My sister-in-law's 93 Expo just failed the EGR valve functional test on her smog check. The car has also had a history of the check engine light coming on and off, a rough idle, and cyclically revving at slow speeds.
I'm pretty sure all this points to a clogged up TPS sensor in the throttle body. I'm going to clean it out and see if that clears up some of these problems.
Does anybody know how to perform a functional test of an EGR valve?