The PF01 isn't a straight forward potentiometer it has a switch built in and is open circuit at idle, hence why you need to open the throttle a bit to see voltage, this signifies to the ECU the driver has taken control via the throttle.

Assuming the throttle cable isn't streteched etc, you can adjust the TPS but there awkward to remove, you have to remove the T/B to do so.

Martin