I actually have a roasted EGR modulator valve here on my desk.
I had a clogged catalytic converter that caused excessive exhaust back pressure, forced the bottom hose off of the EGR modulator valve. It resulted in spewing hot exhaust gases directly into the valve like a blow torch and drilled a hole right through it.
The vac input labeled R goes into the valve and actuates it.
The vac input labled P goes right on through to Q.
If I understand it right, the valve under the intake that connects back to Q acts as a defeat.
A link to a thread with pics and links to other threads that explain how it works better than I could ever do in one post.
http://forum.ih8mud.com/80-series-te...-meltdown.html