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Mercedes Benz spark plug replacement concerns #5425434 05/10/20 10:23 AM
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MajorCavalry Offline OP
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My wife's car is a 2018 Mercedes Benz GLC 300 with the 2L turbo engine. The car requires spark plug replacement at 30K miles. Seems a bit soon with the current generation of iridium spark plugs but that is a different issue.

Feeling fairly mechanically inclined, I was prepared to do this myself. I have changed hundreds of spark plugs over the years. Some more challenging than others. The access does not seem too bad in this engine. I noticed NGK, Denso and Bosch do not list a replacement plug. I thought this was a bit odd. Did some research and found out several things. First, only MB manufactures a plug for this engine. (please correct me if I am wrong). The second, and more troubling, is that these plugs MUST be indexed so the open area between the center electrode and ground wire faces the injector or the engine can fail! A MB service bulletin shows a properly indexed MB plug compared to an improperly indexed NGK plug which resulted in a piston crown melting with catastrophic engine damage that, of course, was not covered under warrantee.

The technique calls for the correct MB plug torqued very carefully to spec which should result in the plug being properly indexed. First question, has anyone heard of this being a requirement for any passenger car engines? I have known about indexing plugs with race engines. Second, how accurate a torque wrench would someone need to be sure to accomplish this task? I am not sure of the accuracy and precision of consumer grade torque wrenches and how often they would need to be calibrated to ensure success. Could carbon or any other contaminant on the threads of the head give false torque readings which would result in failure?

The procedure makes it sound like a crap shoot unless you bring it to the dealer. If they screw up, at least your engine is replaced in warrantee.


Steve

2015 Lexus GS 350 AWD premium CPO
Nebula Grey Pearl
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Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425444 05/10/20 10:33 AM
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KGMtech Offline
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This is really unique from what I've experienced hanging out in car forums, since these have existed. The replacement interval is very short, and the process is risky while the engine is under warranty.

I would not feel bad about getting the dealer to replace, at least until the warranty is done.


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Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425452 05/10/20 10:39 AM
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slacktide_bitog Online Content
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Honda actually did that indexing thing with the first-gen Insight. They list 4 different part numbers for their spark plugs in that application,one for each cylinder. However, the aftermarket only lists one par number. There are no problems with using the same one for all 4 cylinders crzy

Is the GLC300 the same engine as the C300? The M274?
If so, NGK #90654 should work.

Rock Auto lists it for the C300 but not the GLC300, but they may be the same. The notes do mention it being specially indexed.

Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425455 05/10/20 10:41 AM
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skyactiv Offline
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My dad has a Smokercraft boat that has on outboard which requires spark plug indexing.


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Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425459 05/10/20 10:44 AM
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Indexing plugs is a racing thing normally. Wonder if a valve or piston can hit them if they are turned the wrong direction?


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Re: Mercedes Benz spark plug replacement concerns [Re: slacktide_bitog] #5425473 05/10/20 11:03 AM
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MajorCavalry Offline OP
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I believe this engine is from the M260/264 family.

Originally Posted by slacktide_bitog
Honda actually did that indexing thing with the first-gen Insight. They list 4 different part numbers for their spark plugs in that application,one for each cylinder. However, the aftermarket only lists one par number. There are no problems with using the same one for all 4 cylinders crzy

Is the GLC300 the same engine as the C300? The M274?
If so, NGK #90654 should work.

Rock Auto lists it for the C300 but not the GLC300, but they may be the same. The notes do mention it being specially indexed.


Steve

2015 Lexus GS 350 AWD premium CPO
Nebula Grey Pearl
Flaxen interior
30k miles



Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425478 05/10/20 11:08 AM
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I was under the impression that Mercedes and BMW included all maintenance during the warranty period, could be mistaken though. Do you know what Mercedes charges for the spark plug service? Being an inline 4 cylinder engine I wonder how difficult it is to replace the plugs. Though the plug indexing part sounds dicey to get exact.

Whimsey

Re: Mercedes Benz spark plug replacement concerns [Re: Whimsey] #5425490 05/10/20 11:26 AM
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MajorCavalry Offline OP
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This 30k mile service is not included.

Not sure what else is included in the 30k service besides spark plug change, but I was told the cost is $1500!


Steve

2015 Lexus GS 350 AWD premium CPO
Nebula Grey Pearl
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30k miles



Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425494 05/10/20 11:30 AM
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I think BMW includes the maintenance but Mercedes does not. They do sell pre paid maintenance packages and if the spark plugs are required as part of the maintenance, it's included so it might be worth it to get a 2-4 pack. You can buy it from any MB dealer. Some MB dealers mark it up over list price so even though there's not much discounting on it, it can still pay to shop around depending on if the dealer is marking it up or not. If the transmission fluid is required, that would also be done as part of the maintenance package.

Mercedes also went to a mutli spark system and I think initially it did 4 sparks per cycle and the last I heard it was up to 5 sparks per cycle so the plugs are getting a lot more of a work out than regular plugs just do one per cycle.

As for indexing the plugs, I think if you used the right plugs, they can only go in one way so that they're automatically indexed when installing them. They've kinda had this since they came out with direct injection engines back in 2012.

Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425512 05/10/20 12:04 PM
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Quote
Mercedes also went to a mutli spark system and I think initially it did 4 sparks per cycle and the last I heard it was up to 5 sparks per cycle so the plugs are getting a lot more of a work out than regular plugs just do one per cycle.


They also use pizeo (?) injectors for this engine which can inject fuel up to 5 times per compression stroke. Imagine that!

Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425515 05/10/20 12:09 PM
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Just checked my "Prepaid Maintenance Schedule" for the E300 (same engine, 2018) it recommends spark plug replacement at 50,000 miles.

Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425520 05/10/20 12:15 PM
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Match-mark the old plugs to a reference, like the cowl or intake valley. Yes, you won't know where the prongs are in relation, but you can transfer the marks to the new plugs once you get them out.

Torque as specified, then check the marks.

MBZ is still a mass market manufacturer-- they aren't Rolls Royce-- so they'll be able to put the plugs in quickly at the factory through semi-automated means and have it come out right.

Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425550 05/10/20 01:03 PM
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I had a 90's era Dodge Caravan with the V-6. I changed the plugs myself with Bosch plugs stated for that engine. It ran like crap. I took it to a mechanic and he changed back to what plugs Chrysler used. Ran great. He told me that the Bosch open ends were facing the cylinder wall and not open to the fuel injectors. That was the first time I had heard of indexing plugs.
I would think that if you used the MB plugs and torqued them per specs, you would be fine.

Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425558 05/10/20 01:17 PM
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Many marine engines with DI eg E-TEC and Optimax are requiring indexing, no big deal, it take just a bit of additional prep and time.
First make sure you have the correct tools for the particular engine you are working on, that may include a thin wall long 12pt socket, indexing washers if available and an accurate torque wrench. Marking the socket to the ground electrode accurately is easier with just a socket than a universal/extension setup.
Torque wrench accuracy should be within DIN norm which is 4%, eg a 23 nm value will be between approx 22-24nm, this is normal. I just bought a new Norbar torque wrench, PM me if you need the link to it, this one is 3/8 12-50nm and is about 0.5% - 1% (spec is 3% or less) for $120 and its made in England, perfect for work like this and a very nice tool to own.
I will post pics and info in another thread as soon as I have more time.

Do not use anything but an OE MB plug, do not use NGK, its too short for one thing, a picture is worth a whole paragraph.

[Linked Image]

The way I do this job is to first accurately mark the socket to the ground electrode all the way up to the top of the socket, do not use any anti seize on these plugs, they will not be in the engine long enough to carbon seize anyway.
Using the 23 nm as an example set the torque wrench to 22 nm and tighten slowly and steadily once the plug contacts and see where the electrode is in relation to the injector if it is slightly before then tighten to 23 or 24 nm to bring it to center or if it within 45 degrees after center leave it at 22 nm.

If 24nm will not bring it within 45 degrees loosen the plug and tighten it again, sometimes this will crush the washer just enough to bring it within spec. If not use another plug or the washers from the old plug or an indexing washer if available (these are sold by thread diameter not vehicle specific).
Do not beat yourself up trying to get it exactly at zero, it may not happen but it does need to be close. This is not a complex or difficult and easily doable by DIY.
PM if I can be of any help or you need clarification or links.












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Re: Mercedes Benz spark plug replacement concerns [Re: MajorCavalry] #5425630 05/10/20 03:22 PM
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JohnG Offline
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Good info Trav and Elifino. I never knew this was a "thing" with MB, but if I ever get my car to 50,000 miles I will be sure to use one of these two excellent methods!

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