Guys, I've been chasing a high idle and excess rpm problem for a while now.

I need you guys with a good running late model 4.0L and a tach (or preferably a scanguage / scanner / datalogger) to dis-connect your IAC valves electrical connector and then tell me what your idle speed when your in park or neutral (w/o the AC running)

I'm trying to determine if the problem is mechanical or electrical.



btw, any help or advise is appreciated. Just know that I've done a lot already. I have carefully looked at the TB and IAC for leaks. Everythnig is squeeky clean. Even bought a new ford IAC thinking that might be the cause. I've cleaned the MAF with MAF cleaner and have never had a CEL or code stored. Since I have the K&N I even made sure the MAF was sealed properly. I did find a opening and used silicon to seal it. Made no difference at all.
Cleaning the MAF, replacing the IAC, and reflashing the computer did help a decent amount. That cut the problem in 1/2.. But my idle is still a little high (inconsistantly ranging between 800 and 1000 when in gear) and when I'm slowing down in the 20mph to 5mph range it's just wanting to not slow down. I can take my foot off the gas and the truck would pretty much keep the same speed if I let it.
Oh and my throttle when fully closed is at 1.03v (as set from the factory). The TB operates smoothly and closes against the stop with good pressure from the two return springs. Holding a light up to the TB (when its off the motor) shows a nearly perfect gap al the way around the plate.
I'm just perplexed as to why I have a "run on" condition. My idle overall is decent once I come to a full stop. But still higher than what's being commanded by the tune. (700) Its as if the computer is telling the IAC to keep the rpms up too high.
I can see this because the rpms will pretty much match the opening of the IAC. (SCT datalogging)

Oh and I followed the advice of Bob about how to do the quick learn idle. (works very well btw)

Thanks, Rich

 

Rich, did you clean the black stuff out of the TB bore?

 

There was a slight caking of debris around where the plate closes off. I used CRC brand "Throttle Body & Air-Intake Cleaner" on it. Then lightly agitated the bore & plate with a tooth brush while everything was wet with cleaner. Then sprayed the debris away and let it air dry.

IMO nothing that was a permenate type of coating came off. At least I don't think so?

When I have the TB in my hand and have a light on the other side I can see a very uniform gap around the perimeter of the plate. It's not much.. about 0.5mm (0.020")


btw, I saw one of your old posts about how to let it learn to idle before driving. When I reflash the computer this works very well. It's 10 times better than if I reflash a tune and then go out driving/testing. When I come to a dead stop the rpms will drop into the 775 area. But that last 20mph down to 1mph it's just wanting to run on. This afternoon on the way home around a lake (25mph zone) it was holding 26mph on it's own for a solid mile.

Thanks Bob,

Rich

 

Here's the way I heard the story....

Early Ford EFI allowed air to pass around the partially open throttle plate for idle with assistance from the ISC/IAC. It was discovered that, over time, deposits would build up where the butterfly is in close proximity to the bore, changing the flow rate there and through the IAC. To make it more consistent, the throttle plate now has a calibrated hole or two in it for airflow and it is supposed to close completely. During manufacture of the throttle body, a coating is applied around the perimeter of the butterfly, it dries and then is broken. This is intended to make the closed position airflow consistent from part to part.

It is my understanding that if the coating is disturbed, it can possibly cause symptoms like yours.

 

Hmmm.... my '96 2.3 would do that alot. I did discover that if I shut off the motor, then restarted it right away it would idle down fine then the rest of the trip. Sometimes wouldn't do it for a several days, then would do it a couple days in a row. I never could figure out what would make it idle down just by shutting it off and restarting it. Had it to Ford a couple of times and got the "they do that sometimes" story...DUH!!!

 

Thanks guys.

Sounds like I need to maybe play with the gap around the plate. Should be easy to do with the throttle stop as long as I take a good measurement so I know from where I started.


If anyone can unplug thier IAC and then tell me what the *exact* rpms are. I'd sure appreciate it! (Just let me know the rpms with the AC off)

Regards, Rich

 

Hey Guys, I just wanted to give some feedback on this issue so that for future searching folks can find solutions.

I took the TB (throttle body) off and held it up to the sun light & carefully studied the gap around the plate vs bore. IMO it looks fine and was even.. and not excessive.

I took my dremel tool and lightly skined the metal pin that stops the plate when closing. I kept doing this until I had about 1/2 the gap that I had before. Mind you the plate doesn't touch the bore.. it clears by 0.010" the entire way around.

I then put it all back together, disconnected the battery, turn on the headlights & ignition, turned them off, and reconnected the battery.

Then I started the truck and let it fully warm up. I then let it idle in all 4 states that Bob said to in another post on this issue.

Idle learning sequence:
1) Motor fully warmed up.
2) Idle in N ac off.
3) Idle in N ac on.
4) Idle in D ac off.
5) Idle in D ac on.

Now my idle is perfect. I even went in with the x-cal and change the speeds just to make sure it would respond. It does.

So just to recap.. I have a 2006 4.0L, cleaned the TB with "throttle body cleaner" and a tooth brush. I then had strange & inconsistant idle speeds. And the biggest problem was a "run on" or excessive speed/rpms when going slow. (it would drive 20 mph all by itself)

The problem seems to have been a coating on the plate or bore that FORD puts on there. I did not see this coating when I cleaned it. But when I did it made the air passage around the plate/bore excessive which led to the IAC not being able to keep the rpms down.

Whish I had known this sooner. I'd have just washed it with a little water and a soft tooth brush. Oh and saved $70 on a new IAC that I don't need. Oh well, hopefully my experiences will help someone else fix thiers for cheap.

Regards, Rich