I had the same problem also on my 1994 previa supercharge

Watch the video. Third Brake Light Bulb Change Toyota Previa 1991 1997 1993 Toyota Previa...

Check the air filter?

I would check the idle air control valve before replacing anything.
The computer regulates the idle speed by controlling the amount of air entering the intake thru the throttle body.
The valve could be faulty, or the throttle body can be gummed up.
Is the check engine light on ?

I replaced my drive shafts on my 96 Previa LE SC, so I know it can be done. If you don't know the difference between the auxiliary drive shaft and the front or rear drive shafts, then you probably want to have someone do it for you. The auxiliary drive shaft coupling replacement is a difficult job. It is covered in manuals available in auto parts stores. $20 for the manual is worth it if you decide to do it yourself.

well if you are loking for the v.i.n. it will be lower driver side corner of wndshield and on sticker inside driver's door jamb. if it's the engine code your looking for this is it 2.4 L 2TZ-FE I4 2.4 L 2TZ-FZE I4 supercharged. if your previa is a 1995 it would be the supercharged model as that was the only engine available that year. if this was helpful please rate 4 thumbs.

i think that the computer may not be the right one for your car have the supplier check again

Replace supercharger to fix.

This link should give you the info you need. If I knew the engine size I would just tell you :)

http://autorepair.about.com/library/firing_orders/bl-toyota-firing-94.htm

Joe

Check your air intake manifold hose if is crack this cause whistle noise replace the hose.

I do a lot of work on Toyota Previa before; including rebuilt engine, swapping engine and transmission and lot of things in between, but never swap a normal aspirated engine to a supercharged engine or diesel. Since i have some experience in Previa, I would like to share my opinion with you regarding your plan to swapping the normal aspirated Previa engine to a Supercharged engine. When you do a swapp like that, you will not only be dealing with just swapping engine, but some wiring, computer, throttle body, air cleaner assembly, extra frame and pulley on the accessory items, transmission as well as a right rear end to get the correct gear ratio to accomodate the new engine and transmission. I can go on even more, but let me tell you this, beside less headache, it would definitely cheaper if you were to find a Supercharged Previa and fix it up. However, if you are totally motivated to do this project, keep in mind, this engine swap will be a long term and time consuming project.l I wish you best of luck and have fun!