I got a 2001 W210 E240 automatic about 2 weeks ago. There was a tapping sound coming from the engine so I sent it in to have the valvelifters replaced. The mechanic replaced all 12 valvelifters and informed me that the engine is still making the tapping sound. He reckons this is due to the engine running at very high temperatures causing the top of the pistons to shrink. Now I have to decide between replacing all 6 pistons or buying a secondhand engine.

I know the alluminium tops of the pistons can be deformed due to very high temperatures. But how does this cause the tapping sound that sounds exactly like valvelifters...

Is this guy just trying to squeeze money out of me?

heat causes metals to expand. piston slap is caused by the skirts of the pistons, and decreases or disappears as the engine warms up. i suggest another diagnosis.

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I think he probably all ready has !! The mech should have been able to tell the piston noise from the lifters.
When the motor got too hot it has probably damaged the pistons.
When the pistons get too hot they expand too much & then deform as the bore does not expand as much. When the pistons cool back, they are out of shape & so piston noise. Does it breath a lot or use much oil? if not Just drive it!! fix the overheating problem first.

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Probably the reason the car was on the market to begine with. If the oil changes were not done on schedule it might be worn out. If synthetic oil was used you can go by the FSS (oil change reminder system, which has a base setting of 10,000 miles and can go up or down from there), but if non-synthetic was used you want to do it sooner than that.
Problem now is probably keyholing of the cylinders.
Gilly

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He reckons??? Did the engine overheat or not? Was it tapping when you bought it? Has the oil pressure been checked? Does it run OK?

It's true that engine parts expand and contract with changes in temperature but it takes prolonged extreme high temp. to damage the pistons. I've seen lots of these engines survive a typical overheat scenario with no issues whatsoever. When the condition is ignored the damage is obvious in the form of visible damage to pistons and cylinder walls.

I recommend that you have it properly diagnosed, which would include a borescope inspection of the cylinder walls before you throw away good money on a hunch. Make sure the mechanic has M112 experience. That will save you a lot of grief.

Thanks for the good replies and advice.
Aside from the tapping the car runs perfectly. It had too much oil when i got it, but its been drained to the correct level. I noticed the bottom brackets on the radiator fan were missing and had them replaced. The engine gets to 80` c and stays there, so overheating isnt a problem. The previous driver was just a bad one. The tapping sound is exactly the same as the sound my 124 made before i had the valve lifters replaced. One of my friends also drove an engine to the point where it seized, but the only thing he managed to salvage turned out to be the pistons. I have heard of some big diesels with cast iron pistons that deform slightly due to overheating. Deform, not shrink.
The explanation i was offered from the mechanic included the fact that the tapping gets louder at higher temperatures due to the piston shrinking. I dont think that a piston shrinking a few micrometer travelling at that speed would cause a tapping that loud and slow.
Theres no white smoke coming from the exhaust, so no oil is being combusted.

Maybe he just has a spare engine lying around that he keeps tripping over and saw this as a chance to get rid of it and start the year with some money.

could also be exhaust leak causing the noise, or spark jumping from cable to ground. piston slap does not sound like valve lifter ticking/tapping.
also, did he measure your oil pressure?

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I used to have a tapping sound on the M103, it sounded like it was coming from the valve train, it was mild loud at start-up, but would disappear when the engine was warm, but reappear when hot. It drove me crazy, turned out to be a cracked exhaust manifold gasket on cylinder #3. (from radiator to firewall, inline 6)

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(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)

I have decided to revive this thread as I still drive the car and it performs beautifully but the engine tapping noise is still present after all these years. I have come accross at least 3 other M112 engines over the past few years that had an identical sound with the owners having similar stories offered by mechanics. Even on the forums there are a few identical cases with no concluded cause.

A quick summary:
There is a loud metallic tap present whether the engine is hot or cold.
I have done compression tests indicating the rings are still excellent, eliminating piston slap:

W210 Rough idle and missfire in bank 2

I have also bought a USB borescope and the cylinder walls have no scoring, they actually look amazing for an engine with this mileage(Roughly 210 000 km)
Plugs replaced and plug-coil wires replaced recently.
Injectors cleaned as described in above link.
Serviced 2 months ago-very few little metal particles in the oil and filter, basically fine powder.

Recently:

I did a sound analysis on the engine which gave me some interesting information about the cause of the noise.


1. I recorded the sound with the hope of being able to gather information from the recording:
https://www.dropbox.com/s/apncz42who3i4im/kar%20tap2.wav?dl=0


2. I imported the audio file into Audacity(graphical editor for audio files) and spikes were clearly visible where the engine was tapping. I also slowed playback to 19% to verify that the sound I heard matched the pulses on the graph. 19% because the program adjusts playback speed with a slider and this is as close as I could get to 20%.

The complete 45s sample window:
https://www.dropbox.com/s/mtfdaaszij9kadp/Complete%20Window.jpg?dl=0

A 10s Window expanded:
https://www.dropbox.com/s/cf5z9wapy7yz466/10s%20Window.jpg?dl=0

A video of the 19% playback, background clattering is the injectors:
https://www.dropbox.com/s/ih20ygq4j8k7dm7/19%25%20speed.mp4?dl=0


3. I then zoomed in to determine at which times the tapping was taking place. I used the peaks as reference as it is easier to determine the time from them and the time from spike origin to peak should be roughly the same for all spikes. I used 10 spikes to obtain an average and work out the frequency of the tapping and compared it to the engine RPM. I did the test at idle and the RPM was recorded as about 780 RPM using OBD, it fluctuated ± 30 RPM. 780 RPM gives 13 revolutions per second and the frequency of the tapping was calculated as 6.3 per second which is suspiciously close to half the crank RPM - 6.3 Rev/s X 2 X 60s/min = 756 RPM.
I tabulated all these values, it makes more sense that way:
https://www.dropbox.com/s/cl3v1cm9a00v6vc/Klank%20Data.pdf?dl=0

4. I have concluded that the tapping takes place every second revolution of the crank so once per 4-stroke cycle so there are only a few possible culprits.

4-1. One of the valves could be acting up, but I can't see a valve making a sharp noise this loud and they all still look good.

4-2. A con-rod bearing has gone and there is a gap between the crank and con-rod. During compression the top part of the crank will be in contact with the con-rod and detonation occurs pushing the piston/rod downward, top part of crank is still in contact with the con-rod. Exhaust stroke takes place - crank pushes con-rod/piston upward, top part of crank is still in contact with the con-rod. Intake stroke begins and the crank pulls down on the con-rod/piston causing the contact interface between the crank and con-rod to move from the top to the bottom. Because there is a gap due to a crappy bearing the tap is heard. At the transition from Intake to compression stroke this contact surface moves to the top again. I expected to see a second spike 40ms (roughly 158ms / 4) after the first due to the second contact surface transition but can't. This might be due to the sound intensity being very high the first time as the piston is still traveling downward with the combustion force behind it and collides with the crank that is now trying to push the piston upward but during the start of the compression stroke the force pushing the piston down is much less.

I will test the bearing by removing the plugs and turning the crank until I see the piston starting to move down from the top and then pushing it down by hand to try and feel if it moves or if I can hear a sound.
If this doesn't work I will have to get my hands on (or build) a high speed DAC that can record the sound and TDC sensor values.

Any comments or advice?
What else is there that could cause a loud tapping noise every second revolution?

Update: I have noticed a rumbling sound at certain RPM's lately that sounds like a bearing going somewhere...

Thanks for the update and interesting way to analyze noises.

Has the noise gotten louder since 2011? If not it is doubtful it is a rod bearing as it would have failed by now.

Doing the push test is what I do, have the piston down the bore about an inch as 30* crank angle seems to be where most crankshaft wear occurs. ( the crank is the other half of the bearing system )

These cars _might_ have a balance shaft, the problem might be there. I also remember someone posting that cam shaft magnets might making noise.

Electronics for the win!!
I must say it was very effective and on an engine that did not have this problem you would be able to hear the injectors very clearly that will help a lot with faultfinding. I noticed on my injector cleaning episode that my blocked injector sounded different to the others when it was blocked and identical after it was cleaned.

The noise has stayed pretty much the same since I got it. I just checked and have done 45 000 km since I got it and it has always had this noise.

I didnt consider the balance shaft, maybe I should go through the engine in EPC and check what else could be the issue. It does seem that the noise is quite high up in the engine close to cylinder 5.

Dont know anything about camshaft magnets will look into it.

I did however replace all the speakers in the car recently and that has helped a lot with the engine noise...

Thanks for the advice

Good detective work. The 1/2 engine speed tells me it's in the valve train.
Try isolating the source more definitively using a mechanic's stethoscope. You could also start with just a length of rubber heater hose.

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Edmund:

Interesting analysis of load direction on the con-rod bearing vs. half-speed noise (4-2). Consider also that the same load analysis applies to the top end of the con-rod. Engines will quite often run their entire service life with a slightly loose fit of the wrist pin, and clack the whole time.

I have used a rubber hose and it still pointed to the top of bank 2 but it is quite obvious from standing next to the car that the source is probably in this area. I will look at the top of the rod too if Im sure the noise is coming from around there. It's not something that is slightly loose, it sounds like the piston is hitting the valve really hard. But compression is excellent and there are no marks on any of the pistons, only slight carbon build-up.

Im also thinking about the valve train but can't imagine anything on the valve train would make such a loud, sharp noise. This is why I want to use the TDC as a reference signal to know the positions of all pistons and valves when the sound is recorded. The valve train also looks like it was built yesterday by angels.

I dont want to strip the engine to look for the source as I might find something, repair it, re-assemble and end up with an engine that runs 1% better but still has the noise. I would rather like to isolate it to a specific component or function and look there. Luckily modern of the shelf technology allows such tests to be done...I have also designed a bluetooth module that works with these older cars allowing music, navigation, phone calls to be handled through the Audio30 system with additional hardware in sight. Will do a seperate write-up when it has been installed. The bluetooth device will be visible as : Edmund's Mercedes.....

I have read a number of posts where people have replaced lifters and rocker arms only to still have the M112 tick/knock noise. Given that it is not a real problem (you have lived with it for over 4 years) I agree with not doing anything until you are confident in its source.
The stethoscope should help confirm if it is coming from farther down in the block, or higher up in the cam area. Any piston-valve contact would damage something in short order, so I do not believe it is that.

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