low motor pressure 300SD - PeachParts Mercedes-Benz Forum

Brian Lesemann · #1 03-29-2008, 12:54 PM

I have a friend who just replaced a rod bearing on his 300SD. The motor starts fine and has good pressure, but as it warms up a knocking sound increases and pressure drops. We suspect the main bearing might be going bad. Does anyone know any other causes for this?

stevebfl · #2 03-29-2008, 01:03 PM

One bearing? Did he replace the rod? A bearing doesn't get bad enough for someone to contemplate replacement without the rod also needing reconditioning or replacement.

There is no conceivable way one can properly repair a rod bearing problem without removal of the crankshaft and rods.

jt20 · #3 05-21-2008, 03:43 AM

Quote:

Originally Posted by stevebfl

One bearing? Did he replace the rod? A bearing doesn't get bad enough for someone to contemplate replacement without the rod also needing reconditioning or replacement.

There is no conceivable way one can properly repair a rod bearing problem without removal of the crankshaft and rods.

^^ What if no knock is present? How bad is 'bad enough'?

#4 05-21-2008, 03:47 AM

Bad enough = audible knocking.

If you can hear the knock you can bet the crank has impact damage from it.

vstech · #5 05-21-2008, 09:12 AM

the cranks on these motors have a surface preparation done to them that a spun rod bearing would remove... not only does this change the size of the crank, but it weakens it beyond the ability of repair without a serious shop... your next step is to remove the offending rod, then the crank, and replace...

jt20 · #6 05-22-2008, 10:52 PM

I have been told by 2 different shops (and one poster on this forum) that nitride hardening goes down far enough to do the first oversize safely. Not that I would want to, but I don't know if I agree that it would just rub off. If it was that mild of a process, it may not serve it's purpose.

Diesel911 · #7 05-23-2008, 12:52 AM

Quote:

Originally Posted by Brian Lesemann

I have a friend who just replaced a rod bearing on his 300SD. The motor starts fine and has good pressure, but as it warms up a knocking sound increases and pressure drops. We suspect the main bearing might be going bad. Does anyone know any other causes for this?

My first thought is did your friend find out and fix what caused the rod bearing to go bad in the 1st place? Next is why the main bearings were not inspected while the crankcase was open? Last thought what was the condition of the crank journal and the inside diameter of the Connecting rod when the new bearings were installed?

Brian Carlton · #8 05-23-2008, 12:56 AM

Quote:

Originally Posted by jt20

I have been told by 2 different shops (and one poster on this forum) that nitride hardening goes down far enough to do the first oversize safely. Not that I would want to, but I don't know if I agree that it would just rub off. If it was that mild of a process, it may not serve it's purpose.

Typical nitriding goes down about .015". The greatest hardness is close to the surface..........within .005"..........it falls off as you go deeper..........finally reaching the hardness of the parent metal at about .015".

It's possible to nitride to lesser or greater depths.........depending on how much time is spent in the furnace.

jt20 · #9 05-23-2008, 01:42 AM

Quote:

Originally Posted by Brian Carlton

Typical nitriding goes down about .015". The greatest hardness is close to the surface..........within .005"..........it falls off as you go deeper..........finally reaching the hardness of the parent metal at about .015".

It's possible to nitride to lesser or greater depths.........depending on how much time is spent in the furnace.

Then one can suppose that it is safe to say that a high quality Mercedes-Benz diesel engine that meets only the highest of quality standards should indeed have a crankshaft, as the crux of it's power transmission system, of only the greatest depth available in nitride hardening capabilities - worldwide.

all in jest dear moderator.

It is great to finally see the actual numbers.

Thecorrupterx · #10 05-23-2008, 05:17 AM

or just a nitride depth enough to suit their needs, wear characteristics and production rate. remember the engineers are paid to meet specifications, not go over them, going over costs $$$

Brian Carlton · #11 05-23-2008, 09:36 AM

Quote:

Originally Posted by Thecorrupterx

or just a nitride depth enough to suit their needs, wear characteristics and production rate. remember the engineers are paid to meet specifications, not go over them, going over costs $$$

Yes, I'm afraid that this statement is probably closer to the mark. Although, 20 years ago, M/B was not quite as cost conscious as they are today.

pawoSD · #12 05-23-2008, 10:37 AM

Even so, the modern engines seem to last as long or longer than the old ones.....so at least they've kept them durable....

jt20 · #13 05-23-2008, 02:00 PM

if only we knew

http://www.longwear-nitriding.co.uk/htm/technical.htm

http://www.key-to-steel.com/Articles/Art117.htm

Also, issues arise after nitriding since the steel swells (addition of matter?) leaving the reground crank at a larger dimension than measured. The only place I found that I would buy a regrind from was Metric motors in CA.