Misfiring
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Misfiring
My car is misfiring from #2 and #8 , and the check engine light was on I took it to my mechanic first, he replaced the spark plug and coils on both but the car continue to misfire he then looked at the injector and replace it thinking this was the cause but no it was not , however he did notice the spark plugs had some oil on them , he could find out what the problem , so he recommend the dealership for answer , so I took it and they diagnose it with the same thing combustion misfiring of cylinder 2 and 8 has a malfunction so they replaced the coil and spark plugs on both same thing the light still on and misfiring , now they are saying its the injectors on #2 causing the issue , also resealing all injectors on the right engine bank and performing the I injector decarbonization service this will fix the problem but no guarantees , bc the misfire can return I am like why would I paid almost 4k for this services without any assurance this will fix my problem, so I am stuck and need help, I do have 120k miles on my car,
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MBWorld Fanatic!
Joined: Feb 2012
Posts: 2,641
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From: Phoenix
222 S-65
OP You can help us help you by providing the YEAR and MODEL of your car.
Conspicuous by absence is any mention of a leakdown test. When cylinder(s) don't run something is missing. Spark, Fuel or compression. Sometimes a combination of the three.
ASK the dealer for a printout of the 'Quick-Test' the tech performed as a part of diagnosis.
Conspicuous by absence is any mention of a leakdown test. When cylinder(s) don't run something is missing. Spark, Fuel or compression. Sometimes a combination of the three.
ASK the dealer for a printout of the 'Quick-Test' the tech performed as a part of diagnosis.
Junior Member
Joined: Aug 2022
Posts: 21
Likes: 6
2004 SL55
Neither of these two places you have brought the car to are diagnosing the issue and instead are replacing what appear to be perfectly fine parts. Instead of charging you for replacement of spark plug/coil/injector they should have switched them with a working cylinders to see if misfire follows. I wouldn’t trust either of these two places moving forward.
A competent mechanic at this point would be scanning for codes, and if just misfire codes are present then scoping waveforms to be able to tell exactly what is wrong.
A competent mechanic at this point would be scanning for codes, and if just misfire codes are present then scoping waveforms to be able to tell exactly what is wrong.
Junior Member
Joined: Aug 2022
Posts: 21
Likes: 6
2004 SL55
There are several potential reasons for the oil consumption, which could be related to your misfire. Could be valve cover gasket, PCV, valve stem seal, Cylinder scoring/piston rings….and some more.
Simple waveforms (in-cylinder compression, ignition, cam/crank, fuel injector) from an experienced tech who knows how to use a scope and interpret will tell you what exactly is going on. Otherwise, you’re probably going to be doing a lot of individual checks along with parts changing by a less experienced shop to try to narrow down.
Simple waveforms (in-cylinder compression, ignition, cam/crank, fuel injector) from an experienced tech who knows how to use a scope and interpret will tell you what exactly is going on. Otherwise, you’re probably going to be doing a lot of individual checks along with parts changing by a less experienced shop to try to narrow down.
MBWorld Fanatic!
Joined: Aug 2007
Posts: 1,844
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From: Pacific Northwest
2001 S600 V12 Sold, 2011 Jaguar XFR Sold, 2017 S550 4-Matic, 2018 S63 AMG Sedan
My car is misfiring from #2 and #8 , and the check engine light was on I took it to my mechanic first, he replaced the spark plug and coils on both but the car continue to misfire he then looked at the injector and replace it thinking this was the cause but no it was not , however he did notice the spark plugs had some oil on them , he could find out what the problem , so he recommend the dealership for answer , so I took it and they diagnose it with the same thing combustion misfiring of cylinder 2 and 8 has a malfunction so they replaced the coil and spark plugs on both same thing the light still on and misfiring , now they are saying its the injectors on #2 causing the issue , also resealing all injectors on the right engine bank and performing the I injector decarbonization service this will fix the problem but no guarantees , bc the misfire can return I am like why would I paid almost 4k for this services without any assurance this will fix my problem, so I am stuck and need help, I do have 120k miles on my car,
Super Member
Joined: May 2011
Posts: 975
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From: Fort Worth TX
2020 S560, 2014 SLK55 (Prev. 2016 S65, 2013 SLK350, 2008 CLS63, 2006 E350, 2006 CLS500, 2005 C230K)
Well I don't know where you are but this is how the diagnostic should have gone:
1. Move coils
-no change
2. Replace relevant sparkplugs
-no change
3. Compression and leak down test
Don't put any more money into remedies without compression and leakdown results.
1. Move coils
-no change
2. Replace relevant sparkplugs
-no change
3. Compression and leak down test
Don't put any more money into remedies without compression and leakdown results.
MBWorld Fanatic!
Joined: Feb 2012
Posts: 2,641
Likes: 1,224
From: Phoenix
222 S-65
Where there is excessive leakage found in a leak down test.... Where is it leaking to? Out the crankcase? Into the cooling system? Out the exhaust? Out the intake? Or out of the incompetent techs rectum who have looked at it?
Xentry will share a lot of information with the tech. Sure seems like the Mercedes dealer should have been able to go right to it.
If they did a ‘Compression test’ How much pressure did each of the cylinders produce?
Xentry will share a lot of information with the tech. Sure seems like the Mercedes dealer should have been able to go right to it.
If they did a ‘Compression test’ How much pressure did each of the cylinders produce?
MBWorld Fanatic!
Joined: Aug 2007
Posts: 1,844
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From: Pacific Northwest
2001 S600 V12 Sold, 2011 Jaguar XFR Sold, 2017 S550 4-Matic, 2018 S63 AMG Sedan
It's just a matter of time before the obvious is revealed. Scope the cylinder walls, the bad ones and a couple more for reference. You'll find your answer.
Out Of Control!!
Joined: Apr 2019
Posts: 11,993
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From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
My car is misfiring from #2 and #8 , and the check engine light was on I took it to my mechanic first, he replaced the spark plug and coils on both but the car continue to misfire he then looked at the injector and replace it thinking this was the cause but no it was not , however he did notice the spark plugs had some oil on them , he could find out what the problem , so he recommend the dealership for answer , so I took it and they diagnose it with the same thing combustion misfiring of cylinder 2 and 8 has a malfunction so they replaced the coil and spark plugs on both same thing the light still on and misfiring , now they are saying its the injectors on #2 causing the issue , also resealing all injectors on the right engine bank and performing the I injector decarbonization service this will fix the problem but no guarantees , bc the misfire can return I am like why would I paid almost 4k for this services without any assurance this will fix my problem, so I am stuck and need help, I do have 120k miles on my car,
Now you have the basics ignition covered already...
Can you read the FUEL TRIMS... LTFT 1 +2 .
how lean... -20?
Last edited by CaliBenzDriver; Feb 2, 2026 at 07:41 PM.
