nothing fixing a high idle?
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New User
Joined: Aug 2010
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nothing fixing a high idle?
Hey guys, i just picked up a 92 f150 5.0 auto 190k miles. it has a really high idle like 1800 rpm. I just replaced the IAC today because when i unplugged it nothing happened just kept idleing fast. Well when i started it up it ran up to high idle then slowed down to about 1000 rpm, then as it warmed up it started idleing higher right back up to 1800 rpm or so.
I then ran propane around all lines and gaskets looking for a leak, found nothing. So just out of curiosity i pulled the line off of the MAP sensor, it bogged for about 1 second then jumped up to about 2500 rpm. I then plugged it back in and it idled back down to 1800 rpm. What is going on?
If you pull the line off my 88 bronco it instantly runs bad and sounds like it has a monster cam in it. The 92 didnt do this at all.
It runs fine other than the really high idle?
I then ran propane around all lines and gaskets looking for a leak, found nothing. So just out of curiosity i pulled the line off of the MAP sensor, it bogged for about 1 second then jumped up to about 2500 rpm. I then plugged it back in and it idled back down to 1800 rpm. What is going on?
If you pull the line off my 88 bronco it instantly runs bad and sounds like it has a monster cam in it. The 92 didnt do this at all.
It runs fine other than the really high idle?
Elder User
Joined: Feb 2006
Posts: 537
Likes: 1
From: Formerly Nawlins
First time mine was the A/C vacuum lines worn through, those hard plastic lines just don't hold up like they were designed to ... after 17 years mine got brittle.
The next time was when I replaced the IAC, I was sold the wrong one by the dealer for my vehicle based on the info I provided without the old part. Turns out my never been above the Mason/Dixon nor more than 20 miles west of the Mississippi River truck has the California IAC in it???
The next time was when I replaced the IAC, I was sold the wrong one by the dealer for my vehicle based on the info I provided without the old part. Turns out my never been above the Mason/Dixon nor more than 20 miles west of the Mississippi River truck has the California IAC in it???
Fleet Owner
Joined: Oct 2005
Posts: 20,786
Likes: 1,751
From: Kentucky
If the idle did not change with the IAC unplugged then there is a serious problem. A vacuum leak is the first suspect, the other is checking to see if someone has jacked around with the throttle stop screw. With the IAC unplugged the idle RPM should drop down in the 500-600 RPM or maybe a little lower.
If someone has jacked with the idle stop screw then I would suspect the TPS voltage at closed throttle to be high as well. If the TPS measures between 0.6-1.0VDC with the throttle closed then I would start looking for a vacuum leak(s).
If someone has jacked with the idle stop screw then I would suspect the TPS voltage at closed throttle to be high as well. If the TPS measures between 0.6-1.0VDC with the throttle closed then I would start looking for a vacuum leak(s).
Freshman User
Joined: Feb 2011
Posts: 32
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From: Pensacola, FL
Just throwing out some ideas...
Carbon keeping the EGR valve from seating fully is described as a vacuum leak because the ECM thinks the valve is closed, so it doesn't do it's normal tuning to compensate for the added, unmetered air.
Has it done this since you got it?
Maybe a previous owner fiddled with the throttle stop screw?
I have had issues with that before.
How about timing? Have you checked it?
How about the crankcase vent system? PCV with a sealed oil cap... Good valve? Right valve?
Carbon keeping the EGR valve from seating fully is described as a vacuum leak because the ECM thinks the valve is closed, so it doesn't do it's normal tuning to compensate for the added, unmetered air.
Has it done this since you got it?
Maybe a previous owner fiddled with the throttle stop screw?
I have had issues with that before.
How about timing? Have you checked it?
How about the crankcase vent system? PCV with a sealed oil cap... Good valve? Right valve?
Thread Starter
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New User
Joined: Aug 2010
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found the problem, its the upper intake gasket. What is the best way to get the inside center bolt out of the upper intake? and what is it?
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Fleet Owner
Joined: Oct 2005
Posts: 20,786
Likes: 1,751
From: Kentucky
It's a Torx T-40. I found it in this thread: https://www.ford-trucks.com/forums/1...enum-bolt.html
I have used a 1/4" drive Torx bit on a long extension to remove/install that center bolt.
Glad you found the root cause.
I have used a 1/4" drive Torx bit on a long extension to remove/install that center bolt.
Glad you found the root cause.
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