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. A guy told me "it might be what we call "nailing", based on my description of the sound. It has 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? Should I change out all four injectors after only 30k miles? 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.

Valve adjustment?

Does it knock and idle rough all the time or just when it's cold? It could be any number of things, but I'd suspect the injectors first. I have an injector tester at home and it's one of the best investments I ever made. Nowadays, rebuilt injectors are likely to be crap and installing them without testing is asking for trouble. First though, you could try Diesel Purge because that's cheaper and easier. Sometimes, the air filter housing is loose and it makes a knocking sound so you want to eliminate that variable as well.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Run LubroMoly diesel purge per the directions, don't put it in the tank. Then get some Redline diesel additive and see if that helps. Just changing fuel may make a huge difference also. Higher cetane ratings will help reduce nailing. An "italian tune up" might also be in order. Good luck!

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99 E300 133K
98 ML320 99K
87 Saab 900 Turbo Conv 123K

Thanks, guys.
Checking valve adjustment was one of the first things I did.
I've run Diesel Purge through it.
Would the 200 miles I drove the other day, pedal seriously to the metal non-stop, with A/C full blast and temp. gauge at 215F, qualify as an Italian tune-up?
The noise and rough idle are worse now. It's like it's hitting on only 3 cylinders at idle. I will admit that I don't hear or feel anything at speed now (except for the roaring of the wheezle).
Maybe the Bosch reman injector I got is bad. I was told that they install new tips and that's the key.
Can you buy NEW injectors? How much will a 4-pack cost me? Any other suggestions?
Thanks again.
Ted

__________________
Now you're suckin' Diesel!

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