How can I test this thing to make sure it's working? I think Mercedes calls it y31/5 or maybe y31/1.

My son who owns the car did an EGR resistor/diode mod. So it's hard to say that all is ok with that.

He also had the intake off recently. We just pressure tested the boost system and I don't believe there are leaks. But... the EGR diaphragm has a weep hole toward the front of the car. Without vacuum, pressure escapes here. With vacuum, the intake/boost plumbing seems to be airtight or very close to it.

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-E300d '99 350k
-Suburban '93 220k
-TDI Jetta '03 350k
Sold
-F250 '96 7.3
-Dodge Ram 12V
-E320 '95 200k
-E320 Wagon 1994 155k
-300d Turbo '87 187k miles
-E320 1994 200k
-300d Turbo '84 245k (sold to Dan62)
-300d Turbo '84 180k
-300sd '80 300k
-7.3 Powerstroke Diesel 15P Van 500k+ miles
-190d '89 Non Turbo 2.5 5cyl 240k (my first MB)
Tom's Imports of Columbia MO Ruined the IP in changing leaky delivery valve O-Rings - Refused to stand behind his work. Mid-MO MB drivers-AVOID Tom's.