Toyota goofed with the position of the IAC valve in the 1st. gen Sienna - it's on the bottom of the throttle body, where gunk from reversion pulses accumulates, and clogs the valve. Should have put it on the side of the throttle body, like most other cars. A clogged IAC valve will produce low idle, engine may die unless you press on the gass pedal, and it may be intermittent. You do NOT need to replace the valve most of the time - just clean it. You will need an IAC valve gasket, and a throttle body gasket. Once you have the throttle body off, the IAC valve is on the bottom, held on by four Phillips head screws, which may be stuck. I used a Craftsman hand impact driver to remove them. Then, take the IAC valve out, and clean it thoroughly with throttle body cleaner spray, a toothbrush, and paper towels. Do not clean the electric motor, and do not lubricate any thing. While you're at it, thoroughly clean the throttle body. Then, put it back together. Removal of the throttle body allows you to reach the rear spark plugs from the top, if you remove the windshield wiper cowl, so it's a good time to change the spark plugs. Like another poster stated, you do not need to remove the intake manifold plenum.