Denali Denial

Jon Owner/Technician Trussville, Alabama Posted   Latest   Edited  
Case Study
Driveability
2006 GMC Envoy Denali 5.3L (M LH6) 4-spd (4L60-E)
Crank / No Start

Vehicle was towed in with a crank/no start condition. During cranking it attempted to start but could not get enough RPMs to run. After confirming the proper fuel pressures, spark for cylinder number 3 was tested and resulted in no spark. Next the scope was connected to observe CMP, CKP and IC control. During the first test the CMP signal was pegged at 12 volts. After removing the sensor, bench testing the senor and checking wiring, it was discovered that the scope connection at the PCM was not secure! Ooops. The test was run again and it was found that the CKP signal had a problem. What was odd was the the vehicle did try to start a bit. Once all 8 IC circuits had been checked on the scope it was found that 2 cylinders were getting a command to fire, which would explain why the vehicle tried to start.

diag​.​net/file/f3ojc7s4l…

The image shows the drop out in the CKP signal and a command for firing of cylinder #7 ignition coil.

The CKP sensor was removed in order to inspect the tone wheel. After turning the engine over by hand, one of the two tone wheels had a bent tab.

i​.​imgur​.​com/op4M1Mp​.​png

I have heard of these engines having tone wheels that would shift or move and cause issues. So it made sense that it could be possible that the tone wheel shifted and struck the block causing a bent tooth. The engine turned evenly by hand and it would attempt to start on two cylinders, so I thought that getting approval to remove the crank for inspection to repair was warranted.

i​.​imgur​.​com/p5uHzvv​.​jpg

Let's just say it did not come as a surprise that this wasn't a simple matter of a loose tone wheel. With pieces of number six piston bouncing around it is easy to see why the tone wheel got damaged.

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Dave Owner/Technician
Greenbrier, Arkansas
Dave Default
 

I don't mean this as a bad comment but I am seeing a lot of help request/info posts from techs that are going around the world testing sensors,waveforms, and various other time intensive tests when just listening to what sounds the motor is making would point a tech in the correct direction immediately. In this case with a piston blown out the uneven cranking cadence should have been a…

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Jon Owner/Technician
Trussville, Alabama
Jon Default
   

Dave, you should be ashamed for being so nit picky and negative!!! Just kidding, no offense taken. I agree that on the surface it would seem that some basic use of the senses would have led to an immediate judgment of internal engine damage, but in this case I was deceived, kind of. The engine did seem to be firing on 2 cylinders and I did not detect an abnormality in cadence, no knocking noise…

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Was is it the skirt of the piston that broke off, and the upper-half was still there? That would explain still sounding even. If it would have ran, then it probably would have sounded "wrong". Of course we never get the, "it made a loud crash-bang sound and then died" part of the story.

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Jon Owner/Technician
Trussville, Alabama
Jon Default
   

Geoff, bingo! I only found the skirt parts and the oiling ring in the pan. After seeing the lower portion of the cylinder wall missing significant amounts of material I decided to waste no more time sticking my head up a cow's derrie're ;). Out of curiosity, have you experienced this?

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Nope. I was just trying to reason how it could sound normal cranking, and the "compression part" of the piston still being intact explains it. I'm really glad I live in the age of pictures on the internet, though, because reading about is never as good as SEEING it. It's hard to look for things if you don't know they exist...ya know? I wouldn't have guessed the lower end of this engine had the…

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Dave Owner/Technician
Greenbrier, Arkansas
Dave Default
 

My post was not so much directed at you as it was a observation of many techs in general. Another nameless site is overran with posts from so called techs that couldnt find their backside with a manual and a map. I hope this site doesnt degrade the same way. Your post just seemed to be a good example of not using the available senses to diagnose with that my sometimes not to smart self used it…

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Jon Owner/Technician
Trussville, Alabama
Jon Default
 

I understood completely your sentiment. This is a fresh start and we all need to chip in to help it mature. And on that subject I suppose I need to start doing this just in case...... Haeynous Argo, ALLLIBAMMA!

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Stephen Technician
Gallatin, Tennessee
Stephen Default
 

So, I’m wondering, why did a post by someone who had diagnostic experience and wasn’t afraid to use it, trigger a post commenting about techs who are diagnostically challenged being too common? I don’t see any reason to criticize any other’s posts. Although I would like for some better punctuation and capitalization to make them easier to understand.

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LaMont Owner/Technician
Smithville, Ohio
LaMont Default
 

It seems that some on this forum don’t think those who are less experienced and are endeavoring to sharpen their diagnostic skills have the right to do so on Diag​.​net I find it kind of sad. As one sharpens their diagnostic skills they will learn which tests to employ first. Learning how to properly utilize different testing methods, ie. labscopes, and which method is the…

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Ben Mechanic
Chesnee, South Carolina
Ben Default
 

Well, I thought it was a great post. If I had a dollar for every time I was testing out my diagnostic hypothesis and the whole investigation suddenly took an unexpected left turn I could.....well, treat a couple of you to a decent lunch I guess. :)

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