Electrical and Vacuum Testing the EGR Valve's Control Solenoid
 

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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The easiest way to see if the EGR solenoid is working is to simply start the car and let it idle. Then remove the vacuum line from the intake manifold that goes to the EGR solenoid and see what the vacuum reading is. If you have good vacuum then the solenoid is pretty much working as the EGR is almost totally open at idle.

Now that being said, the mod your son did should have disabled the EGR correct? If that is the case then you should *not* have any vacuum on the hose going to the EGR at all.

Honestly it seems that your EGR valve is bad, it shouldn't be allowing pressure from the intake to escape from that weep hole.