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  #1  
Old 05-29-2008, 07:28 AM
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W124 250D knocking when cold

My car is a MB 250D ,year model 1989. abt 500 000km
The problem is nailing when cold.After about 2 minutes running its quiet and the knocking sound does not occur while engine is hot.

Even when cold (and knocking) it does not smoke but runs quite smoothly.It does not consume water or oil.

I have isolated the problem to cyl 2 by opening the injector nuts for each cylinder.Only when opening nr 2 the knocking disappears.
Changing and swapping injectors did not affect but knocking stays in cyl 2.
Compression pressures were even (within 1-2 bars) when warm.Cold pressures still to be measured.

Could this be a mechanical problem?I dont think its a rod bearing problem since there is no knocking when hot .Could the piston pin cause knocking only when cold? I would not expect a worn piston skirt to cause a distinct nailing like this.

Prechamber? I checked that it is not loose.From injector side it is not clogged but on the cylinder side it could have deposits.

Injection pump? Have not checked this except timing which was OK.Could faulty delivery valve cause knocking.No external leaks.



Suggestions how to proceed?
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  #2  
Old 05-29-2008, 08:16 AM
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Since you swapped injectors & there isn't any difference, then moving the pre-chamber would be the next logical step.

I assume that the problem isn't lifter noise related.

Also is that glow plug warming that cylinder? Have you tried a new glow plug on that cylinder?
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  #3  
Old 05-29-2008, 10:19 AM
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Quote:
Originally Posted by M.B.DOC View Post
Since you swapped injectors & there isn't any difference, then moving the pre-chamber would be the next logical step.

I assume that the problem isn't lifter noise related.

Also is that glow plug warming that cylinder? Have you tried a new glow plug on that cylinder?

hello
I had the lifters removed earlier and tried to compress them but they all seemed very hard to compress.
Today I checked the resistance in the glowplugs.The readings were 1,3-1,4 ohms with my cheap meter.
Concerning the prechamber I already tried to remove it from cyl 2 but I could not get that thread ring to move although I tried really hard.Maybe I could try again with warmer engine but Im afraid of spoiling those threads in the cyl head.
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Old 06-01-2008, 01:29 PM
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Today I pulled out the prechamber from cyl 2. It looks Ok,not much carbon and all holes were open.
Also the glow plug looks OK and I could see that it really glows when I looked into the prechamber when the nozzle was removed.
I also checked the cold compression pressures in cyl 1 and 2.Both abt 25bar.

The knocking has been less distinctive and lasted for shorter period during last days.Futhermore it can be heard only at abt 1000-1300revs while it was earlier knocking on all revs from idling to abt 2000revs.Maybe because we have had a little warmer weather (abt +20C) here or because I have been using another fuel brand or...?
Still the knocking stops when I loosen the cyl 2 inj pipe.

Next I might try to open the delivery valve on the injection pump or try another pump although I have no good theory how the pump could cause this.
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  #5  
Old 06-01-2008, 03:52 PM
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I wouldnt worry about it unless its an insane ammount of nailing.
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