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  #1  
Old 05-13-2019, 11:05 AM
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OM616 timing chain noise?

Have a pesky tapping/nailing sound coming from my 1981 240D. I know there are tons of threads already in existence about this, but after looking through them I am still stumped, curious to see if anybody has some wisdom to share.

Here's what has been done so far:

1) Valves adjusted
2) Oil changed
3) Vacuum pump checked. Was replaced by PO and ball bearing look almost brand new. Ran engine briefly without VP installed and noise persisted. Almost certain it is innocent
4) Alternator, water pump, power steering all checked for bearing play and anything else that could make a cyclic sound
5) Injector hard lines loosened one at a time looking for a change in the sound, no change heard
6) Inspected the rocker arms and valve springs for excessive play and/or broken spring
7) Run the engine with the valve cover off to look for obvious problems with the valve components. Did not notice anything, and oil flow was good. The only thing I noticed was white smoke coming from the timing belt compartment. Not sure if this is normal or not.
8) Measured the frequency of the tapping, found it to be ~6 taps per second, which seems to be valve speed rather than crank speed.


After doings the above and still not knowing the nature of the sound, I splurged and bought a mechanics stethoscope at HF.

Using the stethoscope I narrowed the noise down to the valve train or timing chain. Can't pinpoint a specific cylinder, the sound seems to be fairly uniform all across the valvetrain. Noise is not coming from VP or IP. Sound does not change when the engine warms up or when it is cold, and is proportional to RPM.

All signs seem to point to the noise coming from the valves, I just can't figure out why. If a valve had dropped or a spring had broken, it would be pretty obvious by inspection or when adjusting the valves right? Is there something else I should be looking at?

Another idea is the chain guide or tensioner or some other part of the timing chain is worn. Would that cause the 6 taps per second frequency though?

Maybe an intake manifold leak? Or multiple nailing injectors, which is why the injector line test didn't work?

Here's the link to a YouTube video of the noise, maybe that will help.
https://www.youtube.com/watch?v=ssPsRr6qgZA



Any ideas?
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  #2  
Old 05-13-2019, 12:10 PM
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Posts: 677
Check the big balancer bolt on the crankshaft and the small allen head bolts on the crankshaft belt pulleys.

Good luck!!!
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  #3  
Old 05-13-2019, 08:41 PM
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Posts: 46
Possible worn cam shaft/rocker.
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  #4  
Old 05-23-2019, 01:53 PM
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Thank you both for your replies!

I checked the 6 allen head bolts and the big crankshaft bolt, all are tight and the crank and pulley seem to be rotating smoothly.


Here's an update to the source of the tapping:

After some more digging I found that the upper chain guide and the idler gear of the timing chain both have a lot of lateral play in them. These are 85 and 98a in the picture, respectively. Running the engine with the valve cover off revealed that the idler gear visibly moves back and forth, in the direction to and from the radiator.

My question is, would the play in this idler gear possibly the culprit for the tapping sound? If so, what is the procedure to changing only one of the timing chain gears? Haynes has this, but only for a complete engine teardown.

I probably need to check the chain stretch as well.

Thanks!
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Old 05-23-2019, 01:56 PM
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The image I attached may not have gone through, hopefully it made its way to this message at least.
Attached Thumbnails
OM616 timing chain noise?-om617-timing-chain-diagram.jpg  
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  #6  
Old 05-23-2019, 11:37 PM
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Yes, check the chain for stretch, I've seen several break on 240's and it snaps the cam supports at a minimum. If you change the chain use the crimp link style, replace the rails 85, 89 and the chain tensioner.

Also there is a sector shaft type gear that drives the oil pump under the large nut/plug between the front of the injection pump and the block. It is where you would expect an ignition distributor to be if this was a gasoline engine. The bushing in that gear wears out, it is worth checking.

About the idler movement fore and aft I think that is normal. If you change the idler gear it would be a good idea to lock the injection pump in place/time but this requires a special tool.

Remove bolt 94 and see if it is chewed up on the end, if it is the chain is probably loose.

Keep us posted and good luck!!!
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