My '84 300SD has been developing some noises for the last month or two that have me a little bit uneasy. I have some injector nailing going on, which is loud when cold and almost disappears at operating temperature. But that's not what has me a little worried. There's an irregular knock that's in the soup of noises, and I am pretty sure it isn't part of the usual diesel clutter.

A week ago, I changed oil and filter. Today, I replaced the primary fuel filter with a Hengst filter and pre-filled it with diesel purge. That seemed to quiet down the injector nailing some, but that irregular knock is still there and sounds like a loose bearing knocking in its seat or something...

I bought a cheapo mechanics stethoscope, which is kinda/sorta like what doctors use except with a long metal rod at its end. I poked around the engine and listened. It's a pretty neat device for sure.

I put it up to my injectors and tops of prechambers, and all produced the rhythmic ticking, some being quieter and some having more clattering soundscapes. I wasn't really expecting to isolate a nailing injector or a faulty pre-chamber this way, but now I am curious about how the healthy injectors/prechambers are supposed to sound through a stethoscope. Anyone know?

Anyway, back to what I termed the "loose bearing knock." My main fears are the timing chain and/or the vacuum pump failing. Where would be the best spot to listen for abnormal timing chain noises? I put the stethoscope up to the front end of the valve cover and did not hear any unusual noises. I put it up to the power steering pump - sounds smooth. I put it up to the vacuum pump - well, there was a little knocking I could hear, but since it bolts right up to the front of the engine, it made me wonder that maybe these noises just carried over from the engine. I put the stethoscope up to the water pump and heard somewhat louder knocking from it, but again - not sure if those sounds migrate from the engine block, or if my water pump is going bad.

Can anyone help me interpret what I'm hearing via the stethoscope and maybe someone knows how things are *supposed* to sound through the stethoscope... Should I also listen to the bottom end of the engine and see if the knocking could be coming from down there? What are the "better" spots on the engine block where I could put my stethoscope for listening to various parts of the engine?

P.S. When I was listening to the engine, it was right after a cold start - first start of the day. I did not have the chance to listen at operating temperature because it got dark by the time I got back home. Since the diesel clatter seems to subside some at operating temperature, I wonder if I'd have a better chance locating that "knock" then?

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84 300SD - bought in April '06 with 237,000 mi

How these engines develop noises that dissapear is almost beyond me. If you have a noisy rod it should be louder after the oil is really thinned out. One day I even had a suspect cracked flex plate. When that sound self corrected a couple of days later I kind of threw in the towel. If your rod theory concerns you loosen off each injector in turn. It should locate the knock. Then change the injector with the one adjacent to it and see if the knock still exists on that cylinder when you loosen the injector line of the cylinder or has it moved. The main thing with these engines is not to get too excited I guess. I would pay a little more attention if it were number one cylinder though. If it is a rod problem for some reason it is usually number one. Not written in stone though.

Thanks for the reply, Barry.

When I listened with the stethoscope at cylinder 1 injector/prechamber area, I did not hear anything resembling that irregular knock. Maybe I'm listening at the wrong area... Should I put the stethoscope needle up to the lower part of the engine block around cylinder 1?

That knock is heard best from the cabin of the car, with doors and windows closed, especially when the engine is at its operating temperature. Trying to listen to the knock from the engine bay is a more challenging task, apparently. It isn't a "sharp" sort of noise - it sounds sort of muffled/obtuse, and diesel clatter inside the engine bay is really throwing me off from it right now. I'll poke around the engine block again... Maybe try to listen to the bottom end.

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84 300SD - bought in April '06 with 237,000 mi

Here's a wild guess, but check your vacuum pump.

I did listen to it with the stethoscope, and it did sound like it had a bit of knocking going on, but so did the water pump as well... And it made me wonder if I was listening to a local noise or the noise that could have been transmitted from the main engineblock housing...

What sort of sounds would a vacuum pump make when it's starting to go bad? So far, I don't have a problem with vacuum and did not yet notice oil in the vacuum lines... Just out of curiosity, I saw a rebuild kit for the vacuum pump and it was pretty cheap - how easy or hard is it to rebuild?

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84 300SD - bought in April '06 with 237,000 mi

You already know this, but you need to carefully compare the noise when listening to different locations. Yes, they bounce all over in the block. It's an art trying to figure out what's making a particular noise.

I've never rebuilt the vacuum pump, but I've seen lots of posts about it.

The vacuum pump failure I experienced was preceded by a "ticking" sound rather than a knock, but I suppose different failure modes could make different sounds.

Wes

I am having a similar problem. Same as you describe. Did you ever figure what the problem was.