One problem related to gas recirculation valve (egr valve) has been reported for the 1999 Toyota Sienna. The most recently reported issues are listed below.
At 44,000 miles we had to have our cylinder reground and this shouldn't happen. We changed oil every 5,000 miles like the owner's manual said. This is ridiculous that it should happen at all! this is a faulty engine. We should have recieved a new engine under warranty, not had the damned cylinders reground. Now, they're telling us to change oil every 3,000 miles! what is going on? we've owned Toyotas since 1969 and never had a problem with them--and certainly not something like this. This is poor quality of the engine and Toyota should give us a new and improved engine to replace this one.