One problem related to crankcase (pcv) has been reported for the 1998 Toyota Sienna. The most recently reported issues are listed below.
Began issuing blue smoke at startup at about 52000 miles. Dealer finds sludge in engine, refuses to repair under warranty, citing lack of maintenance. Vehicle has had regular oil changes, and an older vehicle (Ford escort) in identical service with same maintenance schedule has no problems. Believe that the failure is due to defective engine crankcase ventilation system design, and that failure is leading to emissions in excess of legal limits.