I see everywhere that it says once your engine is warm, pull your IAC connector and see if the idle drops. Well when I pull mine, my engine dies immediateley. Whats up?

 

The Idle Air Control meters how much air is going into the motor at idle. When you have the A/C running it has to adjust every time the compressor kicks in.
If there is NO CHANGE when you pull the IAC plug at idle, that could indicate a faulty IAC.
If there is a change (or the motor quits, as in your case) that means that the IAC is functioning properly.
Also, Ford recommends not shooting any carb cleaner up the throttle body. I did anyway and killed an IAC shortly thereafter.
Be Cool.
CB

 

I wonder about this IAC test!

My 97 Ranger 4.0L as read by a AutoXray EZ Link showed a IAC duty cycle of anywhere from 18-30% at idle but it was idling anywhere from 1100-1800rpms! The Ford manual stated at warm idle, that it was correct. It wouldn't stall but would slow down.

I have a ton of posts here on my problem of lean codes(P0171/P0174) both banks. This is a new IAC because I thought that I screwed it up while cleaning it. The idle went from 1100rpm to 1500rpm after cleaning it!

As it turned out the gasket under the fuel rail was broken and sucked into the first two intake chambers(one on each bank). So, after the fix, I adjusted the idle speed. It runs like a top, idles so smooth and I'm happy now.

So what's the IAC duty cycle now at warm idle? It reads 0-1% and I doubt that it would slow down or stall if I pulled the connector! It reads 17% at idle with the AC on. I must admit the the scanner I'm using isn't without problems and I'm still dealing with AutoXray about its problems. So, these parameters could be bogus. Just to round out my test equipment for future use, I bought a OTC DAT or a automotive digital VOM. It will read duty cycles but I haven't had the time to connect it to my truck to compare the IAC readings.

I always thought that the IAC duty cycle should be lower but never thought that it would be that low because of the way they say to test it!

 

The engine should NOT die when you disconnect the IAC with a warm engine, it should idle at the base idle speed which is set using the screw on the throttle body. The IAC only increases idle speed due to load at idle, such as AC or in drive with a auto tranny, and cold/start-up conditions. If you would like to read this from the Ford manual I would be glad to scan it.

 

Ken:

In my 97 Ranger book the IAC function test states: Does RPM drop or engine stall? If Yes, proceed to the next step. If No, check throttle body for damage, if none found, replace the IAC.

It's pretty plain and no chance of misreading it either! I know some of the pinpoint tests, like comparing three test points are easily misread! Been there, did that

What do you say about IAC duty cycle at warm idle?

 

Bill,

I stand corrected for newer vehicles but an older truck is done the way I stated in my previous post, which is from the 1988 Ford manual. I would think a low value would be correct but I'm not sure about it. I did look through my 2000 manual and it's not clear what the specification is for duty cycle at idle, did you find it in yours.

Ken

 

Ken:

Yes, I'm at work now but I think that it was Section 6A, specs for different engines. It had IAC duty cycle at warm idle and two or three speeds. I'll have to look at that again and see how they compare at the given speeds.

 

Is yours on CD? Let me know what you find out and what the specifications are, I'm wondering now.

 

[updated:LAST EDITED ON 30-Apr-02 AT 10:13 PM (EST)]No CD, book version. It was Section 6A.

4.0L Auto
Warm Idle, 10.8v, 30%
30MPH, 10.6-11.0v, 35-41%
55MPH, 6-8.4v, 57-68%

4.0L Manual
Warm Idle, 10.7-10.8v, 30-35%
30MPH, 10.3-10.6v, 40-45%
55MPH, 8.8-10.2v, 45-50%

I like that voltage spread or lack of at warm idle, hey? I guess my memory wasn't that good about the duty cycle either! I thought that the spread was like from 20-30% for my manual 4.0L. Hmm, I'm getting curious also. I hope that I get time to connect the OTC DAT and measure the IAC duty cycle/voltage. Since I have a breakout box, backprobing isn't needed to get readings

 

I found that section on my CD. It's strange why they have such a high duty cycle, wonder what the reason is. You have all the good tools, let me know what you find out. Two weeks ago we traded in the 1989 2.9L Ranger for a 2002 3.0 5 speed auto XLT std cab 7' bed, I still have my 2000. Have a good one.

 

Ken:


Didn't have time to connect the DAT but the scanner was still in the glovebox. At 30MPH, I get 32% duty cycle on the IAC. A long way from the 40-45% that the book states!

I went to Helm's site a found a CD-ROM that covers the 97's but has the year of 2001 in it's title. It says that it covers more than one vehicle. The price is 1,200.00! A bit pricey for the average backyard mechanic

 

That is a bit off, has the screw on the TB ever been adjusted? If it runs good and you don't get a code then I don't know if I would worry about it.

My CD covers all 2000 trucks.

 

Ken:

Sure has! When my problem first started, it was idling at about 1100rpm. One day I cleaned the MAF(third time), throttle body and IAC. Idled jumped to 1500rpm! That's when I got with this tech at Ford's High Performance/Racing Div. who faxed me that idle setup procedure which I posted in another thread. It's the one that set the TPS voltage by moving it around but you said that it didn't matter on a OBD-II system. I couldn't get it down from that high speed anyway. The IAC was trying compensate for the broken fuel rail gasket I guess. I don't why they used a screw with such a fine thread on it. Just before I replaced the fuelrail gasket, it was idling at 1800rpm and could hardly drive below 30mph in fourth gear!

The engine mechanical manual mentions that there is a idle speed adjustment procedure in the Powertrain Control Manual but I've never found it! Have you ever found it in that manual or know how it's supposed to be done on a OBD-II system?

 

I have never found the idle procedure in my 2000 Ford manual, I'll look again. I do have the procedure in my 1988 Ford manual for the older vehilces..

 

I found this under section 001 Description and Operation, Air intake system. Check out the note in the TB system and the last sentence in the TB housing.

Throttle Body System Overview

The throttle body system meters air to the engine during idle, part throttle, and wide open throttle (WOT) conditions. The throttle body system consists of an Idle air control (IAC) valve assembly, idle air orifice, single or dual bores with butterfly valve throttle plates and a Throttle Position (TP) sensor. One other source of idle air flow is the Positive Crankcase Ventilation (PCV) system. The combined idle air flow (from idle air orifice IAC flow and PCV flow) is measured by the MAF sensor on all applications.

During idle, the throttle body assembly provides a set amount of air flow to the engine through the idle air passage and PCV valve. The IAC valve assembly provides additional air when commanded by the powertrain control module (PCM) to maintain the proper engine idle speed under varying conditions. The IAC valve assembly mounts directly to the throttle body assembly in most applications, but is remote-mounted to the intake manifold in some applications. Idle speed is controlled by the PCM and cannot be adjusted.

NOTE: The traditional idle air adjust procedure as well as throttle return screw are no longer used on OBD II applications.

Throttle Body Housing

The throttle body housing assembly is a single piece of aluminum casting with an air passage and a butterfly throttle plate with linkage mechanisms. When the throttle plate is in the idle (or closed) position, the throttle lever arm should be in contact with the Throttle Return Stop. The throttle return stop prevents the throttle plate from contacting the bore and sticking closed. The setting also establishes the amount of air flow between the throttle plate and bore. To minimize the closed plate air flow, a special coating is applied to the throttle plate and bore to help seal this area. This sealant/coating also makes the throttle body resistant to engine intake sludge accumulation.