Odd Piggy

Our 2015 GLK350 has now reached the point where the maintenance program recommends changing the spark plugs (46,500 miles). This SUV has been flawless so far. It still idles so smoothly that it cannot be determined whether or not the ECO system has shut the engine down without looking at the tach.

The engine is M276.957

My first question is this:
All my other MBs recommended spark plug changes at 100,000 miles. Why so short an interval on this engine.

My second question is:
Is indexing the spark plugs in this engine really necessary?

The spark plugs available from aftermarket suppliers are labeled "these spark plugs require indexing." The MB spark plugs by the same manufacturer are not labeled this way. Is there a difference in the way that the MB plugs are made so that indexing is not needed? Or does MB just assume that you know indexing is needed?

I am familiar with plug indexing on high performance racing engines where it is done before the heads are installed. Surely there is some way of achieving this without removing the cylinder heads.

If anyone out there has experience with spark plug changes on this particular engine, i would like to hear your comments.

ItalianJoe1

The MB plugs are "indexed" from factory. We install them to the specified torque and that puts them in the right location. Under or over tightening can move them sufficiently to cause misfires and idle quality problems.

The interval should be 60k, as with most other MB engines. Have you been doing your services early? It would be no problem to wait longer.

Odd Piggy

ItalianJoe, thank. Based on your reply I think that I can assume that the spark plugs obtained from an MB parts counter will be manufactured so that they are properly indexed when installed and torqued to the proper spec.

I rechecked my Maintenance Manual for the GLK and it clearly states change spark plugs every 46,500 miles or 4 years. My 2013 C250 does follow the 60K mile recommendation.

Since we put about 25,000 miles/year on the GLK, the service intervals have been every 10k miles as recommended. I may wait until the 50k A service to change the plugs, but do not want to wait until the 60k B service.

mjhawkins2346

I changed my plugs around 90K and the engine is less smooth now. Go figure. I probably should have held off until I needed to.

Touringman

Italian Joe...Correct me if I am over thinking this. Your statement above would seem to indicate that the "end" of the first grove of the thread cut into the head, would be in exactly the same place on all 6 holes on the head. AND furthermore, the "end" of the first ring of thread on the plug would be in exactly the same spot in relation to the ground electrode of every spark plug. That way, once the sealing ring is crushed to the proper torque spec, all the plugs ground electrodes will be in the same relative position in relation to the intake valve in the head.... If that is true, THAT is some DAMN impressive engineering!
Lastly, any idea if the 275.XXX V12 is done the same way?
Thanks in advance!

ItalianJoe1

It is that accurate. CNC machining allows for very precise thread cutting, and yes every head thread and every spark plug thread starts in the same spot relative to the electrode. I don't believe the 275 is like that, but I haven't ever looked. This process started with the 276/278 engines to my knowledge.