miguelcarlos

Hello, Update on the engine knock at startup-
My c300 with 18,000 miles is in the shop and dealer just called and they have taken the engine out and have started to disassemble it to put new bearings, oil pump, gaskets, tensioners, chain etc. I have read on forums where this sound is deemed normal by some dealers but shop foreman at this dealer has been excellent to deal with. He also heard the knock at startup and said that that is not normal at all. He said they hope to finish it up by the end of next week.
Thanks

Glyn M Ruck

Pleased you are getting this sorted. It's not normal.

C300Kid

hmm....a ton of us have that issue..it must be "somewhat" normal.

RLE

The previous M112 3.2 3-valve V-6 made precisely the same noises on startup. I listened to it for seven years. And here we are again with the M272's knocking.

grane

Best of luck and welcome.

Glyn M Ruck

My M112 makes zero knocking noises. Some have a slight tendency to piston slap until hot.

miguelcarlos let us know what they find.

The sources of knocking/ticking/slapping in these engines are:

Bearings - usually small end - can be fixed
Piston slap - very short skirts
Chains & tensioners prior to full oil pressure - can be fixed
Cam adjusters - can be fixed my deburring or replacement
Purge valve - can be fixed

RLE

My M112's startup knock, exactly the same as my M272's, was NOT piston slap.

RLE

Since you are the first one here to get this repaired under warranty, I would be interested to know how this was approved since it is a huge repair.

Was the area MBUSA Rep involved? Is there now a TSB covering this and if so what is it's number?

Glyn M Ruck

Normal = all vehicles do it.

If some do not it is not normal. These engines are not known for startup knock apart from possibly after an oil change until full pressure is achieved with hydrodynamic lubication & full chain tension.

One problem is that one man's knock is anothers tick. Slight knock in a 112 or 272 might not effect longevity but it is not normal & can be repaired.