Troubleshooting 1985 Mercedes 300d knock
 

My 85 300D has developed a knock, see clip:

https://youtu.be/WLWV4niUwpw?si=JftEzR9Qk5blZpx4

I owned this car previously, sold it to someone local who drove it for a couple years. When it started knocking he immediately stopped driving it, thinking it’s a rod knock, and sold it back to me cheap as he didn’t want to deal with it. I’ve seen discussions/resolutions on 300D’s with similar knocks ranging from injector nailing to more serious issues with pistons or wrist pins. Notably, my knock sounds eerily similar to the knock in this old thread, which ended up being a bad piston.

https://www.peachparts.com/shopforum/diesel-discussion/269918-troubleshooting-my-engine-knock-long-ish-1982-300d.html

The knock is present from idle to higher revolutions. It’s very rhythmic, and sounds to my ear like it’s half the engine RPM, so maybe valve train related or power stroke. It sounds like it’s coming from the middle of the valve cover or injectors, but I know this can be deceiving. The car is down on power. The knock sometimes disappears completely on cold start (see additional detail below), but then comes back slowly after a minute or two. I have not driven it enough to get it really hot, but it shows no signs of going away as the car warms up. Car starts and idles fine. Oil pressure is great. Car has about 220,000 miles.

Additional details:

With a stethoscope it sounded loudest / sharpest around #4 cylinder. I listened at the vacuum pump, and elsewhere on the engine and while I could still hear the knock, it was definitely sharpest at #4.

I tried cracking each injector line and the sound seemed to (maybe?) diminish when cracking #4, but that could be my imagination. It certainly never went away, and each cracked line resulted in a notably rougher idle.

What’s been done:

Fuel was drained, filters replaced. Ran Diesel Purge (still knocked on pure DP).

To check for nailing, I pulled the injectors. Two injectors had been replaced previously (2 & 3) and were out of balance with the old injectors. Old popped around 1900, new popped at 2150-2200. #4 is one of the original ones. Some questionable spray patterns, but nothing horrible. Still, I replaced #4 with a completely new injector, others were cleaned and nozzles replaced; all were balanced to pop around 2050.

I inspected the glow plugs and pre-chamber balls, all looked fine. Nothing broken.

I adjusted the valves. Almost all were out of spec, most being too tight. At a glance, everything else under the valve cover (lobes, springs) looked fine.

Timing chain shows about 2.5 degrees of stretch off TDC when the marks on the crankshaft pulley and camshaft are lined up.

While the injectors were out, I did a cold compression test. All cylinders were in the 320-ish range, except #4 which was down around 265. I ran compressed air into #4 (valve cover off so I could see the cam lobes) and heard air out of the intake, but not the exhaust or dipstick. So something is going on with #4 intake at a minimum. In hindsight I’m thinking that having the valve cover off meant air was also able to escape up top, so listening at the dipstick is inconclusive.

I did a wet compression test on #4 cylinder and was able to get it back up to around 320. Assuming stuck rings at a minimum.

After all that I buttoned it back up, restarted the engine, and after 3-4 minutes the knock came back. This is longer than it has taken previously to return (I had brief hope). Maybe the wet compression oil soak on #4 dampened the issue for a bit?

In any case, I’m at the point where I think I have to pull the engine, much as I don’t want to do that.

Questions:

Any ideas on anything else to check short of pulling the engine?

I’ve read that lack of compression on a diesel cylinder in and of itself can manifest as a knock? Can someone verify or deny that? If so, would it be worth doing a soak with Marvel Mystery oil, trans fluid or something similar to try to unstick rings? Would that possibly eliminate piston slap (if that’s what this is)? Or is it just a waste of time at this point?

Worth pulling the pre-chamber and trying to scope #4? I'll need to order the tool for the collar...

Is it worth pulling the head first? What would I be looking for? Or just pull the whole motor?

Open to ideas, opinions or suggestions.

Get a mechanics stethoscope or use a substitute and listen to your vacuum pump and anywhere else on the engine you think the sound is coming from.

Air coming from intake when compressed air is injected, I would check for broken valve
spring. Next burnt or bent valve, also recheck valve clearance, if carbon was stuck under
valve it would be loose, and now after adjustment too tight.

Thanks for the insights. I did use a mechanic's stethoscope and, while the knock reverberated through the engine (I could hear it everywhere) it was definitely most pronounced right around the combustion chamber for #4. Don't know if that's valve or piston (it's not injector). The points Hercules makes about carbon and valve tightness are good ones -- #4 intake was tight when I originally checked it. I'll remove the valve cover and look closer at what's going on there, recheck clearance. Any way to check for burnt/bent valve without removing the head?

Also, anyone have any guess on why it would be quiet on many cold startups and then start to knock after a couple minutes? I would think that piston slap, etc. would do the opposite (disappear as the engine warms up). Seems like that's a clue, but I can't figure it out.

Very hard to bend a valve on this engine, valve opening straight down, unless some thing
was under the valve when piston hit it.
The piston comes very close to the head, if a rod bearing is bad the piston can come in
contact with the head ,it is possible when oil is cold and thicker the clearance on rod
bearing will be taken up by the thicker oil, piston does not hit head, oil thins with heat
piston now hits. Just a possibility, but can happen.

So, I am not a mechanic but that video sound might be timing chain whip. Definitely it is not injector nailing noise.

How's the timing chain and how many miles on it? Although often when the timing chain is beginning to get loose, you can hear a metallic jingle. Cheap insurance to replace the timing chain at 200K miles.

You should check the timing chain guides also for broken pieces etc.

Sorry it get complicated.

Good luck!!