Odd knocking sound when engine is cold
08-14-2011 | 06:51 PM
#1
Thread Starter
Junior Member
Joined: Mar 2005
Posts: 28
Likes: 0
1992 300SD, knocking sound when engine is cold!
I just bought a 1992 300SD with 161,000 miles.
When started from cold there is a knocking sound that disappears when the engine is revved over 1200~1300 RPM's or after the car idles for 15 or so minutes.
My only experience with MB diesels is with a 2002 ML400 CDI, hence I am not sure what to make of this sound.
I would appreciate it if any of you can tell me what that sound may be.
I have included a link to an audio file in which the sound can be heard.
http://www.youtube.com/watch?v=wxKNp3kVH6w
Thank you all.
When started from cold there is a knocking sound that disappears when the engine is revved over 1200~1300 RPM's or after the car idles for 15 or so minutes.
My only experience with MB diesels is with a 2002 ML400 CDI, hence I am not sure what to make of this sound.
I would appreciate it if any of you can tell me what that sound may be.
I have included a link to an audio file in which the sound can be heard.
http://www.youtube.com/watch?v=wxKNp3kVH6w
Thank you all.
Last edited by kayooh; 08-15-2011 at 07:45 AM.
08-15-2011 | 05:49 AM
#2
MBWorld Fanatic!
Joined: Mar 2008
Posts: 3,714
Likes: 55
From: 1 hours drive north of Sydney Australia
2007 W204 220CDI Classic Sedan
When started from cold there is knocking sound that disappears when the engine is revved over 1200~1300 RPM's or after the car idles for 15 or so minutes.
My only experience with MB diesels is with a 2002 ML400 CDI, hence I am not sure what to make of this sound.
I would appreciate it if any of you can tell me what that sound may be.
I have included a link to an audio file in which the sound can be heard.
http://www.youtube.com/watch?v=wxKNp3kVH6w
Thank you all.
My only experience with MB diesels is with a 2002 ML400 CDI, hence I am not sure what to make of this sound.
I would appreciate it if any of you can tell me what that sound may be.
I have included a link to an audio file in which the sound can be heard.
http://www.youtube.com/watch?v=wxKNp3kVH6w
Thank you all.
I reckon its frequency is about 300 knocks a minute.Say the idle speed is around 600 rpm , I would suggest that it is something to do with the valve gear.
As it disappears after the engine & oil is warm (ie everything throughly lubricated) it could be the valve chain tensioner, valve chain or sprockets are worn. The engine oil pressure is higher at 1200 RPM which may also quieten the valve chain gear.
I would see if I could find a stethescope or use a long screwdriver with the metal going through the handle to the ear to pin point the noise.
It could also be your auxilliary belt or any bearings on the equipment it drives.
Some say that you can remove the aux drive belt & start the engine & run it very briefly to see whether the noise is still there.
Good luck & keep us informed.
Last edited by Carsy; 08-15-2011 at 05:51 AM. Reason: add
08-15-2011 | 09:57 AM
#3
MBWorld Fanatic!
Joined: Jan 2011
Posts: 1,747
Likes: 12
From: Central Pennsylvania
2012 S350 Bluetec 4Matic, Diamond White, P2
Diesels can make an awful racked when cold. The knocking you are hearing is very likely the sound of the combustion in the pre-chamber. It is likely not an actual knock. That it goes away at higher revs and when warm, I do not think it is a reall problem. Hell, its a diesel with 169K miles on it. It is just about broken in!!!
