2005 4.0 in a sport trac

[variable70]

Trying to determine why a 4.0 will not idle. It starts up and runs fine but as soon as you let off the gas pedal it cuts off.

 

[RonD]

IAC Valve is not working, clean it and make sure its is plugged in

If you want to replace it you can only use Motorcraft or Hitachi brand IAC Valve

 

[variable70]

Ok Thanks. It's a new IAC. I will check what brand i know its not motocraft. Probably a napa brand.

 

[RonD]

Ford's IAC is a solenoid type valve, creates a magnetic field around a metal rod that has a spring holding it "closed", by varying the voltage the magnetic field is increased and decreased, causing the rod to slide/move and the valve to open and close
The valve controls an air passage that bypasses the throttle plate, computer uses this to set idle RPMs
Its accurate +/- 4 RPMs, so quite accurate
Other brands use Stepper/solenoid type valve, they will cause odd idles used in Fords

There should be 2 wires
The red wire should have 12volts(battery volts) with key on, measured to engine or battery ground
The other wire goes to the computer, its a variable ground, computer Pulse the Ground to vary the voltage, called PWM(Pulse Width Modulation)

You can put volt meter between the two wire and should see about 10-11volts with key on engine off, computer is opening IAC Valve for start up
Engine running, wires connected should be about 7volts
Computer has cold engine and warm engine RPM "target" levels
Cold can be 1,100rpms and then as engine warms up it drops to 800rpms for automatic trans, 700 for manual
Voltage changes of course, and computer "learns" what voltage sets what RPM over time and remembers
So if battery is unhooked longer than say 1/2 hour or a new IAC Valve is installed the idle may wander a bit until computer "relearns" correct voltage again

But engine should never stall

 

[variable70]

Ok thanks for the explanation. I will check those voltages. So, if the voltages check out it's probably the IAC even though its new ? Or, if the voltages check out, one of those off brands would cause weird idle but not stall?

 

[RonD]

Hard to say what symptoms a non-motorcraft IAC will do, wandering idle for sure, stalling maybe

3rd party use stepper motors, they are great, and Ford did start to use them for EGR valves, but they don't react well with Fords Pulse width modulation(PWM) setup used for IAC Valves

Stepper motors are like a "digital" motor, they will rotate a certain degree based on voltage input, and that moves the IAC valve in and out, and its very accurate, but not what Ford uses, lol, many other car makers do use it
Ford uses a magnetic field to pull the valve open or let the spring close it, a solenoid setup, PWM varies the voltage to hold idle at "target" RPM constantly varying the voltage slightly to maintain the "target"

A stepper motor needs a set voltage and it will turn to that point in its rotation and stay there until voltage changes
Solenoid moves based on voltage and voltage can vary a bit without too much of a reaction

Ford computer "Learns" the IAC voltage needed, say 7.0v for 750rpm, and 6.9v or 7.1v is OK as well
On a stepper motor 6.9v, 7.0v and 7.1v are different positions
And it will drive the computer "nuts" as it tries to learn/adjust RPMs when the stepper motor is so "jumpy", its not as fluid as solenoid operation
PWM is just not the correct electrical system to use with stepper motors