hi, I'm not sure if this might help. But I remember seeing an article a while ago, possibly on the yahoo group of the supercharged previa's getting black flakes in the coolant and blocking up one of the pipes where it t's off. I'll try finding the article, but here's a pic showing the pipe I believe it is. I used to have a 95 le s/c and the dealership could never figure out it's high idle in the cold.
scunderhood.jpg