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High Idle on a 1993 F-150 (5.8L)

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Old 07-07-2010, 02:24 AM   #1
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Default High Idle on a 1993 F-150 (5.8L)

I just purchased my first Ford truck and joined the forum.... and now I am in a bit of a pickle....

The truck:
1993 F-150 SWB XLT RC w/ 351, auto, & A/C


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.

What's left?

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.

Any advice is good advice at this point.
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Old 07-12-2010, 08:45 PM   #2
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Try a new throttle position sensor
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Old 07-28-2010, 01:54 AM   #3
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Is it losing power or making a slight whining noise when driving? My brothers truck is doing the same thing and the cat is clogged.
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1993, 351, 5.8l, idle, isc

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