E55 mystery
Thread Starter
Member
Joined: Oct 2013
Posts: 131
Likes: 6
From: NE Phila
E55 AMG
OP what brand of spark plugs?
I have encountered counterfeit NGK plugs. Didn’t pay real close attention when replacing plugs. Had an intermittent rough idle after replacement that never set a check engine light (V-12 cars need something really wrong to set a misfire check engine light). Pulled all plugs and looked at them closely... sure enough plugs that are supposed to be iridium on center electrode and on ground side... some lack iridium in one spot, some on both. Look closely at plugs whilst comparing with information found on the web and yup! A decades long source of parts (obviously) unknowingly sold me eight (two boxes) of counterfeit NGK plugs. Another thing to watch for.
I have encountered counterfeit NGK plugs. Didn’t pay real close attention when replacing plugs. Had an intermittent rough idle after replacement that never set a check engine light (V-12 cars need something really wrong to set a misfire check engine light). Pulled all plugs and looked at them closely... sure enough plugs that are supposed to be iridium on center electrode and on ground side... some lack iridium in one spot, some on both. Look closely at plugs whilst comparing with information found on the web and yup! A decades long source of parts (obviously) unknowingly sold me eight (two boxes) of counterfeit NGK plugs. Another thing to watch for.
sparks are good
Thread Starter
Member
Joined: Oct 2013
Posts: 131
Likes: 6
From: NE Phila
E55 AMG
Hi. The reason that it smooths out is that in the absence of an oxygen sensor, the ECU makes some assumptions and gives you what it thinks is the best air fuel ratio. I don’t mean to be insistent, but that is another clue that the oxygen sensor is the issue. I’ll stop now, but having gone through the identical issue and researching I would really look at the oxygen sensor.
I replaced them
Out Of Control!!
Joined: Apr 2019
Posts: 11,958
Likes: 6,802
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
Appreciate the input. Just to clarify — I replaced the upstream O2 sensors only. The rear sensors are coded out since I don’t have cats. What’s interesting is unplugging the upstream on that bank actually smooths it out, which makes me think the ECU is reacting to a lean signal rather than the sensor itself being bad.
Sensor may be good on a bad harness connections...
You have a perfect setup for comparison bank 1 reference vs. Bk2.
New sensor can be affected by oily harness to cause ECU to lean Bk2 mixture.
Thread Starter
Member
Joined: Oct 2013
Posts: 131
Likes: 6
From: NE Phila
E55 AMG
Can you scrutinize/plot/compare the sensor data as the ECU sees it using scanner?
Sensor may be good on a bad harness connections...
You have a perfect setup for comparison bank 1 reference vs. Bk2.
New sensor can be affected by oily harness to cause ECU to lean Bk2 mixture.
Sensor may be good on a bad harness connections...
You have a perfect setup for comparison bank 1 reference vs. Bk2.
New sensor can be affected by oily harness to cause ECU to lean Bk2 mixture.
thank you will look into to it
Out Of Control!!
Joined: Apr 2019
Posts: 11,958
Likes: 6,802
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
A bit overwhelming...
We keep walking right passed evidence we assume are okay - New upstream O2 may not mean good data.
The best way to go pass that is by looking at live data to understand : " Why is ECU leaning Bk2 ?"
Some odd looking ECU data may help you understand why it's acting up.
Compare your Bk1 vs. Bk2: something will float up.
The best way to go pass that is by looking at live data to understand : " Why is ECU leaning Bk2 ?"
Some odd looking ECU data may help you understand why it's acting up.
Compare your Bk1 vs. Bk2: something will float up.
Last edited by CaliBenzDriver; Feb 28, 2026 at 06:53 PM.
Junior Member
Joined: Mar 2008
Posts: 39
Likes: 11
E550
Curious minds want to know….
Did you solve it? What was the cause? I’m still sticking to the O2 sensor, although certainly the O2 wiring/harness/connector could be the cause. Let us know! Inquiring minds want to know (and learn). Thanks.
Thread Starter
Member
Joined: Oct 2013
Posts: 131
Likes: 6
From: NE Phila
E55 AMG
Solved
We finally resolved the misfire on my E55. Turns out the timing on the misfiring bank was off by one tooth. The head had been removed at some point, and during reassembly, they missed it slightly. I bought timing tools on Amazon and set the crank pulley to 40 degrees. One bank lined up perfectly with the tool, but the other didn’t. We repositioned the cam gear on the problem side by one tooth. Once it all aligned, we reassembled, and now it runs perfectly—misfire is gone!
MB World Stories
The Best of Mercedes & AMG
Manual Mercedes? 6 Times Sindelfingen Let Drivers Have All The Fun
Verdad Gallardo
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Verdad Gallardo
8 Classic Boxy Mercedes Designs That Have Aged Like Fine Wine
Verdad Gallardo
Flawlessly Restored Mercedes 190E Evo II Heads to Auction
Verdad Gallardo
Electric Mercedes C-Class Unveiled: 11 Things You Need to Know
Verdad Gallardo
Mercedes EQS Gets A Major Update: Everything You Need to Know
Verdad Gallardo
5 Underrated Mercedes-Benz Models That Don't Get the Love They Deserve
Verdad Gallardo
Mercedes 300D Has Pushed Well Past 1 Million Miles and It Ain't Stopping
Verdad Gallardo
