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?

__________________
Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

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

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

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.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

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.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

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

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

__________________
1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

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

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.

__________________
Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

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?

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

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?

__________________
1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.