Finally did my compression test (sort of) - Page 2 - PeachParts Mercedes-Benz Forum

mach0415 · #16 01-10-2011, 05:22 AM

Correct me if I'm wrong (CMIIW) but if a rod bearing spun, you would see metal flake in the oil filter folds, and drastic scenario/theory, compression in that cylinder would be drastically lower with diesel tolerances so close, correct? has anyone done a compression test in an engine with a spun rod bearing (it would be a moot point, but none the less...)

t walgamuth · #17 01-10-2011, 05:56 AM

My current 85 300SD came with a comprehensively spun rod bearing. It had pretty good oil pressure as the bearing rolled around the journal and before I got it it had passed a compression test with flying colors.

I agree though this car seems like an unlikely candidate. Change the motor mounts first, it is easy not very expensive and a motor with bad ones sounds like someone is working on breaking rocks by leaning them against your frame and using an 80# jackhammer on them.

The oil lines on the other hand are a lot of work so I would not change them unless they are leaking.

And oh yes, there were large bearing particles in the oil filter.

chasinthesun · #18 01-10-2011, 11:00 AM

The injector has the return feed nipples that usually sit out a bit ,I bend them slightly inwards to get my socket on it.Bend them out when Im ready to put my return hoses back on.

charmalu · #19 01-10-2011, 12:02 PM

Quote:

Originally Posted by chasinthesun

The injector has the return feed nipples that usually sit out a bit ,I bend them slightly inwards to get my socket on it.Bend them out when Im ready to put my return hoses back on.

Using the proper socket and you won`t have to be tweeking your nipples.

this is not a good pracice to do. next time you will have the broken off nipple in your pliers or what ever you use. no fix for this.

Charlie

mikeyfev1 · #20 01-10-2011, 10:03 PM

Quote:

Originally Posted by charmalu

Using the proper socket and you won`t have to be tweeking your nipples.

this is not a good pracice to do. next time you will have the broken off nipple in your pliers or what ever you use. no fix for this.

This is so funny I don't quite know what to say...

240D_MB_1980 · #21 01-11-2011, 04:33 AM

Diesel Weasel,
How would a spun bearing lower you compression????

leathermang · #22 01-11-2011, 09:34 AM

Quote:

Originally Posted by 240D_MB_1980

Diesel Weasel,
How would a spun bearing lower you compression????

Theoretically YES.... if it lost enough from between the surfaces in question to cause the piston to not travel as close to the head as designed..

BUT you will not be able to measure that difference or use it as a diagnostic indicator.

t walgamuth · #23 01-11-2011, 10:24 AM

Quote:

Originally Posted by leathermang

Theoretically YES.... if it lost enough from between the surfaces in question to cause the piston to not travel as close to the head as designed..

BUT you will not be able to measure that difference or use it as a diagnostic indicator.

agreed.

leathermang · #24 01-11-2011, 11:49 AM

I am just going to throw this into the mix even though I read it in the 616 FSM.. ( I do not know whether it is in the 671 manual or not )....
on the back of the oil pump... there is a pressure relief valve.
As with most things on these MB's .. they were changed over the years.. hopefully improving or tweaking them... and it is possible for that valve to fall out of the oil pump... or have the wrong pressure setting and it cause noise in the engine... This may or may not apply to a 617... but if you have the pan off it is something on an old engine to check.

chasinthesun · #25 01-11-2011, 11:50 AM

I gently tweek my nipples with some very soft pliers ,it makes the nipple tweaking a bit less abrasive .The nipple seems to agree since after I dont have to add a lite sheen of oil on them.This is known as " The tweeking nipple procedure" in the manual vs the reverse procedure,"Pulling on the nipple procedure", which is where most of them break off.

leathermang · #26 01-11-2011, 12:03 PM

Quote:

Originally Posted by chasinthesun

very soft pliers ,

?

mach0415 · #27 01-11-2011, 12:26 PM

Quote:

Originally Posted by 240D_MB_1980

Diesel Weasel,
How would a spun bearing lower you compression????

Badly spun rod bearing + increased clearance between the rod bearing journal and the connecting rod = an incomplete TDC travel on the compression stroke, therefore compression is lower.

mach0415 · #28 01-11-2011, 12:28 PM

Quote:

Originally Posted by leathermang

Theoretically YES.... if it lost enough from between the surfaces in question to cause the piston to not travel as close to the head as designed..

BUT you will not be able to measure that difference or use it as a diagnostic indicator.

I would agree that it is not effective as a diagnostic tool, as there would be such a horrendous sound, there would be no question as to what was wrong. BUT, compression would be decreased as well.

leathermang · #29 01-11-2011, 01:44 PM

How thick do you think that bearing is which might have " left the building" ?
I agreed that it would decrease the compression .. but it could not be measured and would be of no value.
Why don't you assume a totally removed bearing... show us the math as to the amount of compression reduction which would occur....
The fact is that just the differences in the temperature of the things in the bore... and the speed of the starter due to battery charge... will make more difference from one compression check to another than the difference of a missing bearing...

mach0415 · #30 01-11-2011, 02:54 PM

Okay. Enough said. Agreed a few times already. See my first post. A bent rod could also do the same thing. The point is that any change in stroke could increase or decrease compression values.The main point to the original post I made was that there would be visual and audible symptoms to glean diag info from if it were a bearing issue.