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2007 chevy trailblazer_ss_3ss
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Discussion Starter #1
Slowing down on the freeway, on the brakes, I have the signature 1200 RPM on the tach. Usually this backs off when you hit the brakes. 1200 - 1500 RPM with slight power while coasting w/o brakes applied and no throttle is normal.

However I have been getting this randomly where it won’t back off until the speed reaches 0. Popping the truck in neutral will get me 1900 – 2000 RPM w/ no throttle that slowly drops down to idle.

So freeway speed to slowing down – drop to N while stopping revs to 1900 RPM, otherwise it is 1200RPM in gear pushing me on.

Stopping with no throttle applied will flare the RPM as the trans downshifts due to speed reduction. (RPM jumps up to ~1500 between the shift.)

Slowing down in 2nd gear on a hill will reach a point where the engine decides to take off – again neutral from second gives me the 1900 RPM. This 2nd gear slowdown and take off is constant, not random.

In all cases it hangs at 1900 RPM and slows down to idle of ~600 after 20 seconds or when I come to a complete stop.

2007 SS is bone stock with 4K on the clock. No codes - No clue at dealer.

At a full stop bring your engine RPM up to 1200 and you will know what I am have to deal with stopping at times.
 

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I had that exact same problem before but it has since been eliminated once I got the truck tuned. I understand your frustration and it pissed me off too, never really figured out what the problem was though. Just wanted to let you know that you were not alone.
 

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2007 chevy trailblazer_ss_3ss
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208 Posts
Discussion Starter #3
"When I say Whoa, I mean WHOA!" Solution to the idle/throttle will not back off issue

Change two tables. This applies to 2007 TBSS, other years are slightly different.
Main difference is by cutting down the Final Idle Airflow Minimum vs. RPM vs. Gear.

Table #1 to change: Final Idle Airflow Minimum vs. RPM vs. Gear (This could use further reduction around 2000 RPM in 2nd gear - but works well enough now.)

Yes this looks nasty on a curve, but it works, the problem is at 1200RPM on up. I cut the entire table and the engine would start to stall at times hitting 535 RPM and having the throttle opened by the stall saver program. So I put in stock values in the idle speed area and smoothed out up to 1000 RPM. Then I broke the kneecaps out of any throttle idle air over 1000 RPM.

Whack the entire table above 600 RPM by 40%. (Select all values and multiply by .6 )
Use 6.30 for the 800 and 1000 RPM rows.

Table #2 Idle Proportional Airflow vs. RPM Error:

Get rid of the zero values in the Idle Proportional Airflow vs. RPM Error: coastdown table from -256 through the -64 RPM

Copy the normal table from -256 to -64 and paste to the Coastdown table.
Select -256 and make it 2. then multiply that value by -1 to make it negative. (I tested -1.5996 and it wasn't enough.) Do not put a "-2" in the other -256 tables like I tried. It will cause your LTFT's to go -17 the second the throttle snaps closed.

This will allow the throttle to snap closed with your foot input. No hang at 1200 RPM and slow down to idle when the PCM feels like it. Rev it in neutral and watch the RPM drop back to 650 RPM immediately.

Test this out by slowing down from 40 MPH in 2nd gear on a slight hill. DFCO will end about 22 MPH and give you a slight bump before finishing the slowdown. Without this 'fix' you will hit 2K RPM and the engine will start to take off. Neutral drop and you get 2K RPM till you stop or 30 seconds pass. With the fix the RPM will drop immediately when shifted to Neutral.

Side effects: You get a driveline 'clunk' when going from no throttle to applying throttle. From engine dragging you down to hitting the other side as you accelerate. Perhaps this clunk elimination is why these tables are so tilted to keeping some engine power on?

If anyone has a better idea I am all ears!
 

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2007 chevy trailblazer_ss_3ss
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ive had that same issue since new.it scares my wife sometimes and dealer says npf.i hope the tune from pcm4less will fix it.
 

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2002 chevy trailblazer_ltz
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3,250 Posts
Have you tried disconnecting the battery for about 45 min so the PCM will reset itself?

It's worth a shot (and it's free! LOL)
 

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2009 chevy
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6,431 Posts
Have you tried disconnecting the battery for about 45 min so the PCM will reset itself?

It's worth a shot (and it's free! LOL)
Not gonna help, that's the way she works.
Warwagon's first post is a bit scary, I think his truck is slow in responding, but idk.
Looking for an answer to this problem is how I found this site a couple of years ago.
No one has given me a satisfactory answer as to why gm programed it this way (lots of guesses).
I am surprised the brakes last as long as they do. I not surprised that they don't stop as well as they good.
I can't understand how protection agencies would permit this programing.
 

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2002 chevy trailblazer_ltz
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Not gonna help, that's the way she works.
Warwagon's first post is a bit scary, I think his truck is slow in responding, but idk.
Looking for an answer to this problem is how I found this site a couple of years ago.
No one has given me a satisfactory answer as to why gm programed it this way (lots of guesses).
I am surprised the brakes last as long as they do. I not surprised that they don't stop as well as they good.
I can't understand how protection agencies would permit this programing.
Just 1 more example of a problem that GM most likely knows about, but doesn't care if it finds the problem for the customers. Been there with GM, experienced that.
 

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2009 chevy
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Just 1 more example of a problem that GM most likely knows about, but doesn't care if it finds the problem for the customers. Been there with GM, experienced that.
Yep, I hear ya. :yes: I've suspect all gm vehicles, with the electronic throttle, have this unusual feature. When I complained to the dealer, they let me try a new truck (mine was 8 or 9 months old when I purchased) and it worked the same. I think about trying one of the cars, but keep forgetting about it.
 

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2007 chevy
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75 Posts
I have this same problem. I also have the Superchips Porgrammer and it didnt fix it. I read somewhere else it might be there to keep the drivetrain sinched up so you dont get the driveshaft clack if you start to accel after you left off. It is really freaking annoying and has almost cost me to hit someone. My next tune will definately address this issue.
 

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2007 chevy trailblazer_ss_3ss
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208 Posts
Discussion Starter #10
Not gonna help, that's the way she works.
Warwagon's first post is a bit scary, I think his truck is slow in responding, but idk.
Looking for an answer to this problem is how I found this site a couple of years ago.
No one has given me a satisfactory answer as to why gm programed it this way (lots of guesses).
I am surprised the brakes last as long as they do. I not surprised that they don't stop as well as they good.
I can't understand how protection agencies would permit this programing.
There is no official GM answer.

Why it was messed up is a guess. The Minimum airflow tables play a part in the cure. (FWIW There is a gear lash setting in EFI live.)

After logging my truck...
When it randomly enters this 'mode', usually after getting on the throttle over 3K RPM and then back off, I have let off the brakes at 9 MPH and the truck accelerated to 12 MPH on it's own. I will add the above is a partial solution, but, hotter weather and the AC clutch running makes the problem worse. Downshifting to stop also adds to the problem.

If you want to see it start with an 80 degree day, AC on high, lock it in 1st. Rev up to 4500 RPM and then try and stop. The engine will hang around 2K. Slowing down to 1600 RPM the engine will attempt to speed up to 2000RPM on it's own. Shift to neutral and watch the engine jump from 1600 RPM to 2000 RPM. Hit 0 MPH and the engine backs off instantly. Otherwise it will hang for over 10 seconds while rolling. AC off the symptoms are not as bad.

This issue adds 10 seconds to compression braking over when it isn't fighting the engine throttle hang up.

Log the trans input shaft speed, brake, ETC pedal position, Trans current gear, and ETC desired and actual position. You can watch the computer command the throttle open or lack of closing and the engine RPM to be higher than the trans input shaft RPM indicating acceleration. Before, during deceleration, it would be engine slower than trans input speed (in the above 1K RPM range.)

I am not sure all 2007's do this, but, I know there is more than mine. However, my 2006 TBSS would back off when the brakes were tapped.
 

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2007 chevy
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7 Posts
mine is a 07 ss and it does the same thing when coming to a full stop the engine will keep pushing at like about 1000 rpms and it goes down realy slow ,
did any body found a solution for this issue.
 

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2007 chevy trailblazer_ss_3ss
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208 Posts
Discussion Starter #12
mine is a 07 ss and it does the same thing when coming to a full stop the engine will keep pushing at like about 1000 rpms and it goes down realy slow ,
did any body found a solution for this issue.
Just changing the airflow tables as shown in the 1st post is the only thing I have found to help with this.
 

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2007 chevy trailblazer_lt
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714 Posts
This happend tome about a month ago just for one trip (about an hour or so worth) thought it was a sticky throttle body so I had top engine cleaner and cleared it out.. and so far I havn't had any issues. but it has been mentally noted.
 

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International Chapter
2004 chevy trailblazer_ls
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3,900 Posts
mine does that too, and randomly :duh: theres no patren for it, just some tims and suddnly out of the blue the riv will jump to 900-1000rpm and i'll have to JUMP on the brakes ! :bonk:
btw i use to have 450 Kilometeres on a full tank, after my 20's and a new fan clutch i get 350 :eek:!!!! what gives ? any ideas ??? :sadcry:
 

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2004 chevy trailblazer_ls
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3,900 Posts
Ask PCMFORLESS to lower the airflow in your tune to help fix this.
hey thanks for taking some time to answer me pal :tiphat but the thing is i cant :no: it'll cost time and money to get that done, i hardly got my tune done, so thats not an option :worried: not to mention its fine but sometimes does that :duh: thanks any way pal
 

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2007 chevy trailblazer_ss_3ss
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208 Posts
Discussion Starter #19
Anyone with HPTuners in your area? If so this could be done over email.
 

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2006 gmc envoy_denali_xl
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Me 2

My 04 envoy xl just started this after i cleaned the trottle body, run the a/c and u'll run a light if ur not on the brakes HARD, same nuetral hits 2000rpms...really sucks this nice of a truck has so may flaws :no:
 
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