I have a 92 300SD and 9k on a rebuild engine, by a shop that when out of bussines allready. Some times I hear some knoking after car been running for a while, knoking is not loud and does not do it frequently, every other day maybe. Is there some thing that I need to check to prevent any further damage? or wait for sound to be came more frequently?. Also once I run car on high speed, oil light come ON, also not all time light will come ON, but oil pressure gauge is normal and have good fresh oil 15/40 Castrol. Any advise

given the very general description you have provided, start with all the turning accessories primarily the belt tensioner and visco fan.

use a stethoscope

listen to the engine all over the head and listen to the lower end.

Keep in mind that tapping is different then knocking - oil pressure suggests you do not have a lower end issue.

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Your intro sounds a little scary with a rebuild done by out of business shop and a recent oil light. Do you know what caused the rebuild? Are you noticing any oil usage? or coolant usage? Can you identify what area of the engine the knocking is coming from? A lower tone usually means bottom end and higher pitch means valve/injector area. If the oil light coincides with the knocking noise, I would definitely avoid running it, except for diagnosis purposes, until the problem is sorted out or at least identified.

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Sam

84 300SD 350K+ miles ( Blue Belle )

The oil light on my '91 meant low oil level, not pressure. Is this the case on the W140 also? If so, and the oil level is fine, likely a bad sensor.

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Gone to the dark side

- Jeff

You want to make sure it's not the vacuum pump first of all. Get a stethascope and listen. It is not good when one lets go. The infrequent nature of the noise indicates it is not in the lower end of the engine. Bearings do not heal themselves. Especially not if noise is quite random in nature.

This is one of those things that if you cannot locate the source yourself perhaps a good independent mercedes mechanic should have a listen. I would not continue to drive it much just in case the item making the noise decides to call it a day. You could land up being pretty sorry.

On the otherhand it could be something as basically harmless as a collapsing valve lifter. You still want whatever it is identified.

Thank you every one for your respond. The noise It comes very once in while and you can barely hear it. Unless you are next to a wall or in close parking structure. Oil is safe level and oil pressure on idle is at 1.5 and running goes to 3.

Where is the vacuum pump localed? and also, can you elaborate on the collapsing valve filter? how it collapse?

The vacuum pump is directly in front of the injection pump. A collapsing lifter is no longer able to operate with the oil supply acting properly internally. So when the oil cushion is not present it is described as a collapsed lifter.

Sometimes dirt can get into them for example. Or they can varnish up. When collapsed it is a ticking sound. They can be intermittent as well. These are just almost wild guesses though as you almost have to be present to locate most noises and even then it can be difficult sometimes.

I will take to Mdz shop to look at it, and find out what it is. I will post what the results will be. Thank you guys again.