Would the throttle positioning sensor make a 1984 Pontiac Fiero idle high between 3000 and 3500 RPMs?

it could but, there are many things you should check first before you start throwing parts at you car.some things you can do yourself.check for vaccum leaks by removing you air cleaner assembly(if 4 cyl.) and listen for any out of the ordanary hissing sounds,also visually inspect all hoses related to vaccum.the beautiful thing about a fiero is it was very advanced for its time. If you take it to a garage they can put it on a scan tool and check for codes, they can also watch the sensors on datastream.this should point the finger at a faulty sensor if that's the problem