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2007 chevy trailblazer_ls
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Discussion Starter · #1 ·
Just purchased a 2007 TB with the I6. Has around 9k miles on it. When I'm in slow traffic (2nd gear) I notice when I apply the gas it immediatly accelerates the truck where you kind of get thrown back a little. Even if I gently apply gas it will do this. All other gears the it's a gentle flow of acceleratation. Does this make sense to anyone? And is this by design?
 

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Sheesh, listening to some of the folks around here, I think they wish theirs did that!!

Otherwise, nope, haven't heard that one before. It sounds though, like it might be at the point where it hits the optimum torque, so it takes very little to gas to launch you!
 

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2003 chevy trailblazer_ls
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well hmmmmmmmm.
if youre in the highway and in overdrive- you step on gas- and you accelerate- but its slow.
if you manually shift to 3, the RPM's go up as you disengage overdrive, you step on gas- and you feel that you immidiately accelerate, theres this really nice response....
in 2nd, im guessing youre feeling the same thing...
why not in 1st? hmmmm
 

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2005 gmc envoy_sle
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You have found the sweet spot.

With the truck slowing like that, it is not yet ready to downshift to 1st on its own. But when you give it gas it immediately drops into 1st and puts you in the 3000 rpm range in 1st, where these trucks like to accelerate. I have done it before, but I don't usually get the chance with the traffic light cycles around here.

ct_mike
 

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2003 chevy trailblazer_ls
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Wish mine would do that. At lower speeds mine sticks in third and then either bogs it's way to speed, or if I give it more gas it slams into second while I cringe.
 

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2005 chevy trailblazer_ls
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gear issues

I have'nt had any problems with my TB it's an 05 and I run synthetics in it. I drive it everyday like I stole it. It shifts like any other suv I've driven with alot of torque in the upper RPM band. If it's grabbing too hard you might want to have it looked at.
 

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2004 chevy trailblazer_ls_ext
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You can have your shift preferences (upshift and downshift) tuned however you like. You can change the shift pressures to adjust firmness, the shift times, the converter lockup points and gears, and you can have them different from normal usage to wide open throttle. Here's a screenshot from HPTuners to give you an idea:
 

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Discussion Starter · #11 ·
I don't think I explained what is happening correctly. This really only happens in like 5-20mph traffic when I'm coasting and give the truck a little gas. When the TB is decelerating and you gently or maybe even tap the gas the engine revs but the tranny does not engage immediatly. When it does engage (pry just a .5 second delay) i get a jolt. It doesn't happen all the time it depends on speed, gear, and accelartion or deceleration. I really hit the gas today and it actually made a noise when the tranny engaged.
 

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My :m2:...I see you have a 2007. I would say that a sudden unintended jolt while driving at any speed could be a safety risk. I imagine you have factory warranty left so get the dealer to recreate it and fix it or tell you that its normal. I think this site and members are very insightful about our trailvoys, and you could get theories about what it may be, but for a traffic safety issue thing, I would say dealership immediately. It is a good idea to know what it may be before going to dealership though to either keep from getting ripped off, or if they are stumped you could offer a few ideas(don't laugh, it has happened) Good luck, and be careful with it!
 

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Discussion Starter · #13 ·
Damn, so I take it knowone else is experiencing something close to this? I wouldn't even think I would notice it if I didn't have a bumper to bumper commute sometimes to work. It takes a specific pattern of variables to occur. But when it occurs if I were to floor it the tranny would get a real PUNCH and I would hear it or so I would assume. I mean if I give it a decent tap on the gas pedal I can get a noise from the drivetrain when it engages.

It only have 9k on it. Just bought it 2 weeks ago used. I will contact the dealer.
 

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Damn, so I take it knowone else is experiencing something close to this? I will contact the dealer.
Never heard of it. I'd definitely get to the dealer ASAP. Tuning to preference is one thing, tuning out a problem is quite another. Sounds like your 1-2 clutch may be sticking or something internal...
 

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Discussion Starter · #15 ·
Before I call the dealer today I wanted to get as much details as possible. So I thought I'd share. Maybe if someone can give me some good suggestions for them to look at if there is a problem.

This will happen at low speeds when the truck is coasting and the engine RPM drop below 1000. If you just barely touch the pedal enough for the RPM's to rise and to catch the drivetrain you will get this jerk. So the delay is the rpm's going up. It's not a nasty jerk unless you really hit the gas. It seems like it's actually in first gear not second like I thought originally. Around 10mph is the area.
 

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Before I call the dealer today I wanted to get as much details as possible. So I thought I'd share. Maybe if someone can give me some good suggestions for them to look at if there is a problem.

This will happen at low speeds when the truck is coasting and the engine RPM drop below 1000. If you just barely touch the pedal enough for the RPM's to rise and to catch the drivetrain you will get this jerk. So the delay is the rpm's going up. It's not a nasty jerk unless you really hit the gas. It seems like it's actually in first gear not second like I thought originally. Around 10mph is the area.
Well, I think that it is related to your torque converter.

here is my disclaimer. I understand the theory behind TC's and trans/engine coupling, but I've never had one apart...

Okay, so here is my attempt:

When you are rolling at 10-20 mph, your transmission is spinning the input shaft that is in the torque converter. The torque converter works by increasing pressure between the engine and trans. It increases when the revs of the motor increase. If the motor is idling, the TC does not develop enough pressure to lock the trans to the engine. Now, my guess is that because your transmission is spinning when you are coasting in second gear, it will require higher pressure in the TC to lock the two together. Therefore, the engine is required to rev higher before building pressure. So, essentially the motor is free reving until it builds pressure in the TC.

A combination of the engine reving quicker AND the fact that the engine will be at a higher rev because the trans is rotating means you will be producing more torque when the trans is "engaged" to the engine.

This doesn't occur in first gear because:

-you probably aren't coasting very fast in first
-the driveshaft has a smaller (more negative) mechanical advantage over the input shaft of the trans.

This doesn't occur in third or fourth gear because

-the trans is spinning fast enough that it keeps revs up (I'm not sure, but the TC might build enough pressure for the engine to "slip" relative to the trans in this situation?)
-the amount of torque required to suddenly accerate a vehicle in a high gear when it's at a high speed is phenominally high.

That is my guess, and I would assume that it is perfectly normal, unless it is very harsh.
 

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I just went to the dealer today for well some oil leakage from between the trans and trasfer case, he he I still have my warrenty so they are going to fix it. But I found out there is a recall on the PCM for the 06 trailblazer mabey they have one for the 07 and the pcm is messed up and causing all your problems. Good luck with it.
 

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2006 chevy trailblazer_lt
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Just purchased a 2007 TB with the I6. Has around 9k miles on it. When I'm in slow traffic (2nd gear) I notice when I apply the gas it immediatly accelerates the truck where you kind of get thrown back a little. Even if I gently apply gas it will do this. All other gears the it's a gentle flow of acceleratation. Does this make sense to anyone? And is this by design?
It happens to me, too. Rarely, but I notice it. On one occasion, I thought I could actually hear a "clank" when it happened. Thought it was normal (my '96 Jimmy did that, too).
 

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'05 Chevy TB EXT
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You have found the sweet spot.

With the truck slowing like that, it is not yet ready to downshift to 1st on its own. But when you give it gas it immediately drops into 1st and puts you in the 3000 rpm range in 1st, where these trucks like to accelerate. I have done it before, but I don't usually get the chance with the traffic light cycles around here.

ct_mike
CAUTION ---> ZOMBIE AWAKENING HERE!
The OP stated he was in 2nd (intermediate) and felt a huge acceleration - and this guy says it's because the transmission dropped to 1st gear ---- and that's wrong.

The GM programming of their latter 4 speed autos is this:

  • If you put the selector in 2nd gear - you are IN 2nd gear and there's no "kick down" or drop to 1st gear allowed.
  • The reason is that you are to use the 2nd gear selection when you have to start from a stop and there's little traction and you don't want to spin your wheels.
  • Your transmission will start in 2nd gear and it will stay in 2nd gear until you shift it to any other gear position.
  • In any other position (except Reverse or Neutral or Park --- natch!) the transmission will start from stopped condition in 1st gear and then proceed to shift up to the position in which the shifter (quadrant) is set.
  • Put it in 3rd at a stop and it will start up in 1st, shift to 2nd and stop shifting once it's achieved 3rd.
GM has always had - for a long time anyway - the option of 2nd gear starts for icy and snowy and muddy conditions. It reduces the torque so the wheels don't spin uselessly.
Now --- let's shoot this Zombie and close the post.
 
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