How can I get my RPMs to drop down, because when I start the vehicle the idle is way to high(2-3000rpm)and I have used this hand held computer device and it says that the IAC valve is not setting the rpms right but i just replaced it and it still gives me problems, plus i just replaced the throttle position sensor and it says that the volts are to low or something but that is also giving me a problem, how can i fix that, I want to sell it but before i do i want to make sure the car is in good running condition... please help

Optional Information:
1991 Chevrolet lumina 3.1V6

Already Tried:
changed both sensor and IAC valve