I have a 1992 f150 with a 5.0 and a 5 speed trans. I’m having trouble with what I believe is the iac. With the iac plugged in the truck will fire up fine and idle high for a little while (from what I hear that’s normal and not the issue) the issue is that sometimes while I’m coasting the truck will stall out, especially after slowing down from higher speeds (70 to 30 or so). The iac is fairly new and never had trouble before. I’ve been running it with it unplugged for about 3 months and don’t have much of an issue except not wanting to idle when cold and I’m tired of babying the throttle until it wants up. I’ve also blocked the and deleted the egr (no it doesn’t leak I’ve tested many times) and all other emissions related things have been deleted. Vacuum lines pulled and holes blocked from them as well. My cruise control and tin can are the only things connected to vacuum anymore and they do not leak either. My question is what causes the iac to open and close. I understand the power in doesn’t change but the ground signal changes in order to open or close it further to adjust idle but what is actually causing the signal to change. Sorry to ramble, any help is greatly appreciated.

 

Personally I don't trust any IAC valve that is not Motorcraft.

 

I agree with above comment. Seems like some parts like engine sensors and actuators is better to go with oem and not parts store or rock auto specialties.

 

I believe the iac position from the ecu is determined by the signals from the engine rpm, tps, maf/map, engine temp, and o2 sensors.

 

You’d be correct assuming it’s not a motorcraft part. But I can exactly swing the hundred or so bucks for one right now. Is there a way I can test the iac to see if it’s the cause of my problem or should I start checking other sensors.

 

Well since nobody is responding I’ll do the only logical thing and add more. I put a set of long tube headers on I got from a buddy and plugged the iac back in and it doesn’t seem to stumble anymore but on start up it shoots to about 2k rpm and sometimes it’ll stay there for about a second and other it’ll stay for 30 seconds. I know these trucks are supposed to have high idle on startup, especially when it’s cold, but that seems a little extreme to me. Can someone at least point me in the right direction on what to do.

 

Either iac or vacuum leak from a hose or intake

 

is there a way to test the iac

 

I'm sure there are several videos explaining if you search around. Have you also looked at your throttle body to see if it is really dirty?

 

Do you have a volt meter?
According to Haynes, you should get 10.5 volts at the harness connector, with it unplugged, testing the engine harness side of the plug, key on, engine off.
You should get 7 to 13 ohms of resistance at the IAC valve electrical terminals, with the valve unplugged.

 

https://www.fullsizebronco.com/threa...ur-iac.206960/

 

Here's a link explaining how to pull engine codes. https://www.ford-trucks.com/forums/1...n-process.html The codes might point you in the right direction.

 

still need to try this but I found an old throttle body I had laying around and it had a motorcraft iac on it. It sat outside awhile so I pulled it apart and cleaned it up. truck idles perfect and on startup it only revs to about 1200 rpm. It causes the idle to drop a little low when I let of the gas but other than that it’s running perfectly. Being I got the sensor out of a scrap pile I still want to buy a new one. I guess I’ll just have to bite the bullet and get a motorcraft. Does anyone know a good site to buy one from. I’m worried about spending a bunch on a ripoff and don’t want that to happen. Thanks.

 

I would recommend just sticking with the one you have on there. Aftermarket parts are questionable.

 

I would check for engine codes. Several sensors and the MAP sensor and ignition system all have to be working properly for the engine to run properly.
Checking the engine codes can let you know which of those sensors is reporting a problem.