Vacuum only reaches the EGR valve when the solenoid closes; otherwise #16 is at atmosphere pressure through #17 hose. #17 attaches between the air filter and throttle body.
One thing I did when testing my code 12 was plug #17 at idle; idle would become rough and eventually the engine would stop as the EGR valve actuate.
#17 is connected to the EGR solenoid (a cylinder-shaped metallic thing with two hose connectors -one on each end); and the other end of the solenoid goes... to a plastic cylinder inside the box if I remember right. When the EGR solenoid closes, then the vacuum produced by the flying-saucer thing would actuate the EGR valve through that plastic cylinder.
If he has vacuum inside the box, then the next step would be whether the disk-shaped thing (can't recall the f... name) inside the box provides steady vacuum (6-8 in HG) for accurate EGR operation.
Oh, and I used the 1989 manual for the testing; the EGR system is identical. Remember, though that there are other things inside the black box and those may differ from 86-87 to 88-89.
Please check with the manual as I'm writing from memory (I have not opened the black box in quite a while) Hope this helps.
I may go to the jy tomorrow morning so I might go hunting for a working EGR if you eventually need one. They run cheap if one knows how to find a working one... let me know.
I also have the flying-saucer thingy, the plastic cylinder and probably the EGR solenoid valve too. I took them from another 3G just in case and I remember they worked -swapped back and forth with mine. I have no use for any of those; I only have to find them.
That reminds me that I should have posted how to diagnose the EGR valve itself...

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