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  #1  
Old 01-21-2004, 12:46 AM
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Motor quiet with no oil pressure, but loud once it comes?!

Well not to start another oil thread but what is the deal with this? My car sounds like a gasser before the oil pressure builds but once it does sounds like any other diesel. Surely this can't be lifter tick could it? Why is it so quiet without oil but so loud with?

Thanks.

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Old 01-21-2004, 12:59 AM
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the 617 engine doesnt have hydraulic lifters...

Interesting question though, I dont experience this, so would liek to read replies. I can only think that if the oil flows back, you have metal to metal contact that is continuous, because just the antiwear additives are protecting. Once the oil pressurizes, and things have a film of oil, everything can knock around on its cushion of oil, so instead of continuous contact it is only sporadic, thus the knocking noises.

Or, if the timing chain is tensioned by oil pressure, perhaps when loose and untensioned, something slides, etc., but when tighter, you hear the teeth of the gears hitting each other instead of the chain just flopping onto the teeth?

But thats just a far out late night guess.

JMH
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  #3  
Old 01-21-2004, 01:04 AM
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Oddly enough, I've noticed the *exact* same thing the last week or so, probibly hapening before then, just occured to me though.

It takes from 1.5 to 3 seconds to peg the oil preasure gauge on cold start, and if you give it about ten seconds after the GP light goes out, it starts and runs smooth. And it's SO quiet, like a gasser, as he said, idle is also a little low for a few seconds.
Soon as the oil preasure goes up though, here comes the cold clatter, which deminishes some after it's warm. Very strange.

I know IP timing will effect the sound of a disel, could the oil preasure tensioner on the chain be related to this? First diesel I've seen do this, my VW's actually sound the same, oil preasure or not.. :p
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Old 01-21-2004, 01:09 AM
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Perhaps it is the IP causing this, it doesn't have enough oil before but afterwards it clatters away? Seems like I heard it is the cause of most motor noise before, quite something....
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  #5  
Old 01-21-2004, 01:30 AM
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Could be.. I've known guys that could set the IP timing on VW diesels by ear, accuratly. Talk about experienced...
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  #6  
Old 01-21-2004, 07:54 AM
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Maybe those idle moments of pleasure reading in the no plot Factory Shop manual have paid off....

In the 616/NA 617 manual it says that if you have " oil noises" ( which I have no idea what they were referring to " at start up then you have the option of screwing in a different ( and implied LATER ) model oil relief in the oil pump. They come with differnt size holes, pointed different directions, and at different BAR ratings....
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  #7  
Old 01-21-2004, 09:18 AM
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Maybe this has nothing to do with the oil pressure at all, and is associated with the glow plugs being on for a few seconds of operation after the engine starts. Once they go out the usual Diesel combustion process takes over and you get the noisier and clankier Diesel typical clattering and clanking?

Sounds odd, but not like an oil pressure cause and effect to me. Good oil films and chain tension make things quieter, not noisier. So, as you get the benefits of oil pressure coming on, something else is going away, and it dwarfs the oil pressure benefit. The only thing I can think of is that the combustion process, minus the benefit of a glowing hot plug tip to get it going quickly, changes and you are left with something noisier until the engine completely warms up. Just a theory. Jim
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  #8  
Old 01-21-2004, 09:24 AM
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I agree with Jim, as the Glw Plugs will syat on for some timer after the light goes out and assists in complete combustion. After they no longer glow and the motor is still cold, the block and head is still really cold and taking away from the affect of compression.
My car does the same thing and was concerned until I found this out as well.
Have fun, Adam
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  #9  
Old 01-21-2004, 10:07 AM
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Ok that makes sense since the GPs' heat plus the compression heat making it silent. After all higher cetane in fuel makes it ignite at once and makes the motor much quieter, so this in a way will also make it combust at once because it is so much hotter than normal. Come to think of it, normally the longest it is quiet is after 5 cylinders fire.

My GP relay is probably original so I don't think it stays on after the car starts (it is only the new ones which do this right?), however I'd imagine there is a certain amount of heat they are still letting off even after shut off.

Thanks for the input everyone.
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Old 01-21-2004, 10:38 AM
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Noticed this same EXACT behavior as well. I also came to the conclusion that it was the GP's shutting down that started the noises.

I haven't thought about trying it for some reason, but this should be able to be confirmed by starting the car while the engine is fully up to temp. If this behavior is gone, then our theory should be confirmed, n'est pas?
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  #11  
Old 01-21-2004, 03:41 PM
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How about starting it without the glowplugs ?
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  #12  
Old 01-21-2004, 03:44 PM
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Yes, I did notice this problem as well on my 300SD. I'm in d'accord with Mr. JimSmith regarding the relation between a glow plug relay cut-off and diesel noise. My thinking about this relation is as follows:

When the glow plugs are still red hot and the atomized diesel fuel is forced (by injectors) into pre-combustion chambers at high pressure, the lack of air in tiny pre-combustion chambers will induce a localized but incomplete combustion/explosion process inside pre-combustion chambers. This process will create a MUCH higher pressure of incompletely burned diesel fuel/air, and forcefully rush this mixture into the larger combustion chambers (with piston contact). Thanks to narrow tubes connecting pre-combustion chambers and larger ones, the Bernoulli's Principle kicks in and this will create an extremely powerful jet of atomized diesel fuel to THOROUGHLY and UNIFORMLY mix with the larger volumes of air in larger combustion chambers, then the heat produced from piston compression will trigger the full combustion process in larger combustion chambers. A thorough and uniform mixture of air and diesel fuel is the ideal condition for efficient diesel combustion process (extracting more energy from same amount of diesel fuel). This explains why the diesel noise is relatively low when glow plugs are still on in spite of a cold engine block.

However, once the glow plug relay cut-off kicks in and the engine block is still very cold (quickly absorbing heat generated from previous short combusion process), pressurized and atomized diesel fuel will not be able to spontaneously burn inside tiny pre-combustion chambers, so the speed and pressure at which diesel fuel enters larger combustion chambers through connecting tubes are MUCH less than before. This causes a partial and irregular mixture of air and diesel fuel, leading to concentration gradients (denser near the exit of connecting tubes and thinner elsewhere). A cold engine block and bad mixtures of air/diesel fuel would adversely affect diesel combustion and may dramatically reduce its efficiency. In fact, I believe that combustion may not occur in one (or two) cylinder for a couple of seconds after a glow plug relay cut-off, especially on old diesel engines with relatively low compression pressure in some of their cylinders. The result is a diesel engine that may idle badly because the combustion process is not complete and may not take place at all in some cylinders. Of course, when the engine block is warm enough, atomized diesel fuel will spontaneously burn (incompletely) under appropriate pressure from piston compression, and will thoroughly and uniformly mix with air in larger combustion chambers to produce efficient diesel combustion, as described above. Therefore, the heavy noise will go away when the engine is warm enough.


Hi BoostnBenz: I'm pretty certain that the original glow plug relay in your 1983 300D does stay on about 30-60 seconds after a car start. You can easily check this by opening the hood and/or turn on the interior light. Start your car and wait for about 1 minute. Within this minute, you will hear a very distinctive click of the glow plug relay cut-off, and the interior light will become brighter (no more current draw from the GP relay).

Hi ck42: It's "N'est-ce pas?"

Sorry for my awkward English

Eric
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  #13  
Old 01-21-2004, 11:29 PM
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Wow Eric!

Must be a slow day at the office...

Your english is getting much better though..
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  #14  
Old 01-22-2004, 12:38 AM
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This is an excellent theory, except for one point in my case.

The noise picks up in a matter of perhaps five seconds after the motor catches and I remove my hand from the key. I recall hearing the GP relay click a good 15+ seconds after the light goes out normally. I'll try and remember to double check, but I'm pretty sure the GP's are still on when mine gets noisier.

But it's a fine theory, may well be the case.
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  #15  
Old 01-22-2004, 03:18 AM
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Eric gets the prize for hardest working imagination........

I don't know what the cause of the noise is... but I am willing to go on the record as thinking it is not the glowplugs.
Due to the timing of the fuel injection I don't believe there is any significant difference between a power stroke caused by a glowplug ignition compared to normal diesel hot compressed air ignition.

Anyone have a theory as to what the Mercedes Engineers were referring to when they said " oil noises" in the Factory Shop manual ?

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