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1 4th November 23:03
survivalist
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Posts: 1
Default 89 F150 302 fast idle


A couple of days ago the idle went from around 900 to 1800 and stays there.
I had some spares so replaced the TPS and water temp sending unit, removed
throttle body and cleaned it, removed EGR and checked it. Checked for vacuum
leaks, checked PCV valve. No joy. Not a linkage problem by the way. Any
ideas? Thanks.
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2 4th November 23:03
spdloader
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Posts: 1
Default 89 F150 302 fast idle


Does it have an idle air bypass valve on it? It may be stuck open.

Spdloader
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3 4th November 23:03
survivalist
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Default 89 F150 302 fast idle


No, I checked and cleaned it. Solenoid could be bad that operates it though
I guess. I know I can pull the wires off of it and the idle doesn't change.
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4 4th November 23:03
spdloader
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Default 89 F150 302 fast idle


If it's stuck, pulling the wires won't change it, but that doesn't mean it's
bad either.
When you have it off, can you move the valve inside?
I think I'd disconnect the solenoid and start the truck, make engine changes
with the throttle, and see if the solenoid is working.

Good luck man,

Spdloader
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5 4th November 23:03
survivalist
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Default 89 F150 302 fast idle


Yes, I pulled the valve off and cleaned it so I know it isn't stuck. Like I
said, I wonder if the solenoid could be bad? Could I tell by pulling it
off, starting the truck and seeing if the plunger is moving?
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6 4th November 23:03
bob
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Default 89 F150 302 fast idle


If unplugging the IAC doesn't change the idle you have 1 of 3 possible
problems.
1. Vacuum leak
2. Stuck IAC
3. Throttle stuck partly open or someone has improperly adjusted the
hardstop.
An easy way to see if you have a vacuum leak is to pull the rubber inlet
ducts off of the plenum and cover the intake with a clipboard while the
engine is running. If it dies immediately you don't have a leak. If it wants
to keep running you should be able to hear a loud hiss coming from the
source of your leak.
Bob
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7 4th November 23:03
survivalist
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Posts: 1
Default 89 F150 302 fast idle


Throttle isn't stuck and hasn't been adjusted, IAC isn't stuck so I am going
to check for a vacuum leak later today. Thanks.
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8 4th November 23:04
lugnut
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Default 89 F150 302 fast idle


Prime suspect for a vacuum leak that large is the gasket
between the upper and lower intake manifolds. Fairly common
and would explain sudden appearance of condition.
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