What is this vacuum hose do??
 

what does the blue vacuum hose that is coming out of the turbo do? on my car (this is not a photo of my car, just a nice example....) this blue hose is not connected (infact, its not even there- must have broke or something) and the rubber L-hose connector to the turbo is not plugged or anything. Does anyone know what the Blue hose does and should I do anything because it is not there on my car?thanks
Scott

Vacuum line to the Air Recirc Valve. You can leave it off, cap both ends, if you can find the other end.

That's the signal line to the air recirculation valve. MB feels that there are some situations in which it's appropriate to leak boost from the compressor outlet back to the compressor inlet.

I don't know where the signal comes from in an SD but if you're going to leave it disconnected, check the inside of the ARV to make sure the plunger seals properly against the compressor housing port. Otherwise boost will leak and you'll rob yourself of power that's duly yours.

Sixto
95 S420
87 300SDL

looks like a 1985 engine. That was added that specific year, there are slight differences in the CA cars. I believe that blue vac line modulates the turbo outlet. Also the 1985 diesels have an EGR "computer" (well, its an analog module) that feeds some of the emissions stuff as well as driving the tach on the clock in the instrument cluster. That makes for a smoother shifting transmission too.
I never set down to understand how it all operates but its there and the car usually drives fine without it! I believe its fail safe, ie. nothing is prevented from operating properly and causing the car to be inoperative. Very unusual compared to today's electronic nightmares!

Ah yes that's what its called, air recirculation valve. I think its the cause of a certain high pitched noise that all the '85s make and can be cause for concern, not me I always liked it mixed with the whine of the turbo it sort of sounds like a lear jet .

thanks guys,
after some more research on the Mshop, now that I know the name of the ARV I have come up with a lot of great info. I think I will pull off the cover to the ARV and permanently lock it shut so it never opens again. anyone want a blue vacuum hose??
thanks
Scott