DFM shudder/vibration getting worse

pittsteel · November 7, 2024

On 3/12/2024 at 6:13 PM, matt grz said:

Vibration is back lmao wtf! I’m not really sure how to check the transmission mounts. I see the tsb pip 5705, but I’m not really sure what I’m looking at. Might have to go back to the dealer. Sick of paying diagnostic fees though! Any advice?

I have been researching and investigating a simuliar rattling sound when DFM kicks in on my 2021 Yukon AT4 w/ the 5.3L. Every time there is a load on the enigine (going up hill or even flat) and I have a little toe on the gas I get it. What I've found so far is there is a exhaust valve mid-way back just before the muffler that isn't working correctly. The value from my understanding and reading helps control the back pressure in the exhaust when DFM is on. This valve is a spring loaded valve that apparetly weakens over time. The flapper inside the exhaust (cant't see it) sounds loose and potentially could cause a fluttering rattle sound that resonates up the exhaust chain. I locked my valve fully open and it killed some of the rattling, but not all of it. Now that the valve is fully open I get a rubbling sound kinda like opening the back window of a car and the pressure inside is fluxating. I suspect this is from the valve being disabled and not working to kill back pressure. I'd rather hear that than the rattle. I also tested this by diabling DFM manually without the need of a tuner. You can take your vehicle and put into manual shift mode. On my Yukon I push the 'L' button on my shifter. Then using the - and + buttons I dial the number up to 9, telling the truck it's max gear and to only use 9 out of the 10 gears. With it in this mode DFM will never come on. If you hit + again and set it to 10 DFM will start kicking in again but only when in 10th gear. To take it out of manual shift just hit the D button to put it back into automatic. Engine response is greatly improved when in maunal mode so I may have some DFM issue going on. When in DFM my engine is a slug. It stumbles and hesitates coming out of 4 cycle mode. Some history with my vehicle. I had both banks of lifters replaced at 13K miles due to lifter failure. This rattle started this year around 30K miles. I'm currently at 32K. It does suck to pay the amount we pay for these trucks and have so many issues. I have an appoint setup towards the end of Nov for my dealer to troubleshoot. But, as you've all have seen - diagnosing a rattle isn't fun! matt grz, not sure if your exhaust update removed this OEM flapper valve. I think most cat back exhaust upgrades do not. You may want to check it out.

Edited November 7, 2024 by pittsteel

SummitSierra24 · December 16, 2024

New here, but I have a 2021 GMC Sierra 1500 AT4 with the 5.3/10speed combo sitting at just about 67k miles. I was lucky enough to not have the notorious lifter failure (knock on wood) but I've also noticed the shudder whenever going from coasting to lightly accelerating to maintain speed, at any speed as mentioned here thanks to the lovely DFM system. I did test this by placing it in "Manual" mode and setting it to 9th gear being the highest gear and it did make a big difference in how smooth it was. The exhaust flapper valve issue is also taking place now and is a noticeable rattling sound whenever accelerating from 1,800 to 2,500 rpm when its warmer outside (65 degrees or warmer). It's pretty aggravating spending the amount we do on these trucks and them having countless issues. I purposely kept this truck bone stock and didn't modify it in any way, and maintained it as well as humanly possible. This truck hasn't even gone over 5,000 miles on each oil change with factory spec oils. Unfortunately yet again, there was never a straight answer given for any issues. The only other thing I experienced was a lower ball joint failure at approximately 62k miles which isn't normal either. That was a lovely bill, especially when I was advised that whenever the lower ball joint fails, the entire lower control arm assembly has to be replaced because it's a "non-serviceable" part and they can't replace just the ball joint. GM severely dropped the ball on this one and I've been a die hard GM fan for years. I wish I never got rid of my 2013 and just fixed the body rot issues. The only vehicle that I had a worse experience with, was a 2017 charger I bought new and kept for 2 years. But even then, it wasn't much worse than what is happening now. Come on GM, you're literally running us out of the company and forcing us to look at other options when considering trucks. Not all of us lease and dump these vehicles on the next person who doesn't know what they're getting in to.

s10dragracer888 · January 31, 2025

On 11/7/2024 at 3:48 PM, pittsteel said:

I have been researching and investigating a simuliar rattling sound when DFM kicks in on my 2021 Yukon AT4 w/ the 5.3L. Every time there is a load on the enigine (going up hill or even flat) and I have a little toe on the gas I get it. What I've found so far is there is an exhaust valve mid-way back just before the muffler that isn't working correctly. The value from my understanding and reading helps control the back pressure in the exhaust when DFM is on. This valve is a spring loaded valve that apparetly weakens over time. The flapper inside the exhaust (cant't see it) sounds loose and potentially could cause a fluttering rattle sound that resonates up the exhaust chain. I locked my valve fully open and it killed some of the rattling, but not all of it. Now that the valve is fully open I get a rubbling sound kinda like opening the back window of a car and the pressure inside is fluxating. I suspect this is from the valve being disabled and not working to kill back pressure. I'd rather hear that than the rattle. I also tested this by diabling DFM manually without the need of a tuner. You can take your vehicle and put into manual shift mode. On my Yukon I push the 'L' button on my shifter. Then using the - and + buttons I dial the number up to 9, telling the truck it's max gear and to only use 9 out of the 10 gears. With it in this mode DFM will never come on. If you hit + again and set it to 10 DFM will start kicking in again but only when in 10th gear. To take it out of manual shift just hit the D button to put it back into automatic. Engine response is greatly improved when in maunal mode so I may have some DFM issue going on. When in DFM my engine is a slug. It stumbles and hesitates coming out of 4 cycle mode. Some history with my vehicle. I had both banks of lifters replaced at 13K miles due to lifter failure. This rattle started this year around 30K miles. I'm currently at 32K. It does suck to pay the amount we pay for these trucks and have so many issues. I have an appoint setup towards the end of Nov for my dealer to troubleshoot. But, as you've all have seen - diagnosing a rattle isn't fun! matt grz, not sure if your exhaust update removed this OEM flapper valve. I think most cat back exhaust upgrades do not. You may want to check it out.

@pittsteel my 2024 5.3/MHT 10 speed has a rattle in the same situation you described….theyre saying its transmission related, but happens under the same conditions your rattle did. I’ll let the tech know to check exhaust flapper, but my truck doesn’t even have 700 miles on it.

matt grz · January 31, 2025

8 minutes ago, s10dragracer888 said:

@pittsteel my 2024 5.3/MHT 10 speed has a rattle in the same situation you described….theyre saying its transmission related, but happens under the same conditions your rattle did. I’ll let the tech know to check exhaust flapper, but my truck doesn’t even have 700 miles on it.

They love to blame DFM. Does your radio cut out randomly like your Bluetooth keeps dropping and reconnecting for seconds at a time?

s10dragracer888 · January 31, 2025

1 minute ago, matt grz said:

They love to blame DFM. Does your radio cut out randomly like your Bluetooth keeps dropping and reconnecting for seconds at a time?

No. Fortunately. I have the pre-refresh dash in mine. It’s the same as the ‘19-‘21’s…because it’s a work truck trim level. I do, however, have an odd, low engine speed vibration or shudder with DFM active. I attribute it to being normal for firing on less than all cylinders. Been a mechanic for 16 years. I’ve seen a few things.