The "other users" on a 210 are the climate control damper actuator pods. There is a "harmonica" valve behind the dash, that controls the pods. I suspect you have a leak in the climate system. I think you can pull the glove box liner and find the harmonica like you can in a 124. Then it's time to break out the MityVac.

I suspect if you cap off the line heading to the passenger compartment your EGR function will return to normal.

The 210 does not use vacuum to run the transmission like earlier models, it is all electronically controlled from the computer.

Door locks and rear headrest dump come off the PSE pump under the back seat on a W210.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech