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2.9 low idle


FredrickPatton

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So I have a 90 ranger with a brand new 2.9 and it runs great except once its warm and not all the time, when idling the idle will drop to about 500 rpm. It doesnt ever die just low idle. And while driving it runs great. Ive gone through most everything possible and ive come to the conclusion that its the iac valve but im not sure. Anyone have any ideas.
 


RonD

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Yes, could be IAC Valve if it is older.
IAC Valve uses a Stepper motor, the computer can send it voltage pulses, like Morse Code, to call one of its 100 "steps".
Step 1 might be rotate 5deg
Step 30 might be rotate 260deg
Step 80 might be rotate 400deg

Each Step opens, or closes, the air valve a specific amount, the computer "learns" what steps set what RPM.
The computer gets the RPMs from the distributor and TFI module.

But it is odd that the computer is not raising the idle back up after it drops, it may be trying to and the IAC Valve is simply not responding.
IAC Valve could also be losing power, IAC Valve closes all the way when key is off(no power), check the wiring at the connector.

When engine is cold the computer opens IAC Valve more, to set idle at 1,000rpms or a bit more, it is temperature based.
As engine warms up computer closes IAC valve more and more corresponding to the the engine/coolant temp, until "Target" RPM for warm engine is set, manual trans target is usually 600-650, automatic 750-800.

ECT(engine coolant temp) sensor is how the computer gets the engine temp, this is a TWO WIRE sensor only used by the computer, dashboard temp gauge uses a ONE WIRE sender, these look similar but are not shared.
I wouldn't think the ECT sensor could cause the drop in idle since idle drops below target idle, if idle was going up then maybe.
 
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FredrickPatton

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Ok that makes sense. I did run the computer for codes and an ect code came up so I replaced the sensor thinking that would do it but It must not have. The code said ect out of range wich probably should have been my first clue that it wasnt the sensor lol. Any ideas on how to truble shoot why its not responding?
 

RonD

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Frayed or corroded power wire on the IAC valve.

IAC valve spends most of its time at target idle position when engine is running so that is where it would wear out.
It would be worth a look could be the valve itself is loose and sliding too far, or just dirty and sticking.
When engine is cold remove IAC Valve, plug in the wires, turn on the key
Valve should open all the way, wiggle the wires, see if it closes.
Unplug the wires and it should close, then plug back in and it should open.
 

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