Yesterday I went to work and all was fine but shortly after starting home the idle went to 2800rpm and wouldn't come down. I made it home, unplugged the IAC and the rpms dropped to normal. I figured I'd throw on another IAC I had laying around, started it up and it idled normally. Took the truck to work today and made it just a few miles from work (30 mile trip) when the problem returned. I popped the hood and unplugged the IAC (no change), disconnected the throttle cable and goosed the throttle a couple times to check for binding but nothing. I replaced the IAC with a new one at lunchtime, startedthe truck and it idled normally again but when I left a couple hours later the problem was back. Disconnected the IAC once home and the idle dropped. The weird thing is that I let the truck run for a few minutes (IAC plugged in) and the idle will rise from 800 to 2700 all on it's own.
Reading through a few threads I'm wondering if the TPS is dropping out. The gas pedal also doesn't feel the same: not as much resistance as it used to but that might just be in my head. My Taurus is my DD to help my wallet. Almost forgot to mention that the check engine light has never come on in all of this although it did flicker a couple times. I disconnected the battery and then thought about seeing of there were any codes
'89 F-150 4X4 302/ M5OD-R2

 

Checking out the TPS would be a good place to start.

 

Update for future viewers: A couple weeks ago I swapped out the TPS with an extra I had on hand. Truck has better throttle response and overall runs better.

As for numbers it looks like the sensor is shorted out internally. Tested against another one I have: bad sensor has resistance between the red and green wires that varies when the sensor is turned. The good sensor remains open at all times. Bad sensor has double the resistance that the good sensor has between the green and black wires. As a side note the insulation on the black wire has pulled away from the sensor end and there is a bit of (green) corrosion on the wire.