My '79 240D has only 117k miles on it. It has a rough idle and one cylinder making a noise. From inside the car it is a ticking. From outside it is more like a bad knocking. It hasw been worst at idle and under hard acceleration. The last time I had the exact same symptoms, I put in a set of Bosch reman injectors from MercedesShop, and the car ran/sounded great. That was maybe 30k miles ago.
This time, I bought one injector from MercShop and tried it in each of the cylinders. The noise & roughness would not go away. I adjusted the valves - no change.
I took it to a reputable independent M-B shop. They diagnosed a bad exhaust manifold gasket and misadjusted idle. When I got the car back, the rough idle was gone and I thought the noise was also. That's when I realized that the noise is still there; it's just quieter now and only noticeable at idle.
I checked the idle speed (easy by ear & wristwatch with the knocking) and it was about 1100 rpm (It's supposed to be 750). I took it back and they set the idle to 750. Now the idle is as rough as it was before and the noise is more noticeable. I asked the shop what they would do next and they seemed clueless - they said "It's not really bad; it IS a Diesel". It sounds really bad to me and I'm used to Diesels. I think their reputation is from working on Porsches and other gas cars.
Should I have compression checked? Could it be a bad prechamber? Any other ideas? I hate to think I have a problem with engine internals.
Thanks in advance for any advice you can provide.
Ted

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Now you're suckin' Diesel!

'88 300SEL
'92 300D 2.5 Turbo
'96 Lincoln Town Car. The Jazzmobile.