LandonC300

Hi all, I recently bought a 2008 C300 Sport with 86K miles. It has the 'knock knock knock' issue at hot startup (after sitting for 1+ hours) that goes away after a few seconds. I changed the oil and filter (Mobil 0W40 oil, Mann fleece filter) to see if that would change the noise but it didn't; sounded the same during first startup after oil change which confirms it is oil pressure related (reading other threads, I suspect crank bearing tolerances as cause). I changed the oil through the drain plug; the drain plug had a thick orange grease pen mark on it that flaked off as I took the drain plug off. While draining the used oil from my collection pan into oil jugs to recycle it, I noticed silver colored debris in the funnel. I'm not sure how concerned to be; my thoughts are that the grease pen mark on the drain plug indicates that the oil has been changed through the dipstick hole up until now, and perhaps the silver debris is normal accumulation that was flushed out of the oil pan when I changed the oil through the drain plug. Any other thoughts or reactions about this?

I might note that the car didn't exhibit the 'knock knock' phenomenon when I test drove it (bought from private seller); I started it cold, no noise. I drove it about 5-10 minutes, pulled into a 7-11 and turned it off and went inside for a few minutes. Came out and restarted it, no noise. Sigh...

I'm also curious if anyone that has the 'knock knock' at hot startup has had an engine failure. I realize this has been asked in the past, but I didn't see it recently and since miles accumulate, I thought it worth asking again...

Thanks for any thoughts or comments about this!

Glyn M Ruck

See response to PM & here.

https://mbworld.org/forums/c-class-w...-pressure.html

Moto_Guzzi

My car do the knock sound. I fill it with Mercdes 229.5 ....5W-40 oil from what I could determine. Originally the dealer had Mobil 1 in it from what they told me. Later that was stopped, add same oil I add now buying from them. I started to heard that from about 15 000Km, complained but no faults found.
This year I cannot remember hearing it so it went away (temporarely) mostly winter time now. I also heard it when it was cold(Some startups).
Maybe if the summer returns I hear it again sometimes hot or cold, so far nothing broke yet at 145 000Km's, I do change the oil at 5 000Km intervals.

This makes me think about oil claimed properties like thickness etc and what you actually get in the can versus the pressure at certain temps or waiting cooled down periods-? Maybe the oil do not have the viscocity etc as claimed at various temps, leving you with oil thats just to thin.