEAD battery, and no code/information clearing by scanner, in order to get any useful code information. Also multiple key-starts in order for the PCM to log codes.
I watched your video above but nothing can be drawn from that video that would be useful. I hear the engine idle increase, then eventually settle down. But we need to know WHAT the IAC is doing, what the engine temps are, what the STIT is doing, what timing is doing... MANY things need to be logged and viewed in order to push you in a specific direction. Start with the simple things-have you scanned it with a good scanner to check codes lately? Even a basic scanner can help you figure out quite a bit. If it truly is a STOCK PCM tune file, and the engine is all stock-then a scanner/code reader may be all you need. Keep in mind you will need at least 100 miles of drive time with NO disconnection of battery, NO DEAD battery, and no code/information clearing by scanner, in order to get any useful code information. Also multiple key-starts in order for the PCM to log codes.
All that said I live/ work in the Santa Cruz, Ca area and would love to get someone with HPT to read the current tune.
I have a persistant P0101 code. This was read on a SnapOn scan tool as well as TorquePro app with cheap BT OBDII thing.
This is my wifes daily driver so it sees plenty of miles and key cycles.
I understand that P0101 is a very generic MAF code which can be triggered by many issues. That said, using the SnapOn scanner, we confirmed that the MAF reading is resonable at roughly 4.8g/s at idle. I don't recall values as the engine speed was increased.
That MAF code is a really big deal! I would STOP driving it until you get it sorted. The MAF also controls LINE PRESSURE in the transmission! You can easily smoke the transmission while driving it with a MAF issue. The ECU will resort to running the engine off the VE Table, but at the same time it can drop the line pressure to MINIMUM inside the trans and you can smoke the 3rd and 4th clutch pack very quickly.
Teching In
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That MAF code is a really big deal! I would STOP driving it until you get it sorted. The MAF also controls LINE PRESSURE in the transmission! You can easily smoke the transmission while driving it with a MAF issue. The ECU will resort to running the engine off the VE Table, but at the same time it can drop the line pressure to MINIMUM inside the trans and you can smoke the 3rd and 4th clutch pack very quickly.
Any advice for troubleshooting the P0101 code? Is there a list somewhere of parameters that can trigger this code.
As I said we looked at the MAF and it's value seemed resonable at least at idle. Also the IAT seemed good. The MAP sensor was found leaking when we performed a smoke check so that was replaced.
Teching In
Joined: Jun 2023
Posts: 9
Likes: 2
That MAF code is a really big deal! I would STOP driving it until you get it sorted. The MAF also controls LINE PRESSURE in the transmission! You can easily smoke the transmission while driving it with a MAF issue. The ECU will resort to running the engine off the VE Table, but at the same time it can drop the line pressure to MINIMUM inside the trans and you can smoke the 3rd and 4th clutch pack very quickly.
Any advice for troubleshooting the P0101 code? Is there a list somewhere of parameters that can trigger this code.
As I said we looked at the MAF and it's value seemed resonable at least at idle. Also the IAT seemed good. The MAP sensor was found leaking when we performed a smoke check so that was replaced.
@Kawabuggy I just read another post of yours and realized that our car has pretty firm 1-2 upshifts so hopefully we are not suffering from low line pressure but I appreciate the warning. For what it's worth, this is a 2003 4.8L from a 2wd truck and a used 4L60e from unknown.
Diagnosing codes for MAF typically means starting with a NEW A/C Delco MAF. OR, taking/borrowing a known good MAF sensor off another GM truck of same/similar years for testing purposes. Your firm shift on 1-2 means little... These units are known (after high mileage) of sticking the 1-2 check ball in the plate (deformation of check ball hole) and once it is stuck-it is going to shift hard/firm on the 1-2 even if you have low line pressure. DO NOT RISK driving the vehicle any more until you get the MAF resolved. Unless you like gambling on a trans rebuild. and... did I read that right.. you bought a USED 4L60E and put it in and it actually worked? That would be the 1st time I've ever heard of that happening.
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From: Ann Arbor, MI
Perhaps one of the tuner here can confirm or correct me, but 4.9g/s sounds high. I recall running more around 3.2 g/s. (Haven't run a MAF in a while and so I forget details.)
In any case, resolve this ASAP. MAFs can just go bad.
In any case, resolve this ASAP. MAFs can just go bad.