Two problems related to engine exhaust system have been reported for the 1998 Toyota Sienna. The most recently reported issues are listed below.
Idle air control valve failure caused vehicle to stall at intersections. To keep from stalling throttle had to be applied with right foot while left foot used to apply brake.
At only 30,000 miles, while vehicle still under full bumper to bumper warranty, smoke began emitting from exhaust pipe. Took to dealer - dealer says it is steam. Next smoke gets worse (its blue) & oil light flashes when braking. Take it back to dealer - dealer says need oil change. Do it. Next - same stuff. Take it back. Dealer says oil leak in valves or rings - will call to set up appt. For repair. No call. I call dealer. Dealer says want oil change receipts. I get them. Dealer says no fix under warranty, not covered, caused by consumer lack of maintenance. I complain to manufacturer. No luck. I look on internet - hundreds of identical complaints on various web sites. Caused by sludge in engine, which analysis of shows water & fuel in oil. I contaact you.