OK, this is gonna be a little long-winded but bear with me.

This is my experience with the non-Ford IAC valves and why they don’t work how they are supposed to. 

A couple months ago, my Ranger developed an issue where it didn’t want to idle below about 900 RPM, which was odd because usually it idols at around 600 RPM once it’s warmed up. After a little bit of research, I replaced the IAC valve aka the idle air control valve.  This initially immediately fixed the problem.  

though maybe two weeks ago I was driving the truck and I pushed the clutch into shift gears and the truck started revving up and holding RPM on its own.    Sometimes while driving, if you took your foot off the gas or pushed the clutch in the RPM would jump up to about 3500 to 4000 RPM and it would stay there for several seconds or until the truck came to a complete stop then the engine speed would drop back down.  But once again it wouldn’t idle below 900 RPM and would typically stay about 1000 to 1100 RPM at idle.    

So I replaced the IAC again, and that seemed to fix the issue. Though the idle stayed a little high around 900. BUT the truck stopped accelerating on its own, so I ignored the high idle problem……Until a week later when the truck began accelerating on its own again. This time I replaced the throttle position sensor And that seems to have healed that issue for me, it hasn’t acted up once at all since I replaced that sensor. 

These trucks have a feature that is supposed to hold the engine RPM for a moment when shifting gears allegedly for emissions reasons is what I read in the forums. When you push the clutch in and take your foot off the gas to shift gears the, the throttle body is closed so the IAC opens to allow more air into the engine so the engine can hold RPM for a moment while you change gears. 

Doing some more reading, the amount of time the computer will hold the RPM varies from source to source some say it’s only a second or two and others say it can be up to 15 seconds depending on the scenario. 

Regardless, there is a potentiometer in the throttle position sensor that tells the computer/IAC what to do when you shift gears or let off the gas. If the potentiometer in the throttle position sensor is bad or reading incorrectly it may tell the the computer to open the IAC far too much, causing the engine to rev way up on its own and hold that RPM for several seconds, because of that stupid feature.  Of course you can combat this by sticking it back in gear and letting the clutch out to bog the engine down or just turning the engine off. Though when this “run away“ situation happens, because this air is only coming through the IAC valve, there is not much volume of power behind it, really it’s just enough for the engine to hold rpm, so it’s pretty easy to keep the engine under control when this happens, though far from ideal.

There are tons of posts on the Internet, talking about their Ford rangers having issues revving randomly on their own when people push the clutch in or let off the gas, but I could not find a single post where someone had actually resolved the issue.  Replacing the throttle position sensor fixed this issue for me, but many people report replacing it not doing anything.  So I’m sorry, but That fixed it for me.  I did see one person mention They believe they resolved the issue by replacing the vehicle speed sensor on the rear axle.

After I replaced the IAC for a second time, and ordered/installed a new throttle position sensor, the truck was fixed other than a returning high idle issue that hangs around 1000 RPM.

Some more reading testing and messing around taught me that the way the IAC works is that the PCM supplies the ground to the IAC valve. The PCM regulates how much ground that Valve sees, the more ground that Valve gets the more it opens and raises the RPM, so because my idol was too high. I knew that either a the IAC was not closing for a mechanical reason or the PCM was asking it to open slightly it to hold the engine RPM around 1000 RPM all the time for some reason.

There is a simple test where you can unplug the IAC if the idle is too high.  A functioning IAC should hold the engine RPM on my truck, which is a 1994 3.0 V6 manual truck , around 600 RPM.    When I unplugged my Parts store I AC valve nothing happened at all. the idol stayed up around 1000 RPM. This means that my PCM was not the issue and it was not asking the IAC to hold this high idle for some bizarre reason.  Which mechanically, doesn’t make any sense, until you start looking at the differences between the generic parts store part and the genuine Motorcraft part.    

With no ground or power supplied, the engine should suck the IAC valve shut and that should bog down the idle speed considerably to 300-500 RPM or even stall the engine in my experience.   This high idle issue can be caused by a dirty IAC that cannot close all the way, but my part was brand new so I know that this was not my issue.

Comparing the parts store aftermarket IAC to the original Motorcraft IAC valve, the parts store version of this part actually actuates the valve inside the opposite direction. Both parts have a coned plastic disc that rests on a chamfered hole to seal. The Motorcraft IAC works against the suction of the engine. This means that the engine is trying to suck that valve shut all the time, and the IAC is using power and ground for the solenoid to keep it open and regulate the idle speed when it is needed. 

If you look at the parts store replacement IAC, the valve actuates the opposite direction as the Motorcraft part does, meaning the internal valve is on the downstream side of the IAC. This means that the engine is always trying to suck this IAC valve open.  Unfortunately, the Parts store IAC relies on spring tension alone to pull the IAC shut against engine vacuum, but the spring that is supposed to pull this valve shut is not as strong as the engine vacuum is.  So while sitting at idle where the engine vacuum is the highest, the engine can actually slightly suck and hold this Valve open, allowing excess air to enter the engine causing an unwanted increase in idle RPM.    I verified this was the case by unplugging the aftermarket IAC while the truck was idling at 900 RPM, when I unplugged the part absolutely nothing happened, the engine just continued to run at exactly the same 900 RPM meaning it was mechanically, incapable of closing . If you do this with a Motorcraft IAC, unplugging it will instantly cause the valve to be sucked shut and my engine will sputter around 350 RPM and eventually stall out.   

I do wonder if you could bolt the aftermarket parts store IAC down 180° the opposite direction, I think it would work better that way. But the wiring harness on my truck is too short to reach if you flip the IAC 180° from how it’s supposed to be mounted.  

In conclusion, the ECM uses the potentiometer in the throttle position sensor to tell the truck where to hold RPM, while shifting gears, the ECM regulates the ground to your IAC to control the idle speed, the more ground. The ECM gives the IAC the more the Valve opens and the more RPM increases

The aftermarket parts are built backwards from the factory Motorcraft parts internally, so they are useless for my engine. If you could mount it backwards, it might work better,  though replacing it with a genuine Motorcraft part instantly fixed all of my problems because (I believe) of the physical design of the part

Though the real mystery is why the first parts store IAC I got worked for a few months and then developed the same high idle problem that the second aftermarket I had.   

Regardless, the genuine Motorcraft part which functions the opposite direction internally does not suffer from this issue. And I will keep this post updated if the Motorcraft part develops the same problem, which I don’t think it will.

If you have to replace your IAC valve, BUY A MOTORCRAFT PART.