Main Knock? - PeachParts Mercedes-Benz Forum

mach0415 · #1 04-14-2010, 04:30 PM

I have been driving my newly acquired 300sd for a few weeks now. I do not remember hearing this before, but between 1200 and 2000 rpms (sweet spot is at 1500 rpms) I hear what sounds to me like a main bearing knock. It is of lower pitch, happens on accel and decel, but seems to be ok at idle and above 2000. You can only hear it inside the car. I locked the throttle in at 1500 rpms, hoisted it up on a lift and listened with a stethoscope. I did not hear it anywhere with a stethoscope. Oil pressure at idle is 2 1/2 bar. I recently did a service on it and found oil to be metal free. Is it possible to have exhaust resonance cause this? I inspected the exhaust, but did not notice it grounding out on the body anywhere, however, it really vibrates at the flex pipe from the turbo at this rpm. Is there anyone else with an SD that has experienced the same concern. Also it nails at idle. Is it possible for it to nail at 1200-2000 rpms?

Service included
- valve adjustment
- lubri-moly diesel purge
- fuel filters replaced
- lube, oil, filter

Aquaticedge · #2 04-14-2010, 04:34 PM

check your oil levels just to be sure. if you can post a vid of it

mach0415 · #3 04-14-2010, 07:31 PM

By that, are you indicating that the oil could be too full, causing it? A vid will not be able to capture the noise , as it is only heard inside and is not actually very loud. It almost sounds like a muffled helicopter might sound (don't laugh at the description, hehe) between 1200-2000 rpms.

Diesel911 · #4 04-14-2010, 11:57 PM

Some thoughts:

It might not be the Engine itself.
Check your Air Filter Housing mount.
How, about the Drivshaft Support Bearing or Flex Discs.

Get it up to speed and try tapping on the Brake Pedal but not Braking to bleed of a bunch of Vacuum and see if the noise shows up. If it does it mignt be the Vaucuum Pump or what drives it.

mach0415 · #5 04-15-2010, 04:34 AM

Thanks, I'll try those today and keep you posted.

mach0415 · #6 04-15-2010, 03:14 PM

Ok so I tried all of those.. with someone in it while I was under the lift to see if there was any change during the following:

I took he air cleaner off and ran it. The mount was checked. I checked engine and trans mounts, exhaust, listened again with a stethoscope, checked the flex joints (found the center support bearing very sloppy and will need to be replaced), tried prying exhaust, engine, trans mounts, loosened the trans mounts to see if they were stressed - - all to no avail. I unplugged the vacuum pump and listened - no change. I listened to the engine, but all I can hear is diesel noise, no lower end knock. But, I can hear a knocking noise inside the car and it is RPM specific between 1200-2000 rpms. Above and below is great. The car has good power and good oil pressure, too. I hate to wait until something really happens, and I do not want to brush it off as a resonance. I do notice that at those RPMs it has a shake as if there is a power imbalance - but it does not miss. I am getting ready to pull what hair I have left out!

rrgrassi · #7 04-15-2010, 03:16 PM

What condition are the engine mounts and shocks in?

mach0415 · #8 04-15-2010, 03:18 PM

They look good, there is no grounding out on metal to metal. I tried to pry them to change any stress, but there was no change. How would I know if the shocks are bad? Could this cause an RPM specific shake and resulting noise?

mach0415 · #9 04-15-2010, 03:21 PM

Also, would one likely hear a main knock over diesel noise outide the vehicle anyway?

rrgrassi · #10 04-15-2010, 03:24 PM

It can, and yes you should be able to here a main or even a rod knock.

Does this happen when driving or when sitting still and holding the engine RPM steady?

mach0415 · #11 04-15-2010, 03:33 PM

It happens both while driving and sitting in park/neutral. The sound is the same in all situations.

rrgrassi · #12 04-15-2010, 03:37 PM

Have you checked the vacuum pump?

Also, does the engine run well, like no injector nailing or fuel starvation?

An engine knock tends to be heard at all RPM's, including idle.

mach0415 · #13 04-15-2010, 03:40 PM

Yes, the engine has some nailing, although I did a diesel purge recently. I listened to the vacuum pump, but I did not hear any knocking. Can injector imbalance cause this type of issue? The engine runs strong. As for fuel starvation, I am not sure. I have listened to youtube vids from others about knocking - rod bearings, but it sounds nothing like those. The mystery is that I only hear it inside the car. Perhaps the cabin filters out the most obvious diesel engine noise. This is not a sharp metalic noise, it is more like how a helicopter sounds going overhead - a deep bumping noise.

rrgrassi · #14 04-15-2010, 04:05 PM

Is it rythmic with the shaking? You should not have engine shake at those RPM's.

How is the tranny mount? Also check the exhaust system and it's hangers.

tbomachines · #15 04-15-2010, 04:23 PM

I do not think it is a rod knock, you would be able to tell at all RPMs and the thing would not sound very healthy...

I have ad a "knock" of sorts and it is almost exactly as you describe a helicopter going overhead, and only heard inside the car. I couldn't pinpoint it directly, but I am about 90% sure that mine is a return fuel line loose and pulsing against something at those RPMs. The only other thing I narrowed it down to was the Flex plate in between engine/trans, not the driveshaft flex disc (commonly confused terms). My flex plate looked good upon inspection though, so unless there was hidden damage, it can pretty much be ruled out.

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