It HAS to be open circuit at idle, and so 5V, the ECU looks for this signal to activate it's idle and overrun strategies, you could unplug the TPS and start the car, as with it unplugged it's akin to how it SHOULD be set at idle.

When the throttle is opened slightly the switch should close and the voltage drop progressivly the further the the throttle is opened.

There is no point in even attempting to get it running properly from what you mention, with the CO that high you will be washing your bores of lubricant, this can also cause other running problems!

Martin