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:
http://www.peachparts.com/shopforum/tech-help/350588-w210-rough-idle-missfire-bank-2-a-2.html
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...