Thanks for replying. I cleaned the head passages as well as I could
with a small brush/rag. The valve had some carbon build-up, but was not
stopped up. I don't know if it means anything, but the valve leaks
carburetor cleaner past the seat in the closed position. I checked it's
operation with a vacuum pump, and it operates with no leak-down. The
first codes read were, "system too rich", "cyl. 4 mis-fire" and two
codes for "EGR insufficient flow detected". Now the only two recurrrent
codes are the two mentioned in the first post.