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I have a 2000 GMC Sonoma Stickshift with the V6 engine (which I recently got for the princely sum of $1900).

Anyways, it has an idle behavior which I find annoying which can be described as "high idle periodically when moving while clutch depressed which goes away when the vehicle stops rolling independent of brake"

In more detail:
  1. Start up, idle briefly goes high then returns to normal.
  2. While driving, pop clutch and idle stays high (sometimes 1000 if driving slow, sometimes 2000-3000 if driving fast).
  3. Sometimes idle stays almost alarmingly high (3000 RPM) with clutch depressed
  4. Sometimes car seems to accelerate with my foot off the gas pedal due to this behavior. I'll notice an instant where the vehicle will go to "idle at 3000 mode" and the car will suddenly start accelerating while in gear.
  5. When the wheels come to a stop, the idle abruptly returns to normal.
A rigorous search of the various forums shows a variety of answers, including "that is the intended behavior for emissions reasons."

I have:
  1. Cleaned the throttle body. It doesn't feel sticky.
  2. Replaced the IAC (this actually made the problem worse as the old IAC wasn't working quite right and the effect was less pronounced).
  3. Unplugging MAF similar problem. Replaced the MAF sensor same behavior.
  4. Done a cursory inspection for vacuum leaks. AFAICT the answer is no since climate control retains defrost for several minutes when engine is not running.
Anyways the behavior remains. My friend found a TSB for a different GM vehicle suggesting that the behavior was ameliorated by PCM reprogramming, but I didn't see a TSB for this vehicle.

Any pointers?

A few things I've turned up:
  1. Someone said "fuel filter"
  2. Someone said PCV
  3. Someone said EGR
  4. Someone said clutch angle sensor
  5. Someone said "intended behavior but aftermarket doodad designed to minimize throttle lag can undo it"
I'm convinced that what I'm seeing is something along the lines of "somewhat annoying feature added by GM for emissions or other reasons that is getting slightly askew input and exaggerating the effect".

I cannot post links to previous threads since it's my first post but I can add them later.
I have a 2000 GMC Sonoma Stickshift with the V6 engine (which I recently got for the princely sum of $1900).

Anyways, it has an idle behavior which I find annoying which can be described as "high idle periodically when moving while clutch depressed which goes away when the vehicle stops rolling independent of brake"

In more detail:
  1. Start up, idle briefly goes high then returns to normal.
  2. While driving, pop clutch and idle stays high (sometimes 1000 if driving slow, sometimes 2000-3000 if driving fast).
  3. Sometimes idle stays almost alarmingly high (3000 RPM) with clutch depressed
  4. Sometimes car seems to accelerate with my foot off the gas pedal due to this behavior. I'll notice an instant where the vehicle will go to "idle at 3000 mode" and the car will suddenly start accelerating while in gear.
  5. When the wheels come to a stop, the idle abruptly returns to normal.
A rigorous search of the various forums shows a variety of answers, including "that is the intended behavior for emissions reasons."

I have:
  1. Cleaned the throttle body. It doesn't feel sticky.
  2. Replaced the IAC (this actually made the problem worse as the old IAC wasn't working quite right and the effect was less pronounced).
  3. Unplugging MAF similar problem. Replaced the MAF sensor same behavior.
  4. Done a cursory inspection for vacuum leaks. AFAICT the answer is no since climate control retains defrost for several minutes when engine is not running.
Anyways the behavior remains. My friend found a TSB for a different GM vehicle suggesting that the behavior was ameliorated by PCM reprogramming, but I didn't see a TSB for this vehicle.

Any pointers?

A few things I've turned up:
  1. Someone said "fuel filter"
  2. Someone said PCV
  3. Someone said EGR
  4. Someone said clutch angle sensor
  5. Someone said "intended behavior but aftermarket doodad designed to minimize throttle lag can undo it"
I'm convinced that what I'm seeing is something along the lines of "somewhat annoying feature added by GM for emissions or other reasons that is getting slightly askew input and exaggerating the effect".

I cannot post links to previous threads since it's my first post but I can add them later.
I have a 2000 GMC Sonoma Stickshift with the V6 engine (which I recently got for the princely sum of $1900).

Anyways, it has an idle behavior which I find annoying which can be described as "high idle periodically when moving while clutch depressed which goes away when the vehicle stops rolling independent of brake"

In more detail:
  1. Start up, idle briefly goes high then returns to normal.
  2. While driving, pop clutch and idle stays high (sometimes 1000 if driving slow, sometimes 2000-3000 if driving fast).
  3. Sometimes idle stays almost alarmingly high (3000 RPM) with clutch depressed
  4. Sometimes car seems to accelerate with my foot off the gas pedal due to this behavior. I'll notice an instant where the vehicle will go to "idle at 3000 mode" and the car will suddenly start accelerating while in gear.
  5. When the wheels come to a stop, the idle abruptly returns to normal.
A rigorous search of the various forums shows a variety of answers, including "that is the intended behavior for emissions reasons."

I have:
  1. Cleaned the throttle body. It doesn't feel sticky.
  2. Replaced the IAC (this actually made the problem worse as the old IAC wasn't working quite right and the effect was less pronounced).
  3. Unplugging MAF similar problem. Replaced the MAF sensor same behavior.
  4. Done a cursory inspection for vacuum leaks. AFAICT the answer is no since climate control retains defrost for several minutes when engine is not running.
Anyways the behavior remains. My friend found a TSB for a different GM vehicle suggesting that the behavior was ameliorated by PCM reprogramming, but I didn't see a TSB for this vehicle.

Any pointers?

A few things I've turned up:
  1. Someone said "fuel filter"
  2. Someone said PCV
  3. Someone said EGR
  4. Someone said clutch angle sensor
  5. Someone said "intended behavior but aftermarket doodad designed to minimize throttle lag can undo it"
I'm convinced that what I'm seeing is something along the lines of "somewhat annoying feature added by GM for emissions or other reasons that is getting slightly askew input and exaggerating the effect".

I cannot post links to previous threads since it's my first post but I can add them later.
Have you checked the cruise control cable to the Throttle body? I have a 96 gmc Sonoma v6 manual, I replaced the intake manifold gaskets, when I took it out to test drive, I had similar issues with the idle going high then staying high for a while after I push in the clutch , turns out the cruise control cable was adjusted too tight so it took some time for the rpms to drop after you stop or push in the clutch.
 

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Discussion Starter #22
Have you checked the cruise control cable to the Throttle body? I have a 96 gmc Sonoma v6 manual, I replaced the intake manifold gaskets, when I took it out to test drive, I had similar issues with the idle going high then staying high for a while after I push in the clutch , turns out the cruise control cable was adjusted too tight so it took some time for the rpms to drop after you stop or push in the clutch.
I'll check that -- I think the issue is unrelated. The idle "hunts" up and down randomly and doesn't correlate to mechanical problems. I have the clutch depressed and it idles up to 3000 then down to 2100 then up to 2900. Looking at the Tech 2 while driving the idle correlates perfectly with the IAC, which is why I'm looking for a vacuum leak first.
 

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Discussion Starter #23
The plot thickens -- garage says no vacuum leak, but that when they drive it the scan tool indicates crazy problems. When they disconnect the TPS, the truck idles normally but while driving the RPM goes up. They said the various modules are throwing errors trying to communicate with each other. Seems to be a deeper problem than I thought...

Now I know why the guy sold the truck ;) Thankfully I have the Tech 2 set up if I have to replace any of these BCM/PCM doodads. Seems like a fun project.
 

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Discussion Starter #24
Interestingly they said the timing advance goes way out of wack when the idle goes high -- up to 40 degrees. He says 35 degrees might be typical under high load. Some sensor somewhere is out of whack I think.
 

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well, all the sensors can be manually checked with a multimeter to confirm your tech2 readings.You can check all the wiring for continuity and resistance,if no problems there, then maybe look for a junkyard pcm on the cheap. Not tryin to be captain obvious here but sometimes ya gotta go old school. You need ODS to chime in on this one, he's probably seen this one before.. Good luck ,Im interested in how this shakes out
 

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Discussion Starter #26 (Edited by Moderator)
well, all the sensors can be manually checked with a multimeter to confirm your tech2 readings.You can check all the wiring for continuity and resistance,if no problems there, then maybe look for a junkyard pcm on the cheap. Not tryin to be captain obvious here but sometimes ya gotta go old school. You need ODS to chime in on this one, he's probably seen this one before.. Good luck ,Im interested in how this shakes out
Yeah that sounds like the move. The PCM looks pretty gross (especially the connectors) and the communication errors are happening between all the major systems, so I'm suspicious that it could be some sort of connectivity issue. Previous owner did suggest computer problemThe weird part is that the behavior seems pretty reproducible which doesn't sound like an intermittent problem and suggest a sensor problem.

I found a PCM on eBay for $70 with free returns so I ordered one of those. I'm going to check the sensors and hit the PCM cables with deoxit.

Does anyone have a list of sensors that feed the algorithms? I suppose I can check them all:
  • CKT -- my understanding is that problems here cause a no-start usually.
  • IAC seems AOK
  • EGR seems AOK
  • TPS has been checked, but if you unplug it seems to restore idle to normal but the high idle while driving persists (tech at shop said 2k rpm instead of 3.1k). Does that kick it into open loop mode?
  • O2 sensor? Interestingly the problem doesn't seem to happen right after start. It's possible it's an O2 sensor problem and that the problem wouldn't happen in open loop mode. I suppose I could try using the Tech 2 to force it into closed loop and see what happens.
 

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To me, if there is no vacuum leak, it's sounding more like the TPS. When my TPS went out, the engine would randomly rev to 2500-3500. I'd start there before a PCM.
 

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Discussion Starter #28
To me, if there is no vacuum leak, it's sounding more like the TPS. When my TPS went out, the engine would randomly rev to 2500-3500. I'd start there before a PCM.
Ah interesting. Why does the faulty TPS do that? Why does the computer even need to know what the TPS sensor says if it’s a full manual throttle?
 

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I agree, and those are easily tested with a multimeter. This issue has me thinking all kinds of "out of the box" ...what if there was a vacuum issue under the plenum? or some way that it could be "gulping" air into the fuel lines at any point? I only thought of that because I've had old diesels "hunt" because of that. I dunno, I'm just spitballing at this point. again -good luck
 

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Ah interesting. Why does the faulty TPS do that? Why does the computer even need to know what the TPS sensor says if it’s a full manual throttle?
My understanding, and I could be wrong, is the TPS reports to the computer, which then tells the IAC where the RPM needs to be. It's how the computer can compensate for things like the AC or the higher RPM during a cold start. I presume the computer also adds fuel to match the air going into the engine, so if it's reading that the throttle blade is open any (it doesn't take much air to rev the engine) when the blade is really closed, the IAC opens more to match what the faulty TPS says it needs.
 

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Discussion Starter #31
My understanding, and I could be wrong, is the TPS reports to the computer, which then tells the IAC where the RPM needs to be. It's how the computer can compensate for things like the AC or the higher RPM during a cold start. I presume the computer also adds fuel to match the air going into the engine, so if it's reading that the throttle blade is open any (it doesn't take much air to rev the engine) when the blade is really closed, the IAC opens more to match what the faulty TPS says it needs.
update: truck drives like a dream after I unplug the IAC.

The TPS sounds most plausible and consistent with what I’m seeing — it’s as if there’s some interference that suggests to the PCM that causes it to read the TPS wrong. TPS is ordered so I'll give that a shot.

There are some other weirdness with the PCM communication between the BCM and ABS modules. I’m going to investigate that a little bit in case the electrical problems are related. I think I'll swap the PCM and try the ABS fix (resoldering the pads that tend to crack on the power supply end).
 

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sounding like you might be on the right path-good. How are you liking your new tech2 clone?Can you tell me what the year range on that is (how New will it do?)Possibly post a link to it?....if its permitted by the mod gods-lol
 

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Discussion Starter #33
sounding like you might be on the right path-good. How are you liking your new tech2 clone?Can you tell me what the year range on that is (how New will it do?)Possibly post a link to it?....if its permitted by the mod gods-lol
I got mine on amazon. It feels pretty cheaply made but it works. Came with the TIS2000 software which will be handy if I need to swap modules.

on at least one of the menus it showed a few model years beyond 2009. I will check.

Lightcolor Tech-2 Flash Tester... Amazon.com: Lightcolor Tech-2 Flash Tester for SAAB/GM/SUZUK/OPEL/Isuzu add 32MB Card (one Model Software for GM tech2 Tech 2 II) Works with GM TECH2, Contain The Latest Software for GM TECH2: Home & Kitchen
 

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I have a 2000 Blazer with a W code 4.3 and a manual transmission, 190,000ish miles. I bought this as a neglected beater from a 'stereo-kid' and have spent a few months getting the interior fixed/replaced. Now that I am finally driving it, it seems to be doing the same thing as yours. Once moving, if I put in neutral, it continues to idle high 2,000-3000 RPM and fluctuate up and down. As you coast to a stop, it will continue to fluctuate until about 4 MPH at which point it drops to a normal idle. Sometimes at the right speed, I can let off the gas pedal and it continues to drive and even pick up speed before slowing down... and then picking up speed again. If this was an automatic, it would be dangerous!

I too have a Tech 2, and it appears that the IAC is causing the idle issues, but it is being commanded by the ECM. It did have a bad IAC, which didn't follow where it was commanded, so it was replaced. The previous 'stereo-kid/hack mechanic' had removed the catalytic converter, but oddly it doesn't throw any codes for it. He also removed that half-round baffle thingy on the throttle body butterfly, but not the rivets. It also appears that they just pulled the intake as it is all clean and has new silicone and gaskets. Maybe it was a leaky spider... maybe they were trying to fix a crazy idle.

I have extensively checked for vac leaks, but can't find any, and the fluctuations don't act like a vacuum leak. I have noticed that the timing is all over as well with the changes in idle. If I remember right (and it's been a week or two since watching this) the timing was about 35 degrees when the idle goes high. I've checked the TPS and it ohms out correctly and reads correctly on the Tech 2. I've disconnected the cruise cable with no change. I've checked the EGR and it functions correctly, but if this were stuck open it would idle down, not up.

The idle issues appears to be related to RPM, which makes no sense to me. The ECM really appears to be causing/commanding it and it only sets the high idle code. Interestingly, I disconnected the VSS connecter at the back of the trans and it drives and idles perfectly, other than the ABS light, and the 4 or 5 ECM codes it throws related to having no VSS signal... If it wasn't for the dash light show, I'd leave this disconnected and go on with life. The VSS seems to be reading correctly, so I'm at a loss.

I've just read a post somewhere tonight that a guy removed the VSS and found the end clogged up with metal debris. He cleaned it off and it fixed his high idle issues. This is what I'm going to try tomorrow.

 

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I have a 2000 Blazer with a W code 4.3 and a manual transmission, 190,000ish miles. I bought this as a neglected beater from a 'stereo-kid' and have spent a few months getting the interior fixed/replaced. Now that I am finally driving it, it seems to be doing the same thing as yours. Once moving, if I put in neutral, it continues to idle high 2,000-3000 RPM and fluctuate up and down. As you coast to a stop, it will continue to fluctuate until about 4 MPH at which point it drops to a normal idle. Sometimes at the right speed, I can let off the gas pedal and it continues to drive and even pick up speed before slowing down... and then picking up speed again. If this was an automatic, it would be dangerous!

I too have a Tech 2, and it appears that the IAC is causing the idle issues, but it is being commanded by the ECM. It did have a bad IAC, which didn't follow where it was commanded, so it was replaced. The previous 'stereo-kid/hack mechanic' had removed the catalytic converter, but oddly it doesn't throw any codes for it. He also removed that half-round baffle thingy on the throttle body butterfly, but not the rivets. It also appears that they just pulled the intake as it is all clean and has new silicone and gaskets. Maybe it was a leaky spider... maybe they were trying to fix a crazy idle.

I have extensively checked for vac leaks, but can't find any, and the fluctuations don't act like a vacuum leak. I have noticed that the timing is all over as well with the changes in idle. If I remember right (and it's been a week or two since watching this) the timing was about 35 degrees when the idle goes high. I've checked the TPS and it ohms out correctly and reads correctly on the Tech 2. I've disconnected the cruise cable with no change. I've checked the EGR and it functions correctly, but if this were stuck open it would idle down, not up.

The idle issues appears to be related to RPM, which makes no sense to me. The ECM really appears to be causing/commanding it and it only sets the high idle code. Interestingly, I disconnected the VSS connecter at the back of the trans and it drives and idles perfectly, other than the ABS light, and the 4 or 5 ECM codes it throws related to having no VSS signal... If it wasn't for the dash light show, I'd leave this disconnected and go on with life. The VSS seems to be reading correctly, so I'm at a loss.

I've just read a post somewhere tonight that s guy removed the VSS and found the end clogged up with metal debris. He cleaned it off and it fixed his high idle issues. This is what I'm going to try tomorrow.
ok let me know. Good to hear I’m not the only one wrestling with this :)

I did some digging online and found a paper going over the control algorithm for the IAC and it lists the TPS, MAF, and MAP sensor. Considering the first two look good, I was going to unplug the MAP sensor tomorrow and see what changes. Interestingly this is proposed for being a similar mechanism to what caused unintended acceleration reports.

Today I just unplugged the IAC after it idled down after start (have to do it while running). Just means you have to depress the gas a bit on cold start. Drove like a dream but it’s still idling a bit high. The control algorithm is trying to run the idle higher. Sticky MAP actually might explain this behavior.
 

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Wow, great paper! My Blazer has a new appearing alternator, so I've also checked voltages, wondering if it was some sort of under-volt issue, but it seems ok. Another thing I've looked into is HP Tuners. A friend does a lot of LS swap stuff and he says that they occasionally have to mess with an 'IAC/Idle cracker table' when they swap cams or swap from an auto to manual trans. This paper was the first other reference I've seen about the idle cracker tables. HP Tuners does support these 4.3 engines, so that is probably my last resort.

I edited my previous post and put in the link I found to the VSS cleaning solution.
 

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Discussion Starter #37
Wow, great paper! My Blazer has a new appearing alternator, so I've also checked voltages, wondering if it was some sort of under-volt issue, but it seems ok. Another thing I've looked into is HP Tuners. A friend does a lot of LS swap stuff and he says that they occasionally have to mess with an 'IAC/Idle cracker table' when they swap cams or swap from an auto to manual trans. This paper was the first other reference I've seen about the idle cracker tables. HP Tuners does support these 4.3 engines, so that is probably my last resort.

I edited my previous post and put in the link I found to the VSS cleaning solution.
Yeah I was about half a second from clicking buy on a tuner but this definitely isn’t a tuner problem. I would be interested to see what the behavior is once I fix this problem and go from there. I’m suspicious that everyone editing those tables is compensating for a bad MAP sensor or something.

So the VSS is used as part of the IAC lookup algorithm. When the speed is low the IAC behaves differently in response to engine behavior. That doesn’t seem like the root cause any more than the TPS does (even though unplugging it seems to help).

My suspicion is that this particular problem is due to a bad sensor — this paper seems like the IAC changes are deliberate but that in this case they’re going awry.

the other hint is that when the shop unplugged the TPS sensor they said the situation improved (idle normalized), which again suggests that a control algorithm is messed up but that when one of its inputs is missing it changes its behavior. They also noted the timing advance, which means the engine thinks it’s under load.

And to think three weeks ago I had no idea how cars worked...at this rate I am soon to be the world expert on the S10 platform idle control algorithm XD
 

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these are not the best out there, but there are many if you want to go thru them.Why guess if its the ecm commanding the iac or whatever?why throw money away ? Please note on the first video what the guy says about funky wiring. As I said before the scanners are great,but, theres nothing like testing components "out of the loop" for a definitive analysis . Hell, I've tested more than one brand new replacement part before installing that wound up being out of spec on the meter...returned it before installation and the second one tested out and worked fine.
 

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Discussion Starter #39
Ok update:
  1. TPS behaves normally
  2. The problem correlated with “engine load” calculation. At idle the car thinks engine load is 3-4% and all of a sudden will jump to 10-15% while driving. This correlates with the engine idle up and IACV opening. The problem is the load calculation or the calculation input.
  3. Unplugging MAP sensor no change
  4. The problem doesn’t reproduce until the engine is warm, even if the car is running in open loop mode (IE doesn’t require closed loop to reproduce.
how does the car calculate engine load? Perhaps I need to do a relearn on cam shaft? Perhaps O2 sensor? But my O2 sensors are complaining about heater problems so I assume the vehicle isn’t using them...
 

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...Today I just unplugged the IAC after it idled down after start (have to do it while running). Just means you have to depress the gas a bit on cold start. Drove like a dream but it’s still idling a bit high. The control algorithm is trying to run the idle higher. Sticky MAP actually might explain this behavior.
This is how I drive my truck now. It can be dangerous in bad weather as I stated before, so do be careful. It becomes more of a hassle when you are in traffic and especially when you run the AC or defrost, and it requires careful foot metering to prevent a CEL for low idle.

I appreciate the investigating you're doing, seems to me that you've made meaningful headway into this deal. Helpful thread, I'll be watching for updates.
 
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