One problem related to engine and engine cooling has been reported for the 1999 Toyota Sienna. The most recently reported issues are listed below.
Oil sludge. Started blowing at stop lights and all startups. Calling Toyota this week. This Sienna is in mint condition. We bought it for safety and dependability for our 7 week old son. Now we'll see if Toyota stands behind it. This is our 1st Toyota,but there are 7 Lexus/toyotas' within our families. This engine is damaged and from what we have read,expect a complete new one.