Duramax Diesels Forum Truck of the Week
  #61  
Old 02-26-2017, 11:09 PM
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I'd like to see this on marks dyno measuring cylinder pressure


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  #62  
Old 02-27-2017, 08:18 AM
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I think you guys are giving GM way too much credit. Having looked through a bunch of stock Duramax and LS tunes, I think GM gets it close, then phones it in once it passes emissions and makes their power target. The stock LB7 tune is a hot mess, especially the early ones.

I don't believe this was done to protect the crank. The amount of fuel is way too low to do anything meaningful. Plus, think of the shock loading the moment you step on the throttle after exhaust braking down a hill. It goes from ~0 mm3 to 40+ instantly. That first cylinder will have a huge differential loading vs the rest (which are still at 0).

I have noticed a recurring theme on stock Duramax tunes that might explain this, though. GM has had the timing for low fuel amounts (0-5 mm3) set really high since the beginning, and what this does is cause a "growl" as the transmission downshifts, especially in tow/haul. My dad (and probably lots of others) really, really like this sound on the downshift. I've played with the timing in that range, and the only thing it seems to do is change the note of the growl. I'm guessing GM did something similar on the L5P, and it isn't supposed to happen except in tow/haul.

Not that anyone asked for it, but that's my
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  #63  
Old 02-27-2017, 09:47 AM
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Yeah, I doubt GM did this for a "growl". I think the best guess is what the ferm mentioned on crank harmonics. It make sense, they only need to inject just enough on random cylinders to upset the harmonics nothing more.
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  #64  
Old Yesterday, 04:07 PM
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My 16 van does it and was concerning but I guess not.
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  #65  
Old Yesterday, 06:10 PM
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Well seeing as this thread popped bag up figure Iíll update it with a bit of info...my buddy and fellow coworkers Ď17 3500 dually with a flatbed and welding setup was making a weird ticking sound following rpm at idle in park/drive
Poured some sea foam in the tank and didnít notice any changes
Turned out to just be in a weird regen cycle we havenít noticed before
He also had a leaking rear differential seal that warranty fixed
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