Engine "knocking" noise- help?
 

Hi everybody,
So I was driving my 1984 Mercedes 300TD wagon (~300k miles) and after about an hour on the road I started hearing this knocking noise coming from the engine. I thought it might be the injectors, so I used 2 cans of diesel purge and it still hasn't gone away. I've included a link to a Youtube video so you can hear it yourself. Any thoughts or tips on how to troubleshoot this? Thanks for your help.

https://www.youtube.com/watch?v=iudxlbCRgcY

You haven't verified if the noise goes away on diesel purge. You also haven't verified if it gets louder with RPMs. All I hear is a rhythmic knocking and have no clue what I'm listening to.

So I can't say anything until either of those are answered otherwise me and everyone else will just talk out of our hind-end.

I can say this.. If it goes away on diesel purge, it's a fuel issue. If not, it's something else internal bottom end or top end. Ever run it out of oil or low?

I haven't watched the video yet... but how close is your power steering pulley to the upper oil cooler line?

OP says noise goes away on a diesel purge.

Your video makes it hard to hear what's going on because the aux fan is running. Can you make another one with the A/C shut off and that fan not running? You might get better help that way.

The noise didn't go away with diesel purge. With higher rpm's, it doesn't seem to get louder. It either goes quiet or just gets drowned out by the engine, not sure which.

I don't run it low on oil or empty, and do oil changes regularly.

I'll check on the power steering pulley.

Clearly I hadn't had enough coffee when I replied this morning. You said in the original statement that it does NOT go away with DP, I blame the lack of caffeine!

It would be nice to have a video without the Aux fan howling away and drowning out everything else out though. It sounds like valve train clatter to me, but hard to tell. Pull the valve cover and do a check, it's part of routine maintenance on a 617. Adjust your clearances to spec and see if the noise is still present.

FYI, Diesel Purge making the sound go away would tell you essentially nothing anyway. It could still be fuel related, bad compression, or any number of other problems and doesn't automatically point to any of them.

I would say the sound staying even when using a quiet fuel (DP, canola) is much more useful information to have, actually.

-Rog

So I listened again, and the knocking definitely becomes faster when I give it gas. Thanks for the help so far.

I think it's lifter tick, when was the last time the valves were adjusted?

I might loosen each injector line in turn. If it is valve line related the noise will remain. If something else it should stop or greatly reduce when you disable the right cylinder.


This may also give you a specific cylinder to focus on. Keep an open mind as it could be something else like a noisy vacuum pump. I am never keen to rev up an engine with an unknown noise. If it is there at idle is usually enough to work with.

It's been about a year and a half since my last valve adjustment, probably about due.

Barry,
So do I just loosen each injector a half turn or so with the engine running the whole time, listen if the ticking stops, then retighten and move on to the next one?

DO NOT loosen the injector. Loosen the injector fuel lines one at a time to find the offending cylinder, if any. Loosening the fuel lines is like pulling a plug wire from a gasser.

I don't think your valves are tight enough to tick. Maybe if they're 40k miles overdue. Loosen the lines, not the injector. Loosening an injector isn't smart while it's its running.

It is overdue, unless you hardly drive the car. Valves should be adjusted annually in the fall, or every 15k miles, whichever comes first.

Not the injector, the injector line. If the noise goes away, the problem is related to that cylinder and is most likely fuel related. If you find that loosening and injector line eliminates the noise, then you can try swapping injectors (need new heat shields) around to see if the noise follows.

Wait, Canola will allow it to run quieter? Fill me in.