High Idle
 

1991 F150 5.0L. After truck is warn it idles at 1000 in neutral. Goes down to about 800 to 850 in drive. Seams too high to me. I have replaced TB gasket, throttle control, cleaned idle air control, removed idle air control while running and coved vacuum--no change, checked temp sensor (by unplugging while running--engine increase idle when unplugged), sprayed vacuum lines--no change, took vacuum line off EGR and capped line--no change. Only thing I can think off is throttle adjustment screw, but I have heard that is not something to be doing. Are those idle numbers too high? Thanks for your input.

Never adjust the idle hard stop screw! The computer is supposed to control idle speed, not the screw. Some things I would recheck. Idle seems too high to me. Look at the decal which is on your air filter box. The idle spec should be listed.Recheck for vacuum leak, especially to brake booster and PCV. Carbon on the IAC pintle and within the bore it resides in.A small piece of carbon can bind the pintle. I use an old toothbrush with carb cleaner. I once had a binding throttle shaft within the throttle body. The hardened shaft roughed up the aluminum area on the lower throttle body which the throttle shaft is inserted. The butterfies were not returning back all the way.

There are no numbers because the EEC controls the idle speed. If you maintain the truck properly, it will idle properly. Have you followed the schedule?

https://www.supermotors.net/getfile/...intsched92.jpg
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Do you use only top-quality parts (not cheapos from the zone) and recommended procedures?

https://www.supermotors.net/getfile/...l/hayneses.jpg
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The more we know about the truck, the more we can help you. Read this caption:

https://www.supermotors.net/getfile/...oorsticker.jpg
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If something doesn't "seem" right, DON'T blindly replace parts. If you still have the originals, I recommend putting them back on. Don't disturb any more than necessary to TEST every sensor & actuator:

https://www.supermotors.net/getfile/...nsorvalues.jpg
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https://www.supermotors.net/getfile/...rs22172dmm.jpg
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Ignore the diagram, but read this caption about the idle speed & throttle body:

https://www.supermotors.net/getfile/...rottleplug.jpg
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But the primary mechanism for idle speed is the ignition timing. Is yours set to factory spec (10°BTDC)? Is the timing chain sloppy?

https://www.supermotors.net/getfile/...chainslack.jpg
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https://www.supermotors.net/getfile/...um-87-5.0l.jpg
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If everything else is right, and it idles consistently high, use this technique to check for vacuum leaks:

https://www.supermotors.net/getfile/...tethoscope.jpg
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This caption describes a common one for that engine:

https://www.supermotors.net/getfile/...ntakev8efi.jpg
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