are the solenoids interchangeable? meaning. i have a extra rats nest laying around but am missing a egr vacuum solenoid can i use another one off the extra rack in its place? they all have the same connectors i was wondering if they all opened under 12v power....

 

Some are closed w/ 12V power. Try to blow air through one. If you can, it's the type that closes w/ power. The EGR solenoid is the opposite (open when energized).

 

which ones open when energized?

 

Probably the relief and switching solenoids. I think the FPR solenoid is the one that is closed when energized.