high idle issue.

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Sep 21, 2020 | 08:15 PM

powersfab

Thread Starter

New User

Joined: Mar 2009

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high idle issue.

my 92 4.9 never comes off hi idle.
no vacuum leaks found.
coolant temp sender ohms out correctly.
if i un-plug the idle air bypass rpm drops to lo idle.
any ideas?

Sep 22, 2020 | 01:25 PM

jas88

Lead Driver

Joined: Feb 2002

Posts: 7,968

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From: Greater Austin, Texas

IAC valve is malfunctioning. Is it a Motorcraft valve? The aftermarket ones that some folks use are crap in my experience.

Sep 22, 2020 | 05:26 PM

ZombieF150

Elder User

Joined: Sep 2018

Posts: 735

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From: NC

Has someone janked with the IAC set screw? thats a no-no , if not that the IAC is probably in need of replacing.

Sep 23, 2020 | 06:50 PM

My4Fordtrucks

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Joined: Jun 2015

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Quote:

Originally Posted by ZombieF150

Has someone janked with the IAC set screw? thats a no-no...

IAC set screw? There is no such thing that I’m aware of. Are you talking about the throttle stop screw?

Sep 24, 2020 | 08:52 PM

ZombieF150

Elder User

Joined: Sep 2018

Posts: 735

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From: NC

Quote:

Originally Posted by My4Fordtrucks

IAC set screw? There is no such thing that I’m aware of. Are you talking about the throttle stop screw?

Yes.. my bad. Too many hours working and not enough sleep.

Sep 25, 2020 | 07:27 AM

rla2005

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Joined: Oct 2005

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From: Kentucky

When you unplug the IAC the idle RPM should drop way below normal or stall out. If the idle RPM remains high or what would be consider a normal idle RPM then you could have a vacuum leak and/or someone has previously jacked with the throttle stop screw.