2016 Denali Rattle in DOD (v4 mode)

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MiPinner

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At approximately 60k miles we detected a rattle coming from the second row passenger side area. My initial thought was heat shield because that's exactly what it sounds like. However, after crawling under the vehicle I could not find anything that was loose. Since I couldn't find anything wrong, and it was under a CPO warranty, I took it to the dealer. They were able to determine that the noise only happened in DOD (v4 mode) and it was "normal". They returned the vehicle with no changes and a printout of some TSB.

Moving forward almost 10K miles. This is my wife's daily driver and she has higher tolerance for things not working correctly than I, but she had apparently reached her breaking point. She came to me and said she could not take it anymore and I could do "whatever" to fix it.

Before I voided my powertrain warranty by disabling DOD in the ECM I wanted to give the dealer one last opportunity to resolve the issue. When I approached the dealer this time I also took my laptop with a road scan from HP Tuners which showed exactly when the vehicle entering v4 mode. Like clockwork after entering V4 the rattle would start and was even being picked up by the knock sensors and pulling timing. With this new information, and them looking over the vehicle once again, the dealer said they did not feel that it was a lifter, but instead a broken baffle weld inside the muffler. To resolve the issue they would need to replace the cat back exhaust system, which BTW was not covered by warranty.

At this point I have several options.
1. Install an entire cat back exhaust system
2. Have the muffler cut out and slap in some aftermarket product
3. Buy a couple HP Tuners credits and turn of DOD

Before I start cutting, disassembling, or tuning I want to make sure I'm targeting the source of the problem.
- Is it a Cat
- Is it the muffler
- Is it the vlom
- Is it a lifter
- Or is it the exhaust adaptive valve (aka flapper valve)

Here is a video I put to gether to demonstrate the issue.


Here is a screen print of HP Tuners VCM Scanner. I had the cruse set on the highway in this example. Here you can see the PID - Displacement on Demand Requested = Yes, and it pulling 4 degrees of timing due to Knock Retard. When it switches back to v8 mode the knock is gone.

upload_2020-12-13_21-22-20.png

Thanks in advance.
 

Tiredmechanic23

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just to try to help out I would check the flapper valve. When in V4 it helps with drone from the exhaust and sits in a more closed position. I would wire or bolt it wide open and see if the rattle continues. It would not be the first case of one going bad.
 

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