A couple days ago I checked EGR valves on the car when idling and lifted up on the diaphragm. Sure enough the idle speed dropped. I even killed it one time. But, the light comes on after 10 minutes of highway action, the b*st*rd.
Today I took the EGR off, squashed the diaphragm in and threw a vacuum port cap on the nipple. Held tight. Did some chores and the diaphragm was almost all the way out when I got done fifteen minutes later. It's not holding the vacuum by itself. It's supposed to, right? I don't have a vacuum gauge so this was my space-age function test.