Hi
I purchased a 2002 CLK 320 recently. It has a knocking sound that starts once the engine is warmed up, and at low RPM, with a slight load (like being stopped with the transmission in D or R). You can hear it even in Neutral or Park, it's just not as loud. Sometimes it's louder. I've replaced the oil with Mobil 1 high mileage 20/50 synthetic, with no help. A local shop looked at it and said it was piston slap, but said it should be OK. I'm surprised because I thought piston slap occurs on cold engines, and goes away once warmed up, not vice versa. The sound is definitely coming from the engine block, not the valves, and it's not pre-ignition. It's a clear metallic clanking sound. Any thoughts on this?
Thanks, Laszlo

Check this thread out. It could be the motor mounts.

Piston Slap?

I agree, piston slap should/must be worse when the engine is cold.

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Prost!

If it was the motor mount, why would the knocking match the rpm exactly? Also, what difference would temperature make? It should then knock when it's cold as well. In that thread I also wondered what the motor mount affects: is it the mount that knocks, or as the result of worn mounts, does something knock inside the engine?

Think about it...If "Something" inside the Engine were caused to be Knocking by
an external force ,such as Motor Mounts...You're in much deeper Kimchi than
"Piston Slap" land.

The other side of that same coin is...You would not believe how many different
distinct noises Motor Mounts are capable of producing in one chassis.

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'84 300SD sold
124.128

On the motor mounts, I'm just passing on what the thread said.

On the temperature issue, piston slap is always worse when the engine is cold because the piston/wall clearance is greater. It is possible that in cars with aluminum engine blocks (M112's) the wall/cylinder clearance doesn't decrease with temperature, since the aluminum piston will have a similar thermal coefficient of expansion as the block, so we'll need to retract that statement in this case. In iron blocks, that was always the litmus test on piston slap.

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Prost!

M112 engine knock when hot-------- Last January we bought a 01 CLK with 70k miles for my wife, it's a beauty like new. We didn't drive it much because of bad weather & we have a 4x4 Jeep. Engine sounded ok during the cold months. Come June when the temp was around 80+degrees she tells me when the engine is hot after a 1/2 hr soak there is a noise when started & under slight load. Sure enough,exactly as she described. Drained out the M1 0-40 which only had 2k miles, checked the fleece filter for any metal particles---none . Installed M1 15-50 ,noise was reduced somewhat but still there if you are listening for it. .........................Made a BIG discovery------ This engine DOES NOT have an OIL PRESSURE warning system ,checked the owners manual, all kinds of other warnings but no oil pressure warning!-------Called Mercedes Benz HQ in NJ & they admitted there is no oil pressure warning system but said "it has an oil level system ". My reply was "the oil level can be full but if the oil pump fails or the pump drive chain breaks at any one of the links it's all over for the engine"! They said they had no reports of failures. Not a good enough answer for me. Checked below the filter,there is a test port plug that can be removed & a test guage can be installed.----------------------------- My opinion is that my noise & yours is main bearing "low thud knock " sounds like lightly knocking your knuckle on a wooden table top. I also suspect that the oil filter housing is a bypass type & not a full flow & is draining back to the pan causing a delay to instant oil delivery to the main & rod bearings ,possibly this may be reason they deleted the oil pressure warning system ??? Owners would be complaining "my oil pressure warning stays on for a few seconds after starting " Easy fix,------ Delete the system ! I don't know how they go away with it. ---------- As you said, if I listen in a quiet place with every thing off ,the windows closed, I can hear that "low thud knock under light load ". Try this,----- I can also duplicate the noise stopped with a "hot engine " ----- selector in gear, left foot on the brake, right foot on accelerator & slowly taking it up to 1.5k RPM & hold it there. I suspect it is main bearing clearance out of spec combined with a thin wall aluminum block that is telescoping the sound into cabin . One of these days I'm going to drop the small oil pan & look for "metal particles " that will tell the story . Sorry for the long winded response ------ Ray

15k RPM? must be a typo :-)

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1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE

I'm thinking "1.5k RPM." would be more like it. An M112 turning at 15k RPM brings one thought to my mind: DUCK!

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"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles

Traditionally Main bearing knocks come under heavy load, and wrist pin tapping comes on light load.

I cannot remember the rule for rod bearing knock.

That said a deep thudding does sound like main bearings.

Yikes!

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I agree with Ray. My '01 E320 wagon has had the same noise since I acquired it with 58K miles. No noise when cold but when warm it has the dull thud of a loose main bearing. Also, after the car is thoroughly warmed up when I restart the car (all noise makers turned off) after 30 minutes or so of having been shut off I can hear and feel the knock for a second or two until the oil pressure has built up. I had the oil analyzed at one point (around 80K miles) and no abnormalities were discovered. At 106K miles the noise has not worsened. My guess is one main bearing journal was set up at the factory slightly loose causing the noise. Based on my experience with this car, I don't believe it will cause a problem with engine longevity.

Mark