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Go Back   Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum > 2002 - 2009 TrailBlazer/Envoy Tech > 02-09 General Tech Q&A > General

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  #1  
Old 10-22-2009, 08:28 PM
convincer253 convincer253 is offline
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2004 Chevy TrailBlazer LT EXT
Summit White 4.2L I6 4X4
Join Date: Oct 2009
Location: Illinois
Posts: 31
Hello Forum

Im new here and hope i can find an answer to a problem question i have...I have a 2004 trailblazer with the 4.2 engine....I have just replaced the t-stat in this vehicle....Heres the problem

I had the battery completely disconnected..+ and -... for 5 hours +....long story but thats when i finally got the install finished...NO the install didnt take long but got called away from the project and thats how long it to to get back to it...

When i got done with the install and hooked up both battery terminal and started it up...it was a successful install...KINDA...no leaks or anything like that...Engine heated up just fine..coolant levels are fine and all appears to be well and done....

I turn the heater on and the engine starts to surge down to 300 revs then up to 1000 revs...up and down and just about dies...if i shut the heater off..it doesnt do this.....its just fine...it sounds like the compressor is locking up..So i start it back up and turn on the heater and it starts to surge again..I unplug the compressor and its all fine...my hunch was correct..but WHY all of a sudden...i cant believe this is the problem...So i call the Chevy dealer and talk to service...They told me that with having the battery disconnected for that period of time that i will have to bring it in and have the computer rebooted...Is this true...or could something else be wrong...When i plug it in to the scanner..I get NO valuts or codes of anytime..Is it just bad luck and the compressor just when bad or is going bad or did the computer loss its programming and needs to be taken to the dealer to be rebooted....Hopefully there is a GM mechanic on here or someone that knows the correct answer to this question/problem...or maybe someine else has had this issue..

Vehicle runs fine as long as the compressor is unplugged...if i plug it back in..the engine surges while in park and will eventually die....This has only happened since i changed the thermostat...Anyone able to help....

Great site and hope to here from someone soon or see a post on this issue....Thanks
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  #2  
Old 10-22-2009, 08:56 PM
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Blazernut Blazernut is offline
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Welcome!

You need to clean your throttle body. Everyone here has done it

https://forums.trailvoy.com/articles....ticle&artid=91
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  #3  
Old 10-22-2009, 10:24 PM
convincer253 convincer253 is offline
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Will try that...but can someone anwser this

Why has it started doing this since and ONLY since i changed the t-stat..It never did it before

Why it doesnt do it when the compressor/dryer is unplugged....Verified this by starting it up..started surging/chugging...unplugged the dryer and it runs just fine...plug the dryer back in and it chugs and surges...did this a few times and each time i unplugged the dryer..it ran perfect...plug it back in and it chugged...is the compressor/dryer directly related to the throttle body...Not questioning anyone...just trying to learn and understand why....Thanks
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  #4  
Old 10-23-2009, 01:11 AM
WOOLUF1952 WOOLUF1952 is offline
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2003 GMC Envoy SLT
Magnetic Red Metallic 4.2L I6 4X4
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Many, many posts on this. It has nothing to do with the t-stat. It is all about disconnecting the battery and the t-body. Seach for dirty throttle body.
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  #5  
Old 10-23-2009, 02:44 AM
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the roadie the roadie is offline
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2004 GMC Envoy SLT "Roadiemobile"
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Geez, I take five hours off for a date with my wife and the world falls apart! (I'm going to the desert for the weekend also - no questions answered Sat/Sun, either. )

Your dealer's service advisor is a guessing, undertrained goof. Reboot indeed.

Article:
https://forums.trailvoy.com/articles....ticle&artid=91

Technical reasons:

1) The I6 engine was designed without a traditional PCV valve, so crankcase blowby gets the back of the throttle body throat horribly dirty/sticky.

2) The PCM has a built-in table in memory that predicts the angle it will have to place the butterfly valve to obtain certain RPMs. It has a feedback loop that tries to keep the RPM constant, especially the 600 RPM idle speed, under varying loads, especially the on/off load of the AC compressor. But it's more successful at nailing the RPM exactly if it customizes this memory table slightly according to what it sees the performance actually being. This is called an adaptive process.

3) The PCM also controls when the AC compressor cycles on and off. So it can bump up the airflow and fuel injector duty cycle to anticipate the compressor load it's about to add. If it didn't anticipate, it would be lagging, and in a control loop that results in overshoots (temporary too-high RPM) and undershoots (temporary too-low RPM).

4) As a vehicle ages and the throttle body gets stickier and stickier, the PCM slowly adapts as best as it can. Eventually, the stickiness causes so much hysteresis that the PCM can't adapt any more, and the former smooth idle performance gets erratic, with overshoots and undershoots, and gets to the point that the engine can just stall and die when the AC compressor kicks in. When the AC is off, the PCM can handle the throttle better because the butterfly valve isn't moving as much, if it even needs to move at all. The PCM can handle tiny RPM changes with fuel injector changes alone, without needing to move the relatively large (physically) butterfly valve. A vehicle whose idle gets bad after 30-60K miles has this going on.

5) A vehicle whose throttle body is getting sticky, but which hasn't gotten to the point of erratic idle, is at risk of losing its adaptive table in memory. This happens when the battery gets disconnected for any reason. The PCM goes back to square 1. It has the factory built-in table of butterfly valve performance, but the throttle body is now horribly sticky. The adaptive algorithms in the PCM programming can't handle real-world performance that's so much different than the engineer's models predicted. If the engineers knew the engine would be depositing so much tar on the throttle body they would have given us a PCV valve. So the formerly well-running engine with an idle table that had been developed slowly over time (even though the throttle body had been deteriorating), is now presented with a too-sticky throttle body and it just gives up. It wasn't programmed to be THAT adaptive, so it reverts to the factory programming. And that, combined with the sticky throttle body, can't accommodate the AC compressor's varying load. But it can do OK with the no-compressor mode.

Sorry for the long technical essay, but so many people demand it that I might as well write it down (again) so maybe I can find it myself in a future search. I agree it *is* a counterintuitive result after a battery disconnect.



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  #6  
Old 10-23-2009, 03:00 AM
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EwingJK EwingJK is offline
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2005 Chevy TrailBlazer LS
Cashmere Metallic U929L 4.2L I6 4X4
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Roadie,
Excellent technical write-up. This is why you are the GMT-360 Guru. I think it would serve the community well to create a new Throttle Body sticky and make this article the first post. The throttle body problem is so common that the Administrators should consider the possibly of making it an advisory for noobies when they first register. I hope you don't mind, but I'm going to copy the write up and save it to my Trailblazer directory on my hard drive. Again, excellent explanation.
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  #7  
Old 10-23-2009, 05:37 AM
convincer253 convincer253 is offline
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2004 Chevy TrailBlazer LT EXT
Summit White 4.2L I6 4X4
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GREAT REPLY...thanks for explaining...guess i know what i will be doing after work today....Will let you know how it turns out.....Thanks again
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  #8  
Old 10-23-2009, 09:25 PM
convincer253 convincer253 is offline
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2004 Chevy TrailBlazer LT EXT
Summit White 4.2L I6 4X4
Join Date: Oct 2009
Location: Illinois
Posts: 31
Cleaned the TB and all is good...Thanks alot to all for the info..
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  #9  
Old 06-14-2010, 07:52 PM
davidfrancisco davidfrancisco is offline
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2005 Chevy TrailBlazer LS
Moondust Metallic 4.2L I6 4X4
Join Date: Nov 2007
Location: Cherry Point/Havelock,NC
Posts: 11
05 tb

I cleaned the tb and it ran good for a bout 2 hrs. I shook the tb and it had a slight rattle. I cleaned it a 2nd time with no luck still the same. I ordered a new tb. Do you have any other suggestion?
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  #10  
Old 06-14-2010, 10:14 PM
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the roadie the roadie is offline
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2004 GMC Envoy SLT "Roadiemobile"
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No. What exactly were your symptoms?



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