Diesel engine noise - broken rings vs bad injection

[JHZR2]

I’m kind of at my wits end with my 1991 MB 300D. I bought it as a driveable classic, not a museum piece. I love the blue interior, AC works, starts and drives great.

I’ve been chasing this horrible sound that is rpm dependent. Here’s an old video:



I’ve replaced all injectors because my pop and balanced set didn’t fix it. Thought maybe there was a different issue inside. Also set timing to FSM 14 degrees, as it was at 15.5.

So now it maybe sounds better. Maybe? But it still has that full of rocks sound off idle to around 1400.



I don’t usually run engines in neutral at high RPMs, but I ran it through the range and note the sound of a misfire or other oddity now that I’ve changed the timing:



So now I don’t know, could it be engine health?? It starts and drives perfectly, good power, >30 MPG, doesn’t consume oil, no issues really. I don’t feel a misfire at 2000+ rpm under load, just when doing this in park, so to me that test may be suspect… but obviously the engine is a bit loud.

And I have verified by smoke machine, cyl #5 has a leak, which I will likely repair, unless I should do some other diagnosis and potentially condemn the engine. Thus this thread.

The reason for asking about condemning the engine? I saw this video - listen to what he’s showing at around 20-40 s or a bit longer:



Makes me wonder if perhaps the issue I have is related to a piston.

What should my diagnosis course of action be then? Compression and leak down cold? Anything else? It starts in the coldest weather immediately, runs great, smooth, I’ve never had a reason to test it or think about it at all. Maybe now I do…

I’d appreciate any thoughts and advice.

 

[Rand]

I liked it better in second video.
How does it sound when driving? (with a load on it?)

 

[Donald]

I would ask around and try and find a real diesel expert, hopefully one who knows cars in addition to (likely) trucks. Often this was a diesel mechanic at a dealership and went out on his own.

Even if you pay him for an hour or two of diagnostic time.

There are diesel experts out there but most mechanics are not diesel experts.

 

[Cujet]

I was fascinated by the other thread with camshaft issues. What a wild thing. Is this engine having similar issues?

Diesel engines make crazy sounds when timing is off, as I am sure you know. Your change in timing may not have been enough? If you have not done so already, you could try mixing in fuel additives and/or two stroke oil, to see what changes that may bring. Tends to quiet noisy engines.

Just another thought, a broken ring can cause a start up misfire. Makes that cylinder difficult to light off when cold.

Edit to add, it sure seems like combustion noise, as during decel, it disappeared.

 

[CleanSump]

I had a 92 that I ran to 348k miles. The only time a knock like that wasn't a failing injector, it was a cracked precombustion chamber. The diagnosis wasn't hard. I did the blead the fuel line with each injector to find the one that changed the knock sound. Swapped to a known good injector and the knock didn't change. Pulled the prechamber with a slide hammer w/adapter and saw the crack between two of the small holes.
You may want to take your vids to Benzworld or Pelican Parts forum for help from those who live these engines. Pelican forum is pretty dead these days though.

 

[JHZR2]

I liked it better in second video.
How does it sound when driving? (with a load on it?)

Second and third videos are the same (current) setup. I tend to agree. Which tells me that timing did have an effect.

The issue is that I need to deconvolute the ~1000RPM off-idle full of rocks sound.

I was fascinated by the other thread with camshaft issues. What a wild thing. Is this engine having similar issues?

Diesel engines make crazy sounds when timing is off, as I am sure you know. Your change in timing may not have been enough? If you have not done so already, you could try mixing in fuel additives and/or two stroke oil, to see what changes that may bring. Tends to quiet noisy engines.

Just another thought, a broken ring can cause a start up misfire. Makes that cylinder difficult to light off when cold.

The thing is there is no sign of a start-up misfire. It will light off smoothly and no smoke. I can smell diesel exhaust but no visible.

The misfire that I could hear and feel in the third video was after a few miles of driving, engine up to temperature. Car ran like a top. No feel of the misfire under load.

FWIW, and not sure if this is a clue, but if I held the rpm, in park, at say 2500 for any amount of time, I’d get white smoke - unburned fuel. And that’s when the misfire gets heard and felt. I’m guessing it’s an artifact of mechanical fuel control, where I’m taking the rack to a certain level, granted without boost, and it’s meter ing more fuel in as a result, and it’s more than is actually needed just to raise the RPMs. That results in white smoke (unburned fuel), and misfire (because it’s in the cylinder perhaps). Maybe that means that there’s something else awry in the fuel system besides injectors.

I would ask around and try and find a real diesel expert, hopefully one who knows cars in addition to (likely) trucks. Often this was a diesel mechanic at a dealership and went out on his own.

Even if you pay him for an hour or two of diagnostic time.

There are diesel experts out there but most mechanics are not diesel experts.

There is a guy that has a lot of sprinters, very popular shop. He doesn’t seem to be the most outgoing person, but may be worth a shot.

I think my plan will be to do a cold compression and dry leak down test tomorrow, as I’m taking a lot of the day off, then I have something to discuss with the shop if I take it there…

 

[JHZR2]

I had a 92 that I ran to 348k miles. The only time a knock like that wasn't a failing injector, it was a cracked precombustion chamber. The diagnosis wasn't hard. I did the blead the fuel line with each injector to find the one that changed the knock sound. Swapped to a known good injector and the knock didn't change. Pulled the prechamber with a slide hammer w/adapter and saw the crack between two of the small holes.
You may want to take your vids to Benzworld or Pelican Parts forum for help from those who live these engines. Pelican forum is pretty dead these days though.

Yes, Pelican is pretty dead. I’ll probably post there and benzworld to give them a try in the diesel forum.

Good point about the prechamber. I have done the release the injector line at each with very limited success in terms of noise. And it’s hard to do it and sweep the rpm range.

 

[TnT_M]

I could be wrong but I think this sounds like an old school preemissions diesel with the good old raddle, raddle, raddle. In my mind it is a diesel doing diesel things at low RPM's

 

[CleanSump]

My '92 sounded like a sewing machine after balancing the injectors and timing the pump. Same with the non-turbo 240D I had before it.

 

[JHZR2]

I could be wrong but I think this sounds like an old school preemissions diesel with the good old raddle, raddle, raddle. In my mind it is a diesel doing diesel things at low RPM's

I have plenty of other idi engines, including the 4 and 6 cylinder versions. None sound like this.

My '92 sounded like a sewing machine after balancing the injectors and timing the pump. Same with the non-turbo 240D I had before it.

This car runs like a sewing machine, it’s such a nice mechanical and pure operation, it’s great. For whatever reason more direct and connected than even the 190d or 350sd, both using the same family of engines. And the lightness and marginally better power of the w124 than the 123 just makes it a great package. But this one doesn’t sound like it!!!

 

[Motu]

FWIW, and not sure if this is a clue, but if I held the rpm, in park, at say 2500 for any amount of time, I’d get white smoke - unburned fuel. And that’s when the misfire gets heard and felt. I’m guessing it’s an artifact of mechanical fuel control, where I’m taking the rack to a certain level, granted without boost, and it’s meter ing more fuel in as a result, and it’s more than is actually needed just to raise the RPMs. That results in white smoke (unburned fuel), and misfire (because it’s in the cylinder perhaps). Maybe that means that there’s something else awry in the fuel system besides injectors.

White smoke in old school diesels is often a sign of retarded timing...but if you have a misfire you will get white smoke too. A sticky injector will give a bad knock, worse than what I can hear there. I'd get the injectors rebuild and see how it goes from there.