I know this has been covered many times, but I have some specific questions about testing an IAC valve. I am on my third IAC valve(one of which was replaced under warranty). I am having high idle problems(only when A\C is off.When the A\C is on the idle works fine.) and have tried about everything else(testing voltage on the TBS, checking and replacing the IAC valve, replacing the PCV valve and tube, checking for vacuum leaks and adjusting the stop screw on my 75mm BBK throttle body.) I replaced the new IAC that I got from Advance Auto because some of the posts on this site said that I needed to stick with Motorcraft. I now have a Motorcraft brand installed and it is acting the same way. My question is this: When testing both valves I notice that they are both stuck open, have 10 ohms resistance and when 12 volts is applied to the terminals(positive\negative or negative\positive) I hear a click, but neither valve moves. I plan on taking the Advance Auto one back in the morning and see if the new one works any better. I am just wondering should the valve completely close when 12 volts is applied and then open when current is reversed?

 

If the IAC motor doesn’t respond properly to battery voltage, the device is faulty.

 

The IAC is not a motor, first.
It is a Solenoid operated Pintle valve.
In unoperated position it closes off bypass air flow around the throttle plate housing.
Do you see the difference between open and closed...properly?
In the closed position it is kept that way by spring pressure. Is it ?
Using a small tool to move the Pintle, does it move freely and return to the closed position?
.
Other info:
Having an after market throttle body might pose an issue of set up.
With the IAC installed and electrical connector removed on a fully warmed engine, the Idle should be in the 500 to 550 RPM range.
If not adjust the Throttle stop to get that idle range then leave it alone from that point on. It becomes the base Idle setting the IAC has to work from 'via' the PCM control signal to maintain idle at about 650 +/- 50 rpm. The difference from the base setting is a range to work with so Idle can be controlled. The correction signal is from the Crank sensor measuring crank shaft rotation time.
Be sure you have the correct IAC for the motor or it's action may not be compatible.
Comment: Why a 75mm Throttle Body if on a stock unmodified motor? There is no benefit.
The system does not reverse operating voltage. Closing is by internal spring and a dash pot diaphragm inside the metal housing.
If the computer has been powered down and /or the motor is cold, the start will be affected as a normal cold start and high Idle until the Cylinder head Temp sensor detects hot coolant and Ox sensors ckt goes closed Loop.
You can't treat this as a one part operation when whole system operation is involved.

It appears you do not have an extended understanding what your working with.
Good luck.

 

Thanks for your observation. My lack of understanding was the reason for my post. First, I realize it is not a motor and thought it to be a solenoid. I am now on my 4th IAC valve. They all seem to act the same way. Right out of the box with no power they are stuck open, they all seem to move fine by hand, but adding power doesn't seem to move the pintle valve. I was just really having a hard time understanding the IAC valve. Surely they are all working because they seem to work fine under load by the A\C.

When I set the idle screw on the BBK to 500-550 rpm I get vacuum lock which causes my pedal to stick. I am going to send the throttle body to BBK for repair.

As for as why am I running a 75mm throttle is because I am running a K&N cold air intake, long tube headers and a Flowmaster Super 40 muffler and wanted to get a little more air in system. I am also running a 93 octane custom tune from 5Star with these upgrade programmed in the tune.

 

When the IAC is on the motor running at idle, it is nearly closed or the motor would race high idle.
The computer adjust/modulates the opening as needed to keep idle at about 750 according to what the Crank Sensor detects as RPM and sends that info back to the computer to apply IAC open correction.. The idle value is in a software table and used to compare to for corrections.
It's a Loop function.
It also has to work for cold weather starts for high idle until the engine warms up.
A simple test is with it on the motor and observe what the idle does with the IAC plug connected and unconnected. That's all the testing you need providing the throttle body is not out of spec in some way. What should be seen with this test is a hot motor IAC plugged up = normal idle. Un plugged, the Idle should fall back to what the throttle plate opening is; meaning the IAC should close down the air bypass port.
Only time the IAC fully opens is at power throttle openings well above a high Idle where it actually has no effect and cannot add any more airflow..

Good luck.

 

That makes sense. I think the problem is in the butterfly valve on the throttle body. I backed the idle out with the IAC disconnect and was able to maintain a good idle, but it sticks something awful. I am going to send it back while it’s still under warranty. I think we’re I got off track was reading a troubleshooting guide that told me to unplug the IAC while the engine is a normal operating temperature and the engine should stall out. I realize now that doesn’t make any sense. Thanks for the help! 😊

 

The Throttle plate has to be open some amount or it will get >stuck closed< when the motor cools because the housing around it will shrink smaller as it coos and lock it in place.
Good luck.