Sense existing vehicle TPS with AVR ADC
Or is this a function of the internal pull-up resistor?


More than likely it's the internal pull-up resistor you have on the AVR. If you couldn't remove that pull-up (for whatever reason) I'd consider using an op-amp as a unity gain buffer. Choose an op-amp that can run from a 5V supply, and has rail-to-rail inputs and outputs for least problems.



Don't forget to wire 5V power and ground to the op-amp. Maybe an AD8605 could be a good start - it's over the top for what you want but will give you an idea.