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96 F150 5.8 High Idle... and I've replaced everything! Please help!

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96 F150 5.8 High Idle... and I've replaced everything! Please help!

Old 11-26-2011
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10-4....

I appreciate all the input, folks!
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Old 11-27-2011
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Originally Posted by Takeda View Post
If you unplug the IAC, does the idle decrease?

Yes....
I unplugged the wiring connector and the idle is normal.

This is a new IAC.


So.... something is telling the IAC to energize?
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Old 11-27-2011
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Here is the ECT (Engine Cooling Temp) Sensor














The block is about 60 deg F.

The resistance is 21K
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Old 11-27-2011
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The 22k reading is normal.
I bought another sensor and it reads the same.

As you heat the sensor, the resistance goes DOWN.

I think I'll check the connector for voltage. NO sensor connected (if the wire was broken) would mean excessive heat... unless the computer sees it as a failure.


CORRECTION.... No sensor would be infinte resistance which would be the COLDEST indication.

Last edited by DEET; 11-27-2011 at 05:28 PM.
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Old 11-27-2011
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With key on, but not running, the ECT connector has 4.66 VDC.

This would mean the wires are not broken going to the sensor.



So what other items would tell the IAC to engage?
The AC is not on.
The ECT is good.

?????
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Old 11-27-2011
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Originally Posted by DEET View Post
Yes....
I unplugged the wiring connector and the idle is normal.

This is a new IAC.


So.... something is telling the IAC to energize?

I don't think so. When the IAC is unplugged the engine should idle extremely slow, to the point of almost stalling. You may want to check the voltage on the throttle position sensor again, to make sure the throttle is going to the rest position.
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Old 11-27-2011
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Originally Posted by DEET View Post
The 22k reading is normal.
I bought another sensor and it reads the same.

As you heat the sensor, the resistance goes DOWN.

I think I'll check the connector for voltage. NO sensor connected (if the wire was broken) would mean excessive heat... unless the computer sees it as a failure.
If the sensor was not connected, this would cause an infinite resistance, which would be in the COLD direction, not HOT.
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Old 11-27-2011
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Originally Posted by DEET View Post
With key on, but not running, the ECT connector has 4.66 VDC.

This would mean the wires are not broken going to the sensor.



So what other items would tell the IAC to engage?
The AC is not on.
The ECT is good.

?????
Did you measure the 4.66V across the 2 connector pins with the connector off the sensor?
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Old 11-27-2011
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Measured the 4.66 with the sensor disconnected.


Thanks for pointing out the "infinite resistance" mistake....
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Old 11-27-2011
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I double checked the TPS sensor and voltage (1.01 VDC) before I bolted the throttle body back on. The throttle cables didn't move the throttle arm. I opened and closed the throttle and watched the voltage change.

It always went back to 1.01 VDC.


Is the IAC "variable" or is it "two position"?


Thanks again for all the help....
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