EGR shouldn’t hold any vac at all which is what the engine vacuum is for. There’s a solenoid that controls when the EGR gets vacuum or not.
Way to test a bad EGR is actually by blowing into it and seeing if any air comes out. If no air comes out, it’s good. If air does, it’s failing. So I was told by a mechanic who was familiar with 70’s and 80’s smog equipment.
Another way to see if everything is working alright is to push up on the EGR diaphragm and see if the engine tries to cut out.
Now i dont know how to test the solenoid to see if it’s good or bad but I can check some of my literature and see if there’s a way to test it.
Usually NOx is an indicator of the O2 getting lazy.