jlb6504

I bought a 2015 E350 4 Matic just off lease a few months ago and a Command Center Notice indicated my A Service was due. Since it was my initial visit with an MB Stealer, we talked about what was involved with the A service and discussed the future B service. I mentioned I heard how ridiculously expensive the spark plug replacement will be when the time comes. Like a good company man that he was, he explained that’s because the engine is a “direct injection” and the plugs must be orientated a specific way so that the side electrode or ground is positioned to facilitate optimum combustion, otherwise poor or no combustion would occur in that cylinder. Now it’s only common sense that there is no way to tighten a spark plug so you know it seats in a specific, desired orientation. Nevertheless, I was polite and gave him a “if you say so” look. Surely this can’t be true, I mean, even for a Mercedes, right? I intend to change my own plugs when the times comes unless by some unbelievable chance this is correct. How many of you change your own plugs? Any issues I should be aware of? Also, when I asked for a free GPS upgrade he said it would not be complementary. That’s when I pulled out the Mercedes Benz flyer I found on this site so he reluctantly accepted it and updated the GPS. I need to establish a relationship with an MB Stealer until my warranty expires but I don’t think it will be with this one.

Mud

The only way I know to index plugs is by using indexing washers and marking the plugs.
I call BS on a standard engine requiring that. We've owned cars with DI engines and a plug change was a plug change. If this dealer is telling you this you are right to walk away.

ItalianJoe1

They are indexed, that is correct.

If you use the correct plugs (OEM), they will line up within range, with the gap pointing at the injector, when installed and torqued to spec. We've verified this in the shop.

There are bulletins specifying to not use anything that isn't oem for this reason, it will not have the correct thread/ground strap alignment, and won't be indexed, causing misfires when the car is in ultra fine multi-pulse injection mode. I attached one of them that shows the basic concept. Def not false information from your dealer.

As far as "no way to index other than marking plugs and washers", you haven't seen how new things are made. CNC production means every thread is cut the same way, in the same position, every time. So it's entirely possible that every cylinder head and every spark plug, when put together, will be well within the cone of acceptable 'index' with no special tricks, so long as they are designed to be.

JALLEN4

Anybody that tries to use the juvenile term "stealer" should be told ridiculous things!

Mud

Joe thanks, I stand corrected. I guess by default since I always use oem plugs any indexing was correct. Good info.

Max_1

Joe,

Thank you for the very detailed input.
That is what can be called "over engineering". We are getting onto the field where even little things has to be done just right to make it run properly.
That was a surprise to me that "regular" street motor is that sensitive. I have seen it before on race engines where every 0.01 Hp matters, but it was not a reliability concern.

Max

injunwiz

Wow, I learned something new. I knew about indexing plugs, but didn't know MB plugs were setup this way!

Jan AMG

ItalianJoe1

Yeah I've done it for performance, they do it for economy.

When the DI motor is in multi-pulse mode, meaning it fires the injector 2 or more times per cylinder firing, it does this to create a tiny puff of fuel in the very middle of the cylinder, as opposed to a homogeneous mixture. What this does, is create a cushion of air without fuel in it around the center of the combustion chamber, which acts as an insulator during the combustion. The flame has to start in the very middle, as there is very little fuel to burn and if the plug is facing away, it may not ignite at all. When all goes to plan, you get an effective power stroke like a smaller displacement engine, with very little heat loss to the metal of the engine due to the insulation of the air at the edges of the chamber which doesn't contain fuel so it doesn't burn. Removal of this quench action doesn't waste fuel and keeps emissions down, which is key. Honda has been doing this for years in max efficiency engines with standard injection even, using small lobes on the VTEC cam low side and other management tricks to get a shrouded combustion for efficiency. Benz is taking advantage of the high-powered Piezo fuel injectors and their fast switching time to do the same thing while still having solid performance.

cetialpha5

Good info on the indexing. I remember this was around back in the 80's. Did it with a turbo engine, you had to mark the plugs and then figure out which washers to use to get them to line up correctly.

Anyway, the other bit with direct injection is that it's now a 60 degree engine instead of the 90 in the previous version. That means the intake manifold has to come out in order to change the plugs which is basically why it's so expensive. You didn't have to do that in the previous engine and plug replacement was pretty simple and cheap.

Mud

Makes me wonder if mfg indexing may be more prevalent than we know if using oem replacement plugs. I changed factory Bosch for new Bosch awhile back in my plain old 2011 V6. Didn't think about the indexing but perhaps this may have been an added benefit of using mfg recommended replacement?