Bay Leaf

The indexing I presume will take care of itself as the threads are designed to match those in the head. The plugs I removed have a distinct colour difference on the porcelain opposite the ground terminal ( sorry if that's the wrong description ) I removed those plugs some 4 years ago at about 32k for inspection purposes and torqued them back down to 23 nm so it looks like they were indexed correctly.

JettaRed

iTrader: (1)

That's the presumption. As long as you don't have OCD, 23 Nm should be correct.

streborx

Attached is a MB techical paper on the topic of indexing. The improperly indexed NGK plug differed from the properly indexed MB plug by 45 degrees, and destroyed the engine. I guess that settles it. I remain skeptical -- a few details are missing.

JettaRed

iTrader: (1)

That document has been around for a while (since 2014, actually) and is a bit of a red herring. It doesn't matter if the improper plug is NGK or Bosch or Autolite or even Mercedes, by definition it is improper. It is misleading and disingenuous to imply this can only happen with NGK plugs. ANY improper plug will potentially cause engine damage. You gotta wonder if the guy who wrote the document either had stock in Bosch or had just shorted NGK.

Still, the point is that the correct plug must be properly indexed and generally this will be at 23 Nm of torque. That is why when I install my plugs, I put a piece of masking tape on my extension and mark the opening of the plug. That way I can tell how close to the plug is to facing the injector directly.

(By the way, the plugs shown in the document are for the M276 NA engine.)

streborx

IMO, the MB document is so badly written that it does more to discredit indexing than to substantiate it. Comparing an indexed MB plug with a non indexed plug of any other brand nullifies the claim that indexing is imperative. Identical MB plugs should have been used with the ground electrode on one relocated so that both could be identically torqued, resulting in the one plug being improperly indexed. Nothing is said about the engine's operation -- duration of test, RPM, loading. Why 45 degrees? What deviation from "perfect" is tolerable? What difference in combustion is responsible for inducing engine damage? If so important, why aren't the plugs marked by manufacturers to ensure proper indexing? If 23Nm is the magic torque that ensures proper indexing, why mark your socket drive, and what do you do if 23Nm versus the mark disagree? Are there documents that provide a more convincing argument?

JettaRed

iTrader: (1)

All great questions and remarks. It really isn't a convincing document. However, it's something I'm going to pay attention to. The spark is going to occur on the axis of the center electrode. Only if the ground strap was exactly between the spark and the fuel injector might there be some disruption of the ignition. Maybe the whole thing is BS, but I don't want to be the one who finds out it is not. The cost is minimal compared to the risk of damage if it's true.

streborx

I'm not an authority on sparkplugs, but I can appreciate the importance and impact of gapping, temperature, material composition, and tapered electrodes. Indexing for me, however, is a stretch. But I concur that if it takes minimal added effort every 40K miles or so, and a slightly more expensive set of plugs, why not?

JettaRed

iTrader: (1)

I use the NGK 96427 (SILZKFR8D7S) plug, and my last purchase was for $12.65 each from RockAuto. Not ridiculously expensive.

Markrtn7

So if you've purchased a used sl and it's running great, how would you know if the plugs were indexed incorrectly?

rmorin49

You won't know and I am in that situation as well. My plugs are the ones installed when the car was assembled in Germany. When I replace them at 50k miles I will ensure the ones I install are properly indexed. For now I'm counting on the engine builders in Bremen installing the plugs correctly.

streborx

If the plugs were installed at the factory by the experts who insist indexing is imperative, and you've accumulated 50K miles without melting the pistons, then they must be OK.

JettaRed

iTrader: (1)

As we discussed above, I will index my plugs just because it makes sense to do so and the relative cost is insignificant, far outweighing the risk that that document is correct. I also do not think having a plug outside of +/-45° is going to melt any piston. Melted pistons are usually a result of running a turbo car very lean. The key factor is having the correct plug. As the description states on the Rock Auto website, "Specially indexed spark plug. Do Not Use Alternatives; OE-Recommended Gap: 0.028"; OE Spark Plug is High Ignitability. For Optimal Performance, Replace With High Ignitability Spark Plugs." I think that is because our plugs can be ignited 3-5 times during the power stroke to ensure complete ignition of the fuel.

TinManMike

So after following this thread for the last few weeks, I finally installed the plugs in my 2014 SL 550. I've completed service on many of my past vehicles, boats, off road toys, etc. and was a little apprehensive at attempting this job. I didn't want to wreck the engine over not indexing the plugs correctly.
However, I marked my socket and extension with tape like @JettaRed advised and tackled it. I purchased the NGK plugs and the first one indexed correctly right off the bat. Second one was 180 degrees off of where it should be, so removed and re-installed. Same result. Pulled another plug from the box and tried again. Same result. 180 degrees off. I decided I wasn't going to fight this issue in every cylinder. I purchased the MB OEM plugs and gave it another shot. Every plug indexed correctly when torqued to the 23 nm spec. The rear plug on both sides was impossible for me to get my torque wrench on, but based on the other plugs, I know the torque is sufficient and the plug is indexed correctly. Car purrs like a kitten now. I didn't have a miss prior to the install and the car was running well, I just wanted to change them per maintenance requirements. FYI, the car has 50,000 miles now.

If anybody is on the fence on whether to attempt this replacement, it is very doable with some patience. It is tight quarters and if my Fred Flintstone hands can pull it off, I'm sure anybody else could too.

One trick I learned the hard way, lay some towels in between the motor and the heat shields just below the plugs. I dropped a coil pack bolt that landed on the skid plate. Luckily I could see it and retrieved it with a magnetic tool. If I couldn't have seen it, it would have been a bear to get it out.

Kind of disappointed that the NGK plugs didn't index for me as I have used them exclusively for all of my other toys in the past with good luck.

JettaRed

iTrader: (1)

Good job! And it makes sense to not proceed if 25% of the plugs did not index correctly. I'm a bit surprised as well since the NGK plugs indexed for me while the Bosch OE plugs did not. It is safe to go with Genuine MB plugs.

rmorin49

Well, this is a conundrum. NGK vs OEM? Who makes the OEM plugs, NGK or Bosch?

TinManMike

@rmorin49 I'm not sure. I'll check the box when I get home and see what info, if any, is on the box.

JettaRed

iTrader: (1)

Three guesses...



I think both, actually. Mercedes probably changed suppliers at some point.

streborx

Did you make note of the plugs you removed? Were they all properly indexed?
In searching WIS/ASRA for information about spark plug servicing on my G550 (M176.980 engine - direct injected), nothing in the procedure says anything about indexing the plugs. Maybe the procedure assumes the replacement plugs are MB branded, and there's no need to note indexing(?). Is there a separate TSB that addresses indexing?

TinManMike

I did not make note if the plugs I removed were indexed correctly. The plugs I installed were the exact same plugs that were removed. I didn't even think to check the indexing of the removed plugs.

JettaRed

iTrader: (1)

There's this.

rmorin49

Well, at least they aren't made in China. I'll probably order from FCPEuro when my SL reaches 50-60k miles.

Baltistyle

I think the misconception with the Mercedes doc on indexing is that it is to compare oem vs ngk. The example given is not finite. The point of the doc is to make sure whatever plug used is indexed correctly. It is the convey that an oem Mercedes plug installed per the manuals torque specs will be indexed properly. My oem plugs required sanding of the washers to ensure this.

It makes sense that if the strap is in the way of atomization, that there could be a drip from the strap or redirection of the fuel before it hits the spark. When the fuel sprays from the injector it’s circular, covering the 45 degree window of the spar plug on its arc and if the strap is open to the “fuel fog”, the fuel will combust before hitting the strap. It makes complete sense to me but I will say that these stupid design choices should get the engineers fired.

JettaRed

iTrader: (1)

I agree and I think we discussed this on another thread but the document seems biased against NGK. Maybe something was lost in the translation. But ANY plug that isn't specifically for these engines could cause a problem. The point is to get the right plug and install it correctly.

I'm not so sure the design is stupid since it drives customers to the dealer. Maybe it's in the German DNA to build things to work precisely without regard for practicality. I will say, however, that Mercedes seem easier to work on than other European cars.

Baltistyle

yep, we’ve discussed this before. I guess we are just both happy to help others!when we can

ngk is likely just a plug of example. The picture may have in fact come from a real in-dealer repair and that’s the plug that came out. I didn’t take it to mean anything towards ngk, just to make sure I got it right. For me that means length, gapping, and indexed. That can be achieved with any plug that matches.

Im being a bit sarcastic about the design but do think that so many new tech failures could be simplified if not for gov regulations and profit seeking.

I too think these are very easy to work on and diy is always my preference if I can accomplish it. Happy to help others save their money when they can.

streborx

The Wikipedia page on Gasoline Direct Injection (https://en.wikipedia.org/wiki/Gasoline_direct_injection) deserves a read (actually 2 or 3 reads to comprehend it all), and while spark plug indexing is not addressed, the methods of direct injection described therein provide some insight into the benefit/necessity of indexing.

Direct injection can implement either a homogenous charge mode or a stratified charge mode. The homogenous mode injects fuel just prior to the intake stroke. Fuel and air are mixed during intake and compression. Ignition then occurs with a homogenized air/fuel mixture, and the location of the ignition spark within this mix is irrelevant (how would the spark plug know where the fuel/air mix originated?). In contrast, stratified injection occurs just prior to ignition (at the end of compression), and because the air/fuel mix is not homogenized, the location of the spark becomes critical (this seems to be the implication of the 2014 MB "technical" paper). I would agree spark plug indexing in an engine designed for stratified fuel injection charge would be critical and imperative.

Indexing spark plugs on an engine that uses homogenous charge mode doesn't hurt anything, but does it help?