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Discussion Starter #1
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.
 

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Discussion Starter #3 (Edited by Moderator)
Ok: as best I can tell after some more digging, this is what’s going on:

the behavior is intentional and annoying. There are “optimizer chips” on eBay that somehow skip this mode and force the PCM to idle down during throttle closed cruise. I ordered one and will report back. It appears that there are PCM recalibration methods and possibly PCM upgrades available. I will chat with the dealership when it opens.
 

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You've already inspected, replaced, or cleaned the main parts that could cause idle issues, so I don't think there's much else you can really do. I too have been frustrated by the high idle deal, and as far as I can tell it is worse in some trucks vs others. Mine is bad enough to make it look like I don't know how to drive a manual transmission: say I go to up shift from 2nd to 3rd at 2,000 rpm, it'll rev to 2,500 rpm after I press in the clutch and stay there until I release the clutch pedal, which of course makes the truck lurch, before settling down again. My truck will also stay at a high idle while slowing down until completely stopped. The most frustrating part for me is my truck is inconsistent as to when and how much it will rev.

I believe it's in the factory programming and though I have no source to back this theory up, I think it has to do a little bit with both safety and preventing drivetrain shock. I have disconnected my IAC (once the vehicle is warmed up) and it'll behave just like my old motorcycle did: let go of the throttle and the engine will immediately drag. Doing this on ice can lock up the rear wheels as I found out at only 5mph. I'm experienced enough as a driver that I immediately pressed the clutch in and regained control, but it still surprised me and newer drivers may not know to how to react.

I suppose it could also have something to do with gas mileage and allowing it to coast more easily, but when you think about it, your truck needs X amount of gas to go Y mph, whether it comes from the computer or your foot.
 

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Discussion Starter #7
You've already inspected, replaced, or cleaned the main parts that could cause idle issues, so I don't think there's much else you can really do. I too have been frustrated by the high idle deal, and as far as I can tell it is worse in some trucks vs others. Mine is bad enough to make it look like I don't know how to drive a manual transmission: say I go to up shift from 2nd to 3rd at 2,000 rpm, it'll rev to 2,500 rpm after I press in the clutch and stay there until I release the clutch pedal, which of course makes the truck lurch, before settling down again.
Mine was up at 3100 RPM. It actually set P0507 high idle

Which makes me think this is slightly off from intended behavior. The one thing I want to try is the "Optimizer Chip" for the OBD-2 port. I conversed with the eBay seller who claimed that the chip convinces the PCM to skip this "idle up during closed throttle deceleration" mode. I'm a bit skeptical but they offer free returns so I ordered it this weekend and I'll report if it successfully ameliorates the problem. I also have a new EGR valve kicking around so I may try that just for kicks.
 

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That definitely seems too high. Mine only goes to 2,500 or so. The only times it has gone higher were to 3,500 rpm when the IAC died which also caused a CEL. Mine being a 2003 I don't have an EGR so I can't report on that.

I'm curious about the chip, too. Definitely report back what results you see please.
 

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Back in the day, there was a TSB about that...can't find it now, but it did address the problem. Maybe someone with better access can find it.
 

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Discussion Starter #10
That definitely seems too high. Mine only goes to 2,500 or so. The only times it has gone higher were to 3,500 rpm when the IAC died which also caused a CEL. Mine being a 2003 I don't have an EGR so I can't report on that.

I'm curious about the chip, too. Definitely report back what results you see please.
Ok. I replaced the EGR, gasket was cracked. Tried driving with it unplugged. No change.

I think this may be a software problem. If I unplug the IAC it idles at 3100, which is the same setting the IAC is getting pegged at. What I think is going on is that the system is behaving mostly normally until the IAC gets pegged then it won't close the IAC until the wheels stop rolling.

I'm going to await the arrival of the PCM coercion chip and if that doesn't work swap back to the partially functional IAC that minimized the effect of this problem. No point in sinking a ton of money into a $1900 truck.

I also have a Tech 2 clone that is theoretically arriving today.
 

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clean the throttle blade and body? fairly common to do it right you need to pull off throttle body so you can clean the blade and surfaces.
 

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Discussion Starter #12
Back in the day, there was a TSB about that...can't find it now, but it did address the problem. Maybe someone with better access can find it.
We found two for a different GM vehicle. One appeared to say "it's a feature, not a bug, drive better." The other suggested a PCM firmware upgrade was required. Should I be able to call the dealership and ask if there are PCM updates?

clean the throttle blade and body? fairly common to do it right you need to pull off throttle body so you can clean the blade and surfaces.
Did that, no dice. It's definitely programmatical, not mechanical.
 

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We found two for a different GM vehicle. One appeared to say "it's a feature, not a bug, drive better." The other suggested a PCM firmware upgrade was required. Should I be able to call the dealership and ask if there are PCM updates?


Did that, no dice. It's definitely programmatical, not mechanical.
Give it a shot....can't hurt.
 

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Discussion Starter #14
Give it a shot....can't hurt.
Called; no recalls listed. Tech said it had been "a while" since he'd seen a Sonoma/S10 platform with stick shift so wasn't able to confirm the behavior. Now to await the PCM-persuasion doodad from eBay...
 

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Discussion Starter #15
The tech 2 arrived and I spent an hour figuring out how the f*$%ing thing works. It would appear that I may still have a vacuum leak since it shows the IAC at 0 with the idle at 800 concomitantly with "Desired Idle Speed" at 650 RPM. Gonna have a local shop check for vacuum leaks since I don't have the wherewithal to assemble a smoke machine at present and the propane trick was not successful.

OTOH I compared the software versions listed from the Tech 2 with the versions from the GM site and they seem to match.

It's possible what I'm seeing is a combination of weird GM algorithm plus slow vacuum leak.
 

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Discussion Starter #17
Sorry to spam everyone:

I drove the vehicle while looking at the tech 2 (totally not distracted driving...).

I think what I'm seeing is the combination of this IAC trying to do funky GM adjustment while driving and a vacuum leak. Basically:
  1. At start, the engine briefly idles up before the IAC ratchets from the 70s down to 0 on the Tech 2. RPM adjusts down but bottoms out at 850.
  2. Driving, the IAC opens up as I drive. The "intended idle RPM" doesn't change all that much even as the IAC opens which is weird.
  3. As I drive, the computer ratchets the IAC open. I see it get into the 110s.
  4. For some reason, the IAC gets quite high but the PCM doesn't ratchet the IAC down until the vehicle slows. I assume that the behavior would be far more acceptable without a vacuum leak.
 

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It should not have the idle flare up. I’ve seen this on a 97 model. Someone played with the throttle plate adjustment because it had a stuck EGR valve. The IAC counts should only go to 0 when the speed sensor hits 35 mph. The IAC valve does a reset to 0 once per startup at 35 mph then goes to max position 125 I think. If your idle is 850 and the IAC counts are 0, air is bypassing the IAC. At 0 the idle should be 450-500. Check for vacuum leaks, misadjusted throttle or a worn throttle shaft.
 

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Discussion Starter #19
It should not have the idle flare up. I’ve seen this on a 97 model. Someone played with the throttle plate adjustment because it had a stuck EGR valve. The IAC counts should only go to 0 when the speed sensor hits 35 mph. The IAC valve does a reset to 0 once per startup at 35 mph then goes to max position 125 I think. If your idle is 850 and the IAC counts are 0, air is bypassing the IAC. At 0 the idle should be 450-500. Check for vacuum leaks, misadjusted throttle or a worn throttle shaft.
Thanks! I'm pretty sure there's a vacuum leak somewhere -- will see tomorrow. EGR replaced and appears to be functioning normally. I cleaned the throttle body and the plate closes well without the pedal pushed. If it were mis-adjusted you'd see it physically open, correct?
 

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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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