1981 240D vibrating/knocking sound diagnosis - PeachParts Mercedes-Benz Forum

JHZR2 · #1 09-04-2017, 03:04 PM

Not diesel clatter, not injectors. Tried to video, see here:

https://youtu.be/2ZRzbiljr70

In addition to typical diesel sound, it sounds almost like something is hitting into something else, it's a faint knock like something hard tapping something soft. Mounts seem ok... kind of like what I recall the air cleaner knock to sound like on a 617.952 when the little rubber mounts are bad, except these aren't.

Thoughts? Can you make out the sound? Its more noticeable in the second half of the video. Thought it was a wheel bearing when I heard it on the highway. Not that...

Thanks!

Diesel911 · #2 09-04-2017, 10:10 PM

Get a mechanics stethoscope or use something like a section of Broom Stick (your ear on on one end) and place it (watching out for moving parts) over suspected areas till you can find where the sound is coming from.

JHZR2 · #3 09-05-2017, 08:32 AM

Easier said than done. Ive been around w123 cars for the last 15 years, have done a decent amount of work on them, this one is tougher to chase.

If I had to claim the most correlated sound, it would be related to the AC being engaged. System runs R12, and had a new compressor in the last 6k due to a seizure. System runs great. No sound via stethescope on comp.

The other correlation is that when the weather/vehicle is cold, no sound. Even with AC engaged. Which points me to a rubber part that gets softer when hot.

tangofox007 · #4 09-05-2017, 09:33 AM

The water pump is one possibility.

JHZR2 · #5 09-05-2017, 09:33 PM

Today I started hearing an occasional squealing sound when the AC compressor was engaged. I drove longer and it was hotter than it has been since Ive had the car out. I haven't driven the car in many months before, so its not like I have a good data point from a few days or weeks ago either...

Hoping its not the compressor. That failed about 3000 miles ago, and sheared the pulley clean off the engine.

AC works perfect and is leak free on R-12, so I really don't want to have it be that!!

barry12345 · #6 09-06-2017, 07:32 AM

Harmonic balancer still tight?

Diseasel300 · #7 09-06-2017, 11:08 AM

You sure it isn't a belt? Squealing when the compressor engages is certainly a clue! I've had a loose belt make a very loud knocking noise on one of our belt-drive machines at work. No noticeable flapping and tension SEEMED ok, but when tightened slightly was silent as a mouse.

JHZR2 · #8 09-06-2017, 11:29 AM

Ive gone by belt tension/deflection.

May be prudent to change them all.

Still, the squeaking is not the knocking/rubbing sound of the second half of the video linked above. Very hard to chase.

funola · #9 09-06-2017, 03:20 PM

Quote:

Originally Posted by JHZR2

Today I started hearing an occasional squealing sound when the AC compressor was engaged. I drove longer and it was hotter than it has been since Ive had the car out. I haven't driven the car in many months before, so its not like I have a good data point from a few days or weeks ago either...

Hoping its not the compressor. That failed about 3000 miles ago, and sheared the pulley clean off the engine.

AC works perfect and is leak free on R-12, so I really don't want to have it be that!!

I find that hard to believe w/o seeing a pic. The pulley on the engine (crankshaft) is a pretty stout MF.

JHZR2 · #10 09-06-2017, 03:59 PM

Quote:

Originally Posted by funola

I find that hard to believe w/o seeing a pic. The pulley on the engine (crankshaft) is a pretty stout MF.

Of course Photobucket doesn't host anymore.

Help stranded belts flew off

Alec300SD · #11 09-06-2017, 04:45 PM

Here's a somewhat grainy photo of my 83 W126 300SD to show some potential areas that may be the cause of noise.

1) air cleaner bracket
2) oil separator return tube
3) exhaust manifold heatshields

Even though my rubber isolation mounts are new, engine vibration caused the threads to strip out on the rearmost threaded hole of the air cleaner bracket.
The rattling air box left shiny metal wear evidence on the air cleaner bracket in four different spots.

I got tired of the rattle and of replacing brackets so I installed an 85 W126 fender mount air box in place of the original.

Remove your air cleaner, hook up a temporary cone filter to the turbo inlet, and start your engine.
If the sound disappears, you know you have a rattling airbox or a rattling oil separator return tube.

The oil separator tube is supposed to be anchored in three spots.
One is a bracket attaching the tube to the engine block...these usually fail from metal fatigue, or the securing bolt is loose or missing.
The other two sposts are the nipples on the bottom of the air box and the nipple on the oil sump.
The two little o-rings on each nipple that prevent the tube from vibrating are probably hardened or missing.

Another area to look at is rattling of the heatshields on the exhaust manifold.
The bolts securing the heatshields can be loose or fall out, and the bolt holes in the heatshields can get enlarged from rust.

An inexpensive mechanic's stethescope can be fashioned from two funnels and a few feet of hose that fits the funnel tips.
Put an earplug (cotton ball) in one ear to block out extraneous noise, cover the other ear with the mouth of one of the funnels.
Point the other funnel at the engine (be wary of hot or moving parts) to pinpoint the sound source.

Hope this helps.

funola · #12 09-06-2017, 05:14 PM

Quote:

Originally Posted by JHZR2

Of course Photobucket doesn't host anymore.

Help stranded belts flew off

I was able to look at your pics by copying their url's and pasting into a new window. The pics were poorly taken with bad lighting and do not show the details of what happened. The crankshaft pulley is held on by 5 Allen head bolts and there is NFW they can shear off by a seized AC compressor if all 5 bolts were there and torqued to spec. Did you or someone r&r the pulley and did not torque it correctly? That's the only way the pulley could shear off, by losing bolts/ running with a loose pulley, slowing cutting through the bolts.

JHZR2 · #13 09-06-2017, 05:30 PM

Quote:

Originally Posted by funola

I was able to look at your pics by copying their url's and pasting into a new window. The pics were poorly taken with bad lighting and do not show the details of what happened. The crankshaft pulley is held on by 5 Allen head bolts and there is NFW they can shear off by a seized AC compressor if all 5 bolts were there and torqued to spec. Did you or someone r&r the pulley and did not torque it correctly? That's the only way the pulley could shear off, by losing bolts/ running with a loose pulley, slowing cutting through the bolts.

OK, argue with me over something like that... LOL.

I took it with an iPhone 3 stranded on the side of the road 7 years ago... High quality pictures were not my first objective.

The pulley was loose. The only thing changed was the AC compressor. Everything else on the engine worked and still works fine on there.

EVERYTHING was OE original until the pulley fell off. If someone overtightened the belt in the 20 ish years before i got it, I couldn't say - nothing in my receipts and records indicate anything of the sort. All I know is the pulley fell off, the screws sheared off, and that was that.

If someone else pulled the pulley before I owned it, I also couldn't say... WHunter mentions that these screws will shear off if retorqued: Harmonic balancer Dowel related data

funola · #14 09-06-2017, 06:53 PM

Quote:

Originally Posted by JHZR2

OK, argue with me over something like that... LOL.

I took it with an iPhone 3 stranded on the side of the road 7 years ago... High quality pictures were not my first objective.

The pulley was loose. The only thing changed was the AC compressor. Everything else on the engine worked and still works fine on there.

EVERYTHING was OE original until the pulley fell off. If someone overtightened the belt in the 20 ish years before i got it, I couldn't say - nothing in my receipts and records indicate anything of the sort. All I know is the pulley fell off, the screws sheared off, and that was that.

If someone else pulled the pulley before I owned it, I also couldn't say... WHunter mentions that these screws will shear off if retorqued: Harmonic balancer Dowel related data

I am not arguing with you whether your pulley fell off or not. It is obvious it did fall off. I was stating the obvious: the pulley bolts would not have sheared off unless they loosened up.

How many bolts fell off and how many sheared? It didn't happen overnight but over a long period. Normally a seized AC compressor would smoke the belt, not shear the crankshaft pulley bolts.