Since I'm new to this car, I wanted to ask about some knocking sounds that I hear which are a little bit unsettling. The engine idles perfectly and starts within less than a second of turning the key. But at around 1,200 to 1,900 RPM, I can clearly hear some low-frequency knocks coming from the engine. When you open up the throttle wider, these knocks get faster and get blended with the rest of the engine noise, but at lower RPMs they are more obvious. I thought that maybe these are transmission-related sounds, but these sounds appear both in drive and in park.

What are the possibilities? I thought maybe it's valve-related, but from what I read the car would have more difficulty starting if it's the valve issue, right? Is it from a timing chain getting loose? When the engine is cold, it definitely makes these distinct "glass full of nails" sounds which go away when it gets warmed up. Maybe it's typical to these diesel engines - I don't really know.

I'm sure it's not that serious (just makes the engine sound a bit "chuggy"), but I do want to keep on top of noises in this car, so if you could help me figure this out or at least give me some ideas as to the possible origins for these sounds, that would be appreciated.

P.S. Previous owner told me that valve clearances were checked and needed pretty much no adjustment, but that's his word.

____________
Denis

1984 300SD

The valves typically get tighter with age and they don't make any discernable noises above the remainder of the diesel clatter.

You've probably got some injection issues........a dripping injector.......or a bad prechamber.......that's causing that noise.

a dripping injector is solved by replacing it, i presume? could well be - i'd imagine they have a lot of miles on them.

what's a pre-chamber?

i would ask if timing needs to be checked, but other than those knock sounds the engine works alright. it's got a decent pick-up... it doesnt smoke much (i can only see the smoke at night when people behind me have their headlights on)...

Well, the new thought is to change the nozzle with one of the new Bosio nozzles. The replacement Bosch injectors are a hit or miss proposition for longevity.

The injector sits in the prechamber. The fuel is injected into the prechamber and starts combustion prior to entering the cylinder.

When it's knocking, try to isolate it to one cylinder by cracking the injector lines......in sequence.....until the noise goes away. That's the injector to change.

Trouble is, I can't really detect this noise at idle... Maybe I should listen to each cylinder with a stethoscope at idle.

How do you mean "by cracking the injector lines"? (sorry, noob alert...)

The hard lines from the IP to the injectors have a nut at the end where it attaches to the injector.

"Crack" this nut.....about 1 turn or so......to open the line and kill the injector. Then the noise will cease.

You can also do this at higher engine speeds, but the fuel will run out all over the side of the head from the cracked line.

Yea that's what I thought you meant... Interesting method to disable an injector.

Have you ever had to address a noise like that in your experiences? I was a bit surprised when you said injector because I was thinking it might be something mechanical... But I don't really know any better

I have no where near as much as experience as Brian but I'll throw my $0.02 in. My '85 that is now wrecked had a timing chain that was around seven degrees stretched. It had a miss on startup and had an irregular knock till around 2000 rpm, at which point it either went away or blended in with the other engine sounds. The valves were all within specs (checked them all myself twice) but I haven't tested the injectors yet but I will soon to see if I have an irregular spray pattern. The reason I mention chain stretch is because my new car, the '84 has no measureable stretch by the aligning the marks method(I know that's not surgically acurate but it reassured me) and it fires right up with no miss or knock regardless of temps.

__________________
Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo

Those are good 2 cents. I was having the same thoughts on that. But definitely I'd check the injectors first, as Brian suggested. If it's not related to the injectors, then I'll have to see how stretched the chain is.

Should I do this injector business on cold or warm engine, or does it even matter?

Before you put money into new (or rebuilding) injectors, try running a can of Lubro-Moly Diesel Purge through the system first. I do it about twice a year. Takes about 30 minutes. It quiets my engine down every time. (As in down to no knocking at all.) About $10.00 a can, but pretty impressive stuff.

__________________
1980 300D - Veggie Burner !

I really like diesel purge. That and a tank of B100.

Jay.

__________________
On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er

The previous owner did a Lubro-Moly diesel purge not long ago, so I doubt it would help this particular issue...

Yep......and it works like a charm.

I chased a similar noise on the old W123 that I had. But, I never isolated it......it was a fleeting noise..........however, other members of the forum had great success with the procedure.

It's related to spray pattern. If the injector is clogged or the needle is damaged, the pattern won't be satisfactory and the combustion will be off. This can also occur if the prechamber ball is damaged or missing. Many of the very high mileage vehicles have prechambers that are marginal.......at best. I replaced mine at 180K. The opinion at the time was that they would never make it to 280K.

Don't ignore the diesel purge procedure. It's possible that the procedure was not done properly by the previous owner and it was not successful. For all we know, he dumped the purge in the tank and called it a day. You need a proper container and come connection hoses and you must understand the procedure thoroughly before you do it. Many folks won't bother.

I know that the previous owner didn't just dump the purge in... He explained to me the procedure how to do it right, but of course since it was the first time, I don't really remember. I have a reason to believe he did the procedure correctly - knowledgeable, intelligent that guy is.

But perhaps I can try to re-purge and see if that makes any difference. Can't hurt and it's pretty cheap, eh...

Cheaper than a new injector, and easier to do.
I'd do it again.

If you order it from Performance Products, it comes with instructions and the hose you'll need. All you need to provide is a jar, a screwdriver and the ability to operate your primer pump.
I bungie the jar inplace over by the IP. That way you don't have to stand there holding it, or worry about it vibrating out of position. Fill it. Prime it. Start the car. Walk away. I come back to check it every few minutes. I like being able to actually hear how good it works. (or maybe I should say: "not hear.")

__________________
1980 300D - Veggie Burner !