Knocking Sound 2014 E350 M276
MBWorld Fanatic!
Joined: May 2018
Posts: 6,601
Likes: 6,547
From: Jakarta-Indonesia
2014 - W212.065 - E400 ( M276.820, 3 liter Turbo) RWD not Hybrid
See if your s-park plug tool lacks the top side gripper, some uses rubber insert as gripper to prevent spark plug from not being centered.
Below is a simple but good video , where the ceramic may crack
Below is a simple but good video , where the ceramic may crack
Newbie
Joined: Apr 2024
Posts: 4
Likes: 6
W212 E350
Hi everyone, my W212 E350 does exactly the same sound. This model was not sold in France (it is a German import) so the mechanics here are not use to the M276 and they just suggested me to replace the motor (because they suspect a rod issue)....
@Geep 250GD @2015-E350-Wagon do you confirm that after the spark(s) replacement the knocking sound is gone? (it should be cyl #2 for me as I had a few sporadic misfires here in the past)
Thanks!
@Geep 250GD @2015-E350-Wagon do you confirm that after the spark(s) replacement the knocking sound is gone? (it should be cyl #2 for me as I had a few sporadic misfires here in the past)
Thanks!
Newbie
Joined: Apr 2024
Posts: 4
Likes: 6
W212 E350
Newbie
Joined: Apr 2024
Posts: 4
Likes: 6
W212 E350
The issue was the spark plugs (two in my case, cyl #2 and #3) !
Engine sound before:
Engine sound after:
And here is one of the plug in question:
I suspect that the torque (and so, the index/orientation) was not respected by the previous mechanics...
Thanks again for this thread, it saved my engine (!)
Engine sound before:
Engine sound after:
And here is one of the plug in question:
I suspect that the torque (and so, the index/orientation) was not respected by the previous mechanics...
Thanks again for this thread, it saved my engine (!)
Out Of Control!!
Joined: Apr 2019
Posts: 11,924
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
OMG!
Thank you for these great video. 
That failed sparkplug is really something to see it slide up/down!!!
Overheated ignition coils and boots are known to require replacement to prevent misfirings.
That failed sparkplug is really something to see it slide up/down!!!
Overheated ignition coils and boots are known to require replacement to prevent misfirings.
Newbie
Joined: Jul 2025
Posts: 1
Likes: 1
Same fix!
I experienced the exact same issue with the knocking sound and the problem also turned out to be two broken spark plugs.
Mercedes listened to it and suspected a major engine problem, with anticipated need to take the engine out for proper diagnosis. I showed them this post and they checked the plugs and found the issue.
Thanks for sharing in this forum - it most likely saved me thousands of EUR and a lot of time 🙏
PS. My car is also an E350 cabrio from 2014. Only 42000 km and allways maintained at Mercedes. So, a damaged engine sounded unlikely.
Mercedes listened to it and suspected a major engine problem, with anticipated need to take the engine out for proper diagnosis. I showed them this post and they checked the plugs and found the issue.
Thanks for sharing in this forum - it most likely saved me thousands of EUR and a lot of time 🙏
PS. My car is also an E350 cabrio from 2014. Only 42000 km and allways maintained at Mercedes. So, a damaged engine sounded unlikely.
The issue was the spark plugs (two in my case, cyl #2 and #3) !
Engine sound before:
https://youtube.com/shorts/I_B_sXU4yaY
Engine sound after:
https://youtube.com/shorts/3WzBw1RlF1k
And here is one of the plug in question:
https://youtube.com/shorts/TQv8leWjQPU
I suspect that the torque (and so, the iindex/orientation) was not respected by the previous mechanics...
Thanks again for this thread, it saved my engine (!)
Engine sound before:
https://youtube.com/shorts/I_B_sXU4yaY
Engine sound after:
https://youtube.com/shorts/3WzBw1RlF1k
And here is one of the plug in question:
https://youtube.com/shorts/TQv8leWjQPU
I suspect that the torque (and so, the iindex/orientation) was not respected by the previous mechanics...
Thanks again for this thread, it saved my engine (!)
Last edited by Thomas Gamst; Jul 25, 2025 at 11:54 AM.
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Out Of Control!!
Joined: Apr 2019
Posts: 11,924
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
MB PLUGS FAILURE
I experienced the exact same issue with the knocking sound and the problem also turned out to be two broken spark plugs.
Mercedes listened to it and suspected a major engine problem, with anticipated need to take the engine out for proper diagnosis. I showed them this post and they checked the plugs and found the issue.
Thanks for sharing in this forum - it most likely saved me thousands of EUR and a lot of time 🙏
PS. My car is also an E350 cabrio from 2014.
Only 42000 km and allways maintained at Mercedes.
So, a damaged engine sounded unlikely.
Mercedes listened to it and suspected a major engine problem, with anticipated need to take the engine out for proper diagnosis. I showed them this post and they checked the plugs and found the issue.
Thanks for sharing in this forum - it most likely saved me thousands of EUR and a lot of time 🙏
PS. My car is also an E350 cabrio from 2014.
Only 42000 km and allways maintained at Mercedes.
So, a damaged engine sounded unlikely.
Extreme heatsoak likely involved with thermal shock.
Last edited by CaliBenzDriver; Jul 25, 2025 at 02:37 PM.
Newbie
Joined: Feb 2026
Posts: 2
Likes: 1
2012 MB R350
Thank you for posting your findings. Just wanted to chime in, in 2023 I replaced plugs in my 2012 MB R350 with 6 Bosch ZR6SII3320 plugs.
I heard a distinctive knock on passenger side. This post gave me a good direction to follow, and sure enough, when I removed the passenger side plugs, I found cylinder 1 plug failed where the inner portion would move resulting in no proper gap.
The other plugs from cylinders 2 and 3 had a wide brown band on the ceramic portion, indicating a blow by gasses making it through the plug.
Interestingly enough, the car ran fine otherwise and at least there was no a visual indication of misfire, I did not go into scanner to confirm it either way.
The only other thing I want to add, is that I ran into this situation once before. I was working on 2017 Jetta and I bought Bosch plugs. Over short amount of time, 2 of those plugs failed in the exact same manner, except car
was misfiring badly and it was pretty obvious what the issue was. I replaced Bosch plugs with NGK equivalent and the issue never returned.
Unfortunately, there is no NGK/Denso approved replacement for the MB that I found, so I replaced all 3 passenger side plugs with another set of Bosch.
All 3 failed plugs hail back to 2022-2023 manufacturing, so hoping that the defect was addressed since.
I heard a distinctive knock on passenger side. This post gave me a good direction to follow, and sure enough, when I removed the passenger side plugs, I found cylinder 1 plug failed where the inner portion would move resulting in no proper gap.
The other plugs from cylinders 2 and 3 had a wide brown band on the ceramic portion, indicating a blow by gasses making it through the plug.
Interestingly enough, the car ran fine otherwise and at least there was no a visual indication of misfire, I did not go into scanner to confirm it either way.
The only other thing I want to add, is that I ran into this situation once before. I was working on 2017 Jetta and I bought Bosch plugs. Over short amount of time, 2 of those plugs failed in the exact same manner, except car
was misfiring badly and it was pretty obvious what the issue was. I replaced Bosch plugs with NGK equivalent and the issue never returned.
Unfortunately, there is no NGK/Denso approved replacement for the MB that I found, so I replaced all 3 passenger side plugs with another set of Bosch.
All 3 failed plugs hail back to 2022-2023 manufacturing, so hoping that the defect was addressed since.
Out Of Control!!
Joined: Apr 2019
Posts: 11,924
Likes: 6,776
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
spark plugs work
Thank you for posting your findings. Just wanted to chime in, in 2023 I replaced plugs in my 2012 MB R350 with 6 Bosch ZR6SII3320 plugs.
I heard a distinctive knock on passenger side. This post gave me a good direction to follow, and sure enough, when I removed the passenger side plugs, I found cylinder 1 plug failed where the inner portion would move resulting in no proper gap.
The other plugs from cylinders 2 and 3 had a wide brown band on the ceramic portion, indicating a blow by gasses making it through the plug.
Interestingly enough, the car ran fine otherwise and at least there was no a visual indication of misfire, I did not go into scanner to confirm it either way.
The only other thing I want to add, is that I ran into this situation once before. I was working on 2017 Jetta and I bought Bosch plugs. Over short amount of time, 2 of those plugs failed in the exact same manner, except car
was misfiring badly and it was pretty obvious what the issue was. I replaced Bosch plugs with NGK equivalent and the issue never returned.
Unfortunately, there is no NGK/Denso approved replacement for the MB that I found, so I replaced all 3 passenger side plugs with another set of Bosch.
All 3 failed plugs hail back to 2022-2023 manufacturing, so hoping that the defect was addressed since.
I heard a distinctive knock on passenger side. This post gave me a good direction to follow, and sure enough, when I removed the passenger side plugs, I found cylinder 1 plug failed where the inner portion would move resulting in no proper gap.
The other plugs from cylinders 2 and 3 had a wide brown band on the ceramic portion, indicating a blow by gasses making it through the plug.
Interestingly enough, the car ran fine otherwise and at least there was no a visual indication of misfire, I did not go into scanner to confirm it either way.
The only other thing I want to add, is that I ran into this situation once before. I was working on 2017 Jetta and I bought Bosch plugs. Over short amount of time, 2 of those plugs failed in the exact same manner, except car
was misfiring badly and it was pretty obvious what the issue was. I replaced Bosch plugs with NGK equivalent and the issue never returned.
Unfortunately, there is no NGK/Denso approved replacement for the MB that I found, so I replaced all 3 passenger side plugs with another set of Bosch.
All 3 failed plugs hail back to 2022-2023 manufacturing, so hoping that the defect was addressed since.
After 50kMi or way prematurely defective like 20kMi ??
-- Now if this is your engine, you should really replace Cyl.4+5+6 to run both banks nearly identical.
-- Also replace the plastic BOOTS that have the carbon-stack resistors built-in.
There must be a perfect NGK/Denso upgrade!!
Top Japanese brands license the Bosch 3.5L-V6 engines design they "make in Japan" with Swiss precision.
flat top design
Flat-tops used to connect NGK requires:
- the matching flat boots connections
- the correct electrode temperature
- the right physical dimensions
What's the right cross reference part No??
Last edited by CaliBenzDriver; Feb 12, 2026 at 01:37 PM.
Newbie
Joined: Feb 2026
Posts: 2
Likes: 1
2012 MB R350
How soon did you see these Bosch plug fail ?
After 50kMi or way prematurely defective like 20kMi ??
-- Now if this is your engine, you should really replace Cyl.4+5+6 to run both banks nearly identical.
-- Also replace the plastic BOOTS that have the carbon-stack resistors built-in.
There must be a perfect NGK/Denso upgrade!!
After 50kMi or way prematurely defective like 20kMi ??
-- Now if this is your engine, you should really replace Cyl.4+5+6 to run both banks nearly identical.
-- Also replace the plastic BOOTS that have the carbon-stack resistors built-in.
There must be a perfect NGK/Denso upgrade!!
If you watch any of the posted video's in this thread, my failure was identical, therefore knocking noise that I heard coming from head #1.
I don't disagree with your suggestion in regards to replacing plugs 4-6 on driver side head, but will leave it alone for now.
I looked far and wide, but I am not able to cross reference NGK/Denso plugs.
I looked at RockAuto and FCPEuro, and both show Bosch/MB plug. Rock also showed Champion plugs which I am definitely not using here.
Going through Denso and NGK catalogs shows no equivalent plugs.
Napa/autozone does show an NGK plug, but I can not find any supporting documentation that its in fact compatible.
Car in question is 2012 MB R350 3.5L gasoline engine - M276
