Indexing spark plugs
Banned
Joined: Nov 2025
Posts: 477
Likes: 187
From: US
Mercedes SL R231
I think the confusion may be because there are two types of engines: those that use stratified combustion and those that use homogeneous combustion. In the US, homogeneous combustion is used and in the rest of the world, stratified combustion is used. I believe indexing is critical for stratified combustion engines and less so for homogeneous combustion engines.
Here is another document that discusses the two approaches.
Here is another document that discusses the two approaches.
MBWorld Fanatic!
Joined: Aug 2021
Posts: 1,536
Likes: 765
1983 Nissan Shltbox
This is a common-sense issue that has been beat to death ad-nauseum since it's conception.
Just don't shield the spark from the injector. What is the discussion?
A far, far better question, is why, after all these years of intense, hair-pulling frustration do I still insist on replying to threads on this same, road-dead dog subject?
Just don't shield the spark from the injector. What is the discussion?
A far, far better question, is why, after all these years of intense, hair-pulling frustration do I still insist on replying to threads on this same, road-dead dog subject?
Super Member
Joined: Feb 2009
Posts: 713
Likes: 214
From: Brooklyn, NY
2007 S550 4Matic 2013 S550 Base
Banned
Joined: Nov 2025
Posts: 477
Likes: 187
From: US
Mercedes SL R231
Banned
Joined: Nov 2025
Posts: 477
Likes: 187
From: US
Mercedes SL R231
Banned
Joined: Nov 2025
Posts: 477
Likes: 187
From: US
Mercedes SL R231
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
10 Most Reliable Mercedes-Benz Models You Can Buy Used
Verdad Gallardo
MBWorld Fanatic!
Joined: Aug 2014
Posts: 5,317
Likes: 1,673
S500
I think the confusion may be because there are two types of engines: those that use stratified combustion and those that use homogeneous combustion. In the US, homogeneous combustion is used and in the rest of the world, stratified combustion is used. I believe indexing is critical for stratified combustion engines and less so for homogeneous combustion engines.
Here is another document that discusses the two approaches.
Here is another document that discusses the two approaches.
now that makes for interesting (and confusing) reading
the way I read ONLY the V6 gets the different engine for USA vs the Europe one - check page 8 and 9 no ref to the 278
Banned
Joined: Nov 2025
Posts: 477
Likes: 187
From: US
Mercedes SL R231
Good point. In that case, the M278 and other V8s require indexing. Personally, I index my plugs regardless since the WIS says to torque to 23Nm, which is supposed to get the plug in the correct orientation to the fuel injector. Remember, correct orientation is within +/- 45°.
Super Member
Joined: Feb 2009
Posts: 713
Likes: 214
From: Brooklyn, NY
2007 S550 4Matic 2013 S550 Base
Banned
Joined: Nov 2025
Posts: 477
Likes: 187
From: US
Mercedes SL R231
Super Member
Joined: Feb 2009
Posts: 713
Likes: 214
From: Brooklyn, NY
2007 S550 4Matic 2013 S550 Base
Thread Starter
MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,141
Likes: 1,293
From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
All four plugs that I installed so far stopped with the electrode opening pointing 90 degrees short from the direction of the injector with 23 Nm torque without using any thread compound.
My copper seal pre- crush is to make plugs turn just the needed angle. Two of them turned all 90 degrees, two only about 45 degrees where I left them. Motor runs good with the Bosch plugs from FCP and new coils from AutoZone.
My copper seal pre- crush is to make plugs turn just the needed angle. Two of them turned all 90 degrees, two only about 45 degrees where I left them. Motor runs good with the Bosch plugs from FCP and new coils from AutoZone.
on the passenger side of the engine I used a separate strong thick washers on an all thread. This was a M12x1.75 all thread that I got from the Kmac kit. Tightened to 23 Nm and after that turned 99 degrees. Washer thickness went from 1.5mm to 1.3mm as of this turned the plug to about 30degrees from TDC.
I will replace the four plugs I first did first using the method I explained above.
MBWorld Fanatic!
Joined: Jun 2021
Posts: 3,110
Likes: 1,346
From: Fleriduh
W212 E63S Wagon - GSL580 - E63 - E350 - C300
Does anyone have anything more than antidoteal evidence this is a real issue????
Assuming you plug is seated (with or without a torque wrench)?
Assuming you plug is seated (with or without a torque wrench)?
Banned
Joined: Nov 2025
Posts: 477
Likes: 187
From: US
Mercedes SL R231
Honestly, whether it’s a real issue or not, I find no harm in complying with the warning to index. 23Nm gets me within that 90° window. Of course, my 6 cylinders are probably easier to work on. I do mark the extension and socket to see how close I am to 0° when hitting 23Nm.
Thread Starter
MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,141
Likes: 1,293
From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
All we have is the Mercedes document I posted above about indexing direct injection engines, though some people believe it doesn’t apply to their M278 or M157 because their car model wasn’t listed.
Honestly, whether it’s a real issue or not, I find no harm in complying with the warning to index. 23Nm gets me within that 90° window. Of course, my 6 cylinders are probably easier to work on. I do mark the extension and socket to see how close I am to 0° when hitting 23Nm.
Honestly, whether it’s a real issue or not, I find no harm in complying with the warning to index. 23Nm gets me within that 90° window. Of course, my 6 cylinders are probably easier to work on. I do mark the extension and socket to see how close I am to 0° when hitting 23Nm.
I try to get my new plugs close to same. I don’t see there is any harm doing that either.
MBWorld Fanatic!
Joined: Aug 2014
Posts: 5,317
Likes: 1,673
S500
Every spark plug I took out, the originals with MB Star logo on them, had the plug spark opening directed right up towards the injector. I don’t think this is coincidence to have all eight plugs the same way.
I try to get my new plugs close to same. I don’t see there is any harm doing that either.
I try to get my new plugs close to same. I don’t see there is any harm doing that either.
most likely a learnt trick from the i people, marketing drivel to scare you in to using over priced servicing by franchised crooks - who actually employ sacked burger flippers
Banned
Joined: Nov 2025
Posts: 477
Likes: 187
From: US
Mercedes SL R231
The interesting part is that for me the OE Bosch plugs didn’t come close to being properly indexed, but the NGK (also OE) did. I returned the Bosch plugs to FCP and got the NGK ones from RockAuto.
MBWorld Fanatic!
Joined: Jun 2021
Posts: 3,110
Likes: 1,346
From: Fleriduh
W212 E63S Wagon - GSL580 - E63 - E350 - C300
To REALLY play internet devils advocate...unless someone takes off their head and checks from below, they really are not sure. Granted, the margin of error is massive here (45deg?) . We still have NOT ONE REPORT of an owner saying "yeah, my plugs were not indexed and I kepts getting an odd CEL about a misfire". It simply has not been posted by anyone (on any forum I can find).
MBWorld Fanatic!
Joined: Aug 2014
Posts: 5,317
Likes: 1,673
S500
it won't throw a code just knock and destroy the engine
the nasty limits of early DI just damaged the engine - hence everything went backwards and now "modern engines" get port injection for normal work and hit DI in the emission lab during fake driving conditions - plus now they use special oils to try and keep the thing alive as start stop and chocolate camchains do their best to help you need a new vehicle
the nasty limits of early DI just damaged the engine - hence everything went backwards and now "modern engines" get port injection for normal work and hit DI in the emission lab during fake driving conditions - plus now they use special oils to try and keep the thing alive as start stop and chocolate camchains do their best to help you need a new vehicle
Thread Starter
MBWorld Fanatic!
Joined: Oct 2010
Posts: 5,141
Likes: 1,293
From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
you can bet Merc didn't play with washers - so the torque values they quote are a misleading joke - do them up till the electrode is where its supposed to be, without going one turn too far and hitting 60lbs ft
most likely a learnt trick from the i people, marketing drivel to scare you in to using over priced servicing by franchised crooks - who actually employ sacked burger flippers
most likely a learnt trick from the i people, marketing drivel to scare you in to using over priced servicing by franchised crooks - who actually employ sacked burger flippers
I think the difference between plugs with the MB logo and regular Bosch from parts stores is where the plug thread starts. I tried to look at this but it is very difficult to see from the plugs.
But it really can only be this as I measured the copper washers the same 1.5mm thickness between MB logo plugs and the ones I bought from parts store.
MBWorld Fanatic!
Joined: Aug 2014
Posts: 5,317
Likes: 1,673
S500
but plug washers can be squashed loads (one and half turns easily past tight enough, without going mad and hitting 80 - on a std plug), so they index and you end up post washer crush with a plug done up about right..... that said still interesting it hits 23lbs ft exactly
