I am having an issue with a high idle 1500-2100 depending on load on engine (accessories). When I put it in gear it idles down to 1100 or so. I did ODB1 paperclip diagnostics and came back with a code 12 while checking only ignition.
Code 12: Idle Speed Control motor or Air Bypass not controlling idle properly (generally idle too low) - ISC
While running it displayed the following code
Code 412: Idle speed system not controlling idle properly (generally idle too low) - ISC
So (despite it being too high idle instead of low) I removed and cleaned ISC. No change. Believing the ISC was an original 17yo part I ordered and replaced it... no change. In between each attempt I did disconnect the battery to reset the computer.
The truck has 122,300 miles and is not displaying a check engine light BUT it has to pass local emissions testing (inspection is expired) I got very good deal on the truck because of the idle and a leaking rear main seal. The leak I can deal with but this idle risks to tear up a solid transmission, burn mucho petro, and overheat a strong engine.
Last owner was meticulous about maintaining records and its had thousands of dollars or parts replaced and work done.