So I've had a high idle for a good 3 or 4 months now and have been picking away at what the heck it might be, now it's getting to be getting worse. When I start the truck it immediately revs up to 2000 then settles to about 1700. once I put it in gear(automatic) it drops down to 1100 and I drive away and everything is great. Once I get to my destination I put it back in park and it goes back up to its 1700 or so RPM.

This is a 1993 5L 302 4x4 with a little over 350,000k. I've had the truck for 3 years now and bought it for cheap and am slowly putting some money into it. Other than the idle issue it runs great.

A fella from work scanned it for me: 411, 412, 538, 632.
I was really hoping for a TP sensor or something of the like that would be a nice easy fix but that's not what we got. I really don't know what to think of these other codes since they don't give me any concrete answers.

411: cannot control RPM during KOER low RPM check
412: cannot control RPM during KOER high RPM check
538: Operator error
632: Overdrive cancel switch not changing states (E40D)

Any help or advice would be appreciated, thanks!

 

No sense running the KOER tests until you can see what is happening or passing the Key On Engine Off (KOEO) tests. As part of that process the truck will display any code stored in Continuous Memory (CM).

Unplug the IAC, if the idle drops down and/or the engine stall out you most likely have a sensor caused problem. If the idle remains high you most likely have a vacuum leak(s).

 

X2 on what rla said.
It sounds like the IAC is malfunctioning or the O2 sensor could be fault. But with OBD1 systems, 1 bad sensor can cause other sensors to report faults like a dominoe effect.