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.

If the other suggested stuff does not work listen with a piece of broom stick or a mechanics stethoscope to see if you can locate where the noise is coming from.

The Vacuum Pump having issues can make noise.

Removing the small steel oil pan and looking in the bottom for metal fragments might also tell you if something is destroyed.

When I first got my Car the engine would shake and the Air Filter Housing would pound on the intake manifold. Some where there is thread that tells you to put a piece of 3/4" or 1" heater hose between the manifold and Air Filter Housing.
The noise was bad and sounded like something was going to break off.

The same as above but caused by a broken Air Filter Bracket.

Well, just as long as the canola is in the fuel. Diesel Purge is the same, the engine runs quiet while you've got it running through and people think it's fixed something but then the noise returns more often than not. Both are quieter because they burn easy, I guess.

The first time I dumped some old (but unused) cooking oil in my tank I thought the engine died when I was idling through a parking lot it was so quiet.

-Rog

I think the flame front is more gentle or slower. Reducing combustion noise to some extent. Just what I think. Not proven.

So I pulled all the injector return lines one at a time and there was no change in the noise. Air cleaner mounts look solid, and the noise seems to be coming from under the valve cover. Should my next move be to pull the valve cover and go ahead and plan on adjusting the valves while I take a look?

Do a cold valve adjust ~ once I apparently left a lock nut un snugged and a valve began to tick noisily .

FWIW : tight valves are silent as they burn .

It runs better off of diesel purge because of the higher levels of cetane than fuel. It's a more complete combustion and can mask bad injectors, or other related issues to fuel. It's great for diagnosing fuel issues. I'm not sure why oil combats better and makes the engine quieter. I'd assume lubricity properties.

@ the OP. Why did You remove the injector lines and not crack them just a quarter turn? Also, your next step should be to do the valve adjustment with feeler gauges.

Yes, but because it burns so well it also gives you the same "fix" if you have bad compression, incorrect timing, et cetera. So if it runs quieter no matter which you have I don't see how it's automatically a fuel problem. It could be any of those.

-Rog

Mainly because I have no idea what I'm doing. I pulled the injector lines off all the way, which made me realize I need new lines. Going today to pick some up.

If I wanted to adjust the valves in the next day or two, are there any stores that sell the adjustment wrench or do I pretty much need to look online and wait for shipping?

Order them online. Why do you say you need new lines? Are you talking about the metal hardlines? If you aren't sure of what you're doing, before you do anything, ask questions! Before you break or make a big mistake that could cost you the engine. Before you do tbe adjustment, do consult.

I've never seen it correct compression. if compression is bad enough the purge wont mask it. Same with timing. It's mainly used to mask poor nozzles. I only say that cause I did it on my 92 300d before I swapped it and it didn't mask the low cylinder compression misfiring.

No, that's wrong. The return lines are only there to return any unused fuel to the fuel system. The correct test is to loosen the nut of the metal injection line at the top of the injector. I believe it is a 14mm nut.

On the OM60x engines it is 14mm, but on the older diesels I think it's 17mm. Either way the hard injector line has to be loosened, not the rubber fuel return line.

The OP needs to pay attention to what is being said by people trying to help him. There have been multiple iterations of the same suggestions that have so far gone completely ignored. It's really hard to help someone who refuses to help themselves.