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  #1  
Old 12-23-2009, 08:23 PM
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1980 240D not idling well (I think)

Hello,

I have a 1980 240D that I think is not idling correctly (although maybe it's perfect and I just don't know it). The body has 322,000 miles on it, but the engine is unknown since I swapped it out about 2000 miles ago. I have no information as to the past history or condition of the engine, other than that it runs, and I adjusted the valves soon after the engine went in (so within the last 2000 miles).

The car starts well when cold and when warm, and seems to idle OK when it's cold and I have the idle adjustment turned all the way up. Here are two videos of a cold start, which illustrates the difference with the idle adjustment all the way up and all the way down.

http://www.youtube.com/watch?v=o13CjqmNKv0

http://www.youtube.com/watch?v=n5Gjb8XklLA

When the car is warmed up, however, I hear a knocking or tapping noise that I have not been able to place. It sounds like something is loose and rattling but I haven't found anything like that. It is most noticeable in gear when the RPMs dip down a bit. It can especially be heard at around 0:15 in this video when I put the car in gear, then gets pretty obvious when I turn down the idle adjustment.

http://www.youtube.com/watch?v=NVcZzPERMyU

This to me certainly does not sound like the healthy MB's of the same era that I have heard. Does it sound to anyone here like there are some issues?

Thanks for any advice!

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  #2  
Old 12-23-2009, 11:39 PM
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The first two sound fine. I don't know what rod knock sounds like, and I don't think that depends on temp, so probably not, but something definitely sounds funny in that third one.
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  #3  
Old 12-24-2009, 12:24 AM
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Runs good cold and not hot could be Valve Clearances that are too tight.

Have you listened with a Mechanics Stethoscope around the Engine (or with what I like to use a 2-3 foot piece of wooden Broom handle) to localize where the sound is coming from?

Be sure to listen to the Vacuum Pump and the Timing Chain Tensioner.
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  #4  
Old 12-27-2009, 09:49 PM
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JEBalles: Yeah, I don't think it's a rod knock since it isn't very rhythmic and it doesn't happen all the time, only when the car is warm.

Diesel: I haven't done that yet, but it's a great idea. I'll have to give it a try.

Is it appropriate that it sounds that rough when I lower the idle adjustment knob?

Any other ideas? Thanks!
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Old 12-27-2009, 10:09 PM
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It kinda sounds like a exhaut manifold leak in the video, but its hard to tell.

The motor doesn't sound like it has rod knock.
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1979 280CE 225,200 miles
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1976 240D 190,000 miles
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1976 300D 195,300 miles
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  #6  
Old 12-27-2009, 10:59 PM
Bryan Automotive
 
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When you hear the knocking sound take a 17 mm wrench and loosen the injector line at the injector on each cylinder one at a time. The idle will dip down a bit when you loosen the line. If your knocking noise goes away then note which cylinder it is. Pull the cam shaft cover (valve cover) and check the valve clearances for that cylinder. I would bet it is tight (real tight). If still in spec or close to it then time to pull injectors and pop test.
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Old 12-27-2009, 11:02 PM
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Thanks I'll check that. If it helps though, the noise was present before and after the last valve adjustment I did, with no appreciable change. I also swapped the injectors out with the set from the old engine, with no change to the noise (note I did not check those injectors at all, just swapped the injectors and put in new heat shields)

Thanks!
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Old 12-27-2009, 11:05 PM
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loosening the lines one at a time will not take but just a few minutes to do and see if the noise goes away. If you have time to do it now I will wait for you.
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Old 12-27-2009, 11:11 PM
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Thank you for the offer but I need to run to a family dinner. I will do it later this evening or tomorrow.
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Old 12-27-2009, 11:16 PM
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No problem. You can pm me in the a.m. have a wonderful evening!
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  #11  
Old 12-27-2009, 11:39 PM
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car sounds fine. Something is up in the third video but I wouldn't worry about it. Could be a zillion different things shaking, spinning, etc. Our cars are loud, shake a lot and are old
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Old 12-29-2009, 06:34 PM
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Quote:
Originally Posted by Diesel911 View Post
Runs good cold and not hot could be Valve Clearances that are too tight.
What is the reason for this? The valves on my 300D was not adjusted for over 30+K and was out of spec big time ( too tight ) and it ran good during cold start. Now I adjusted it properly and it runs rough for a little while in the morning or stalls if I do not press the pedal. Is the valve adjustment meant to compensate for the metal expansion? You explanation is appreciated.
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  #13  
Old 12-29-2009, 08:03 PM
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Quote:
Originally Posted by 79Mercy View Post
It kinda sounds like a exhaut manifold leak in the video, but its hard to tell.

The motor doesn't sound like it has rod knock.
X2. Sounds like an exhaust leak.
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  #14  
Old 12-30-2009, 02:54 AM
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Would an exhaust leak only present when the engine is warmed up? I plan on going through some of the diagnostic steps recommended here tomorrow. Any tips on testing for an exhaust leak? I plan on a visual inspection of the manifold, and checking the connection from manifold to exhaust pipe since the engine is a replacement.

Thanks!
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Last edited by WM20650; 12-30-2009 at 03:01 AM.
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  #15  
Old 12-30-2009, 11:30 PM
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Quote:
Originally Posted by WM20650 View Post
Would an exhaust leak only present when the engine is warmed up? I plan on going through some of the diagnostic steps recommended here tomorrow. Any tips on testing for an exhaust leak? I plan on a visual inspection of the manifold, and checking the connection from manifold to exhaust pipe since the engine is a replacement.

Thanks!
Watched the videos again.
Observations:
Your oil pressure is pegged when the engine is cold. It drops a bit when warm. I think it might be the reason for the difference in sound.

Still *sounds* like exhaust from one of your cylinders. The stethoscope idea would be worth trying to pinpoint the source of the sound. Interesting issue.

I'll say this, the engine seems smooth.

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1983 300DTurbo w/sunroof.
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