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1991 560sec. 1991 560sel 1969 280SL
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I would imagine it’s positive crankcase ventilation.
 

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Discussion Starter #3 (Edited)
“Blue” area is where PCV hose goes to intake. Red goes to idle valve / cold start injector pipe work. I’m just not sure what it specifically does. After doing an EGR delete, I’m sorting out what’s needed and what isn't. Cam cover will be off shortly so should be able to figure it out.
 

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1991 560sec. 1991 560sel 1969 280SL
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The cold start injector would have a fuel line connected to it coming from the fuel distributor. To delete the egr function all you need to do is disconnect the vacuum hose to the egr valve and plug it.
 

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Discussion Starter #5
In the parts manual it simply called a breathe tube, others have called it a metering orifice ?
Still have no idea of its purpose over and above the breather pipe that runs to the air intake box.

I believe you can thread it out, so will do so and have a closer look.
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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Usually the cam cover is supplied with filtered air for the ventilation.

I don't know what the airfilter looks like but you can see the fresh air lines [34 & 43] for each in the diagram.

Looks as though there could be a 'metering orifice' [63] in one of the images.

M

Screen Shot 2020-07-26 at 7.30.33 AM.png Screen Shot 2020-07-26 at 7.31.06 AM.png
 

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Discussion Starter #7
Thanks for that,

Yes mine had the same hose from intake box to cam cover.

I'm not sure what a metering orifice actually is ? Its about the only thing on the entire engine that wasn't obvious to me in its purpose, everything else was pretty straightforward. Do the V8's have a similar setup ?
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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Yes, that’s a really good diagram

Very clear
 

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Discussion Starter #10
Most engines I've worked on always have a pre throttle plate PCV hose, as does this engine to the airbox, not much more effective than a vent to atmosphere system as far as vacuum. The vent valve feeds directly to the intake ports via the IACV hose which is under full manifold vacuum post throttle plate of course. Still questioning if it is really necessary over and above the vent pipe to airbox. After completing a fairly exhaustive clean up process on this intake I don't want any more crud moving past the valves or intake runners.
 
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