After working with my truck for a while (and things are doing well for the most part, thanks for the group here!), I want to see if I fully understand the IAC function...

Does the computer watch the engine RPM and have the IAC compensate to maintain a certain engine speed (depending on what the computer determines is needed)? If so, how is this scenario I've seen play out with several people not compensated for by the IAC:

An air leak gets into the system (manifold, hose, etc) and the engine revs up. The IAC is unplugged, the IAC closes, and the engine dies. It would seem that correct of the idle speed is certainly well within the range of the IAC at this point. Why didn't the computer simply command the IAC to compensate for it?

Next, I see where people change out one IAC after another with all sorts of varying results, and the common group response on this is to be sure to use a motocraft IAC, not aftermarket. Are they various aftermarket brands somehow calibrated differently? I recently went through this with NAPA brands. One after the other. 3 units that all behaved differently from each other. Again, sorta based on my first question, if calibration is different, why is it the computer doesn't simply command the IAC to whatever position that is required to meet the desired RPM?

Thanks for any feedback, just want to better understand this whole IAC mystery better :-)

Dave

 

Maybe the third party IAC doesn't respond to a command in a way the PCM likes.

I had a '97 Ranger that had a intake leak and was idling over 1500RPM. The only reason I able to still drive it was the fact that it was a 5-speed manual shift. I had to downshift to get below 30MPH.

 

Hi Bill,

So it sounds like the ECU in your truck wouldn't command the IAC to compensate, either. Do you remember, if you unplugged your IAC, would the engine die?

Dave

 

I know that it's supposed to but don't remember if it did. I was getting lean codes but only got one IAC code for over limit or something like that. I remember removing it, leaving it plugged in and having the open port covered also. Long time ago, sold it in 2008 after getting my 2009 Escape.

Found the problem by spraying Quick Start around the upper intake which I found out later is a disaster waiting to happen! Use carb cleaner, etc. The gasket below the fuel rail was sucked into one intake port.

Quick Start is banned from use in some shops, too many accidents caused by its use.

 

I've replaced a couple of IACs, on different vehicles, and all were aftermarket. No problems as of yet, and I did notice that they did have a allen screw at one end, and I've tried adjusting the ones I've replaced before I bought new. Another thing is, you are talking about thousandths of an inch adjustments that this valve does. I think that if you replaced one even with an aftermarket brand, and you still have issues-I think you have more than an IAC problem...Possibly throtle body issues, and DO NOT MESS WITH THE SET SCREW FOR THE THROTTLE PLATE!!! You will make more probs than you want!!! TPS can give one fits too. My '99 Ranger would sometimes idle down below 500 RPM sitting at a light-I would clean out the IAC-still the same result. Idle smelled rich, especially cold start ups-changed out the upstream O2 (before cat), and the idle down stopped, throttle response is much better, and gas seems to last longer, as I haven't done a mileage check in a while.

 

I had a '97 Ranger and while playing with a scope on the O2s, I left one disconnected by mistake. At idle it never got a CEL and I think it was an upstream too!

 

I put my scan tool on, and was looking at the data for the O2s in the run mode, and the numbers were always in the high .8 to .9 range for the upstream-figure it was time to change out. If it gets to 1. its supposedly throws a code. That thing was a booger to get out after being in there so long!! And then the new one didn't want to thread on correctly-had to find a tap to recut the threads. I cut the wires to the old one so I could use a socket to remove it-no soap!! No room for a socket with the 2.5. Had to wrench it all the way out with an open end. Never saw a code for this.

 

I replaced one as well, cut the pigtail to use a socket and had to worked it back/forth a million times to get it out. Luckily it was the easy one to get to but I have a O2 socket now.

You'd think if disconnect that it would throw a heater circuit fault.

 

Yesterday, I replaced the upper & lower intake gaskets and all of the vacuum hoses. I tested the injectors while the lower manifold was off, for dripping/leaking with the ignition "on". Also installed a new set of coils I bought a while back and was waiting for the opportunity to do them, new PCV valve, cleaned the throttle body, tested the TPS, and installed new gaskets on the throttle body, EGR, and IAC. She runs like a new truck, now. The IAC is controlling just fine and it's idling at about 650 rpm. It looked like two of the gaskets on the bottom manifold were leaking, as the contact surface of them were dirty. 3 hours, start to finish, thanks to all the help and education here on the forum.