Not sure whats going on with it, but something cant be right.
I started another thread on my CEL and all that mess, and I finally got everything running good, got the vacuum leak sealed, and shes idling below 1000rpm some where in the 8-900 range, its hard to tell with the factory tach.

Anyways, idle is down, but when I unplug my IAC valve my engine rpm's does not change.
I read it was supposed to make the engine idle rough.
Does that mean my IAC valve is bad??

No codes by the way.

 

Sounds like your base idle screw is turned up too high. Unplugging the IAC valve doesn't make your engine idle rough, it just idles slower, at the base idle speed, which is about 400-500 RPM. Adjust the base idle screw to that speed with the IAC unplugged and then reset your computer (disconnect battery for 10 minutes) and let it re-learn your idle and see what you get. It may take up to a week for the computer to completely smooth out the idle after this change. This would be a good time to clean the IAC valve while you are at it.

 

^ +1
get yourself some crc throttle body cleaner to clean it and the TB while you are at it. Most sprays will work, but the crc leaves a dry lube which should help.

 

Base idle speed should be 650-700rpm, and that is controlled by the computer via the IAC valve based on feedback from the TPS. That throttle valve set screw should not be touched, if it has thread locker on it it is at the factory setting.

 

Thanks for the replys guys.
Yesterday I backed the screw off one full turn and that help a little bit, but I have not unhooked the battery yet, will do ASAP.

I already took apart the iac valve and cleaned it while I was replacing the TPS sensor, and cleaned the tb while I was there.

So I will keep tinkering with the screw till I can get her adjusted right.

 

Conaski, do you think my iac valve is bad then?
I double checked for vacuum leaks after I tightened down the upper intake bolts.
Anything else that you know of that will cause a slightly high idle?

 

The TPS should read close to but not over 1v when closed(0.9 -0.99), if I were you I would put the screw back to where it was as close as you can, and then probe the TPS wires to see what voltage is produced at closed throttle. If it's slightly over that range and you can't get it to the right value by shifting the TPS sensor on it's mounting screws then back out the throttle plate stop screw a bit. If the TPS voltage is way off it's either installed incorrectly or faulty. You will notice I didn't mention anything about the IAC.. that wasn't by accident.

 

I've had this discussion with Paul before - suffice it to say, he and I do not agree on the "sacredness" of what I call (and Ford used to call) the base idle screw. I say it's no big deal to adjust the screw as long as you understand you are not setting the idle speed for your truck with it. Paul disagrees. Your truck, your choice.

 

Well.. I don't totally disagree about the "idle screw" thing... I have messed with mine too and learned a few things like the fact the computer takes a TPS measurement of closed position every time at startup and uses that as a baseline for that drive cycle, so it can deal with things that aren't at the ideal settings. My post above was to help the OP get things baselined and to verify that the TPS is installed correctly.

 

It's just my opinion that these trucks are 20-25 years old and have usually gone thru several owners. What Ford originally set it to is not known anymore and so is therefore, IMO, irrelevant.

 

When I installed the TPS, I checked the volts, I had the 5 volt ref, and my TPS read 1.019, so I called that pretty good and went on.

So much misinformation its pisses me off, I read its supposed to read 1v and that also the idle is supposed to run rough after unplugging the IAC valve, whose right?

I will leave it where she's at for now and see what the computer will do.

 

Any progress?

 

I haven't drove the truck since this post.
I still need to get it inspected and titled first, then I will drive it more to see whats its going to do.

I did start her up tonight and she was idling just under 1000rpm which is better, this weekend I will do some tune up work on her, maybe that might help?.

 

The 1 volt TPS signal at closed is throttle myth is just that...A myth. For reference: http://oldfuelinjection.com/?p=30 cut from the same article: "Closed Throttle can be anywhere between 0.6-1.0 volts"

The idle is supposed to drop to the 600 - 700 RPM range with the IAC disconnected. If not there is something wrong.

Common causes:

Faulty IAC (stuck open or "sticky", plunger does not seat properly)
Throttle stop screw has been tampered with
Sticking throttle cable
Vacuum leak(s)
Throttle body (dirty, blades bent, broken, throttle shaft wore, etc.)

 

Have you changed the gasket yet?
Quote short bed:
"So I suspected a vacuum leak, so I sprayed some carb cleaner around and found the center bolt on the upper intake was leaking."