Just out of curiousity, and to help me diagnose a problem,
I was wondering how exactly the Idle Air Control Works. ( or what makes it work).

At the Moment, I beleive my IAC is not working properly, Ever since I cleaned it when I did the throttle body, The engine is slow to respond when I release my foot from the accelerator.

(ie. When approaching an intersection or etc. ) I will take my foot off the accelerator, Pull the stick out of 3 or 4 (whatever gear i'm currently in) and the engine will continue to rev in neutral at the last speed the engine was reving at when it was placed in neutral .. ususally between 1900 and 2500 rpm. it will stay like this until I push the clutch IN then the revs wil drop back to normal (900-950) sometimes it will act like this and other times it will act like it should.

I put an ohmeter across the solenoid itself and came up with the same resistance value no matter which polarity I probe it.. ( the haynes manual says I should get about 10 Ohms one way and less the other due to a diode inside the solenoid.. I read approx 10.5 ohms both ways..

I guess What I am asking is what causes the solenoid to Open/Close and what is its "at rest" state.. Does the PCM increase or decrease voltage?

KOE-Off voltage form the connector is 12.25V ( batt voltage )

With all that, I figure it may not be closing or opening completly when I release My foot from the accelerator.. Are there any other tests I can do to try and figure out if it is MY IAC for sure that is causing this problem.. Or can their be something else that may causing this problem that works in conjunction with the IAC that may be at fault. I do not get any DTC"S from the PCM (unless I outright disconnect the connector, it will throw a "IAC current High" Code.. ( can't remember the code)..

If my IAC is at fault, will one from a scrapyard work or would it be better
to buy brand new.. seeing as how I cleaned it and now this problem.. I dont want to clean another one and screw it up..

P.s. All I used to clean the IAC was Throttle Body Cleaner and an Air cmpressor..

if anyone can help me shed some light on this it would be greatly appreciated..

Thanks..
Jim

 

It's only active when the TPS reports idle. Then the computer adjusts the IAC's position to maintain a steady idle, near 750RPM.

Don't get a junker - buy one with a warranty since they're known to go out occasionally.

 

So in other words, When the engine is off.. And I Remove the IAC, I should not be able to blow air from one port to the other??

I would assume it is sealed tight until 1) Engine Cranks, 2) IAC Opens a little For initial Start, 3) IAC Adjusts itself once the engine is running to maintain its preset idle acording to load on engine at idle..

What I want to know is

(A) What would be considerd "Normal" voltage readings from the connector with the engine Running and manually opening and closing the throttle plate

(B) Should the connector be off the IAC or should I backprobe it while it's still connected and

(C) does the PCM Fluctuate the voltage at the conntector according to the TPS position.. and I guess

(D) With the Symptoms that I described in my original Post, Should I also be looking at the the TPS inaccuratley reportingan idle as being a culprit?

Sorry for all the in-depth questions, I have a good understanding of basic Engine principles, but I am really starting to take an interest in more of the detailed PCM Sensor Stuff. ( that area)

I like to have as much information as I possibly can before doing even the stupidist of things..

Believe when I say I can come up with a 1000 different questions for changing a gasket ... well you know what I mean..

You know what they say knowledge is power..
(it also keeps me from looking like an idiot when I get asked what would be a simple question from someone who knows what they are talking about..

thanx again for replying..
Jim

 

Before this thread gets any BIGGER.. I must apologize if its in the wrong forum....

 

this seems like a fuel injection question to me... I can't really offer any insights, but I am interested in keeping this thread active, as I am having a similar problem with my 99 f-150 4.6. The revs actually increase before they drop when I depress the clutch.

 

what yr/engine/miles? sounds like tps to me.

 

Well From What I understand so far the IAC Maintains the idle depending on the TPS position and the Load on the engine..

IE.. When the A/C cycles ON and OFF, the PCM Sees a Load on the engine and "Magically" Signals the IAC to compensate for the demand by increasing the Air going past the throttle plate to increase the Idle just enough so that it won't drop below 750.

I say magically for lack of better words, because I do not yet know exactly how the PCM signals the IAC..

I think this also justifies why the engine wont drop back to normal idle when the stick is in neutral until I push the clutch to the floor disengaging the transmission.. I think that the PCM sees the input side of the transmission as some sort of a load and keeps the idle high until the load is gone - like when i push the clutch in.. hmmm....

At least on mine (at idle) when the compressor kicks in, the RPM drops to approx. 700 RPM then increases to about 1000 RPM then drops back to 750. (all taking place in about 1 second)

Also From my own findings I believe the IAC also helps smooth out shifts..

Just for a test I unplugged the IAC connector and went for a drive around the block.. The instant I pulled it out of gear,and off the gas, the RPM's dropped so fast, it made it difficult to shift into the next higher gear. I had to feather the accellerator a bit to get the gears to mesh.. ( shifting without the clutch)

Before this stupid problem the gears would mesh like butter and now because i believe the IAC is stuck open or slow.., the RPMS are always too high for it to shift nicely into the next gear, it holds it way too long and the RPMS dont drop they just stay at 2000 RPM when I pull the stick out of gear

so I believe the Secondary duty of the IAC ( especially in standard vehicles) would be that the instant the TPS sees a closed position the IAC "holds" the engine at the last highest RPM it was at, for the gears to engage (approx 1 "Jiffy" ) then releases, and then the process starts all over again...

Only in my situation, a "jiffy" is more like 5 -10 seconds, not 0.5 seconds like it should be.

Am I right or even close on this? If I am, I came up with that all on my own... if i am not well I guess I put my foot in my mouth and return to my cave....

thanx,
jim

 

Sorry 03f150man, I currently have 101,500 KM the rest is in my tagline..

 

The PCM varies the pulse-width of current to the IAC to open it more or less against its return spring and the vacuum of the engine. It ONLY does this when the TPS reports idle, regardless of what the A/C is doing, but there is an A/C input to the EEC. The EEC monitors the PIP for a variety of reasons, but that's how it knows RPM.

I think your A/C may be low on refrigerant, but that could be explored in another thread.

The IAC doesn't "seal"; it merely closes, but a small leak isn't significant or a problem. The engine vacuum helps to close it, so just blowing thru it won't necessarily yield useful results since your lungs can't get close to as much pressure (or vacuum) as the engine.

The clutch pedal should have no influence on idle - that indicates a problem, or the change in idle is just coincidental. Have you ever tried waiting EXTRA long before pushing it to see if the idle will drop on its own?

Your unplugged test drive only showed that your throttle stop screw is set close to correct, but the fact that the engine idled with it unplugged shows someone has messed with it. With the IAC unplugged, the engine should ALMOST idle, but die within ~2 sec. That could be your whole problem, since the EEC doesn't monitor the IAC's actual position, but uses preset values to control it based on TPS & PIP inputs.

 

hmm. Stuff to definatley ponder about I wil try a few more experiments today to see what I can come up with.. Thank you for explaining how the IAC works to me..

Jim

 

Sorry guys for the delay... I got called in to work on my day off and ive been screwed for time ever since.. work, work, work, I'm still trying to find time to get back at testing the TPS..

I took a look at the idle set screw, and like you said Steve83 it had been mucked around with, Looking closely at it, their were 1 - 1 1/2 threads with a different shade of "weathering" on it (i guess weathering would be an appropriate word.. ) I havent checked the TPS idle voltage yet , But I would assume it wouldnt be .9V like it should be.

I also think the clutch problem is coincidental, the amoount of time it takes for the engine to revert back to idle at neutral varies from normal to almost 5 seconds.

I also have another problem I want to correct first before I rush out and buy anything.. I constantly get a code P0442 Emmisions EVAP Control Sytem leak detected. I have a funny feeling it may be my purge solenoid. It may be contributing to the original problem, it may not. but i'd think it be better to start on that first.. before going nuts on new sensors etc.

anyways just letting you know i'm still here, just trying to find time hopefuly after the weekend I should have my results.

Jim

 

Either the purge solenoid, a leaking gas cap, or a hole in the emissions tubing will cause that code.

 

overfilling the fuel tank can damage the evaporative fuel cannister i.e. filling it up to the neck can dump raw gas into the cannister and "foul" it. A bad cannister causes weird driveability problems that are difficult to diagnose. If the obvious checks dont reveal a problem, keep the cannister in mind.

 

uh oh..

I have overfilled it a few times to the point where it is spilling on my toes.. now that I thnk of it I probably could have damaged the canister by overfilling it.

When I Checked all the emission lines to see if i could find any leaks, I didnt see any signs that fuel was on the outside of the canister, is there any other tests that I can do to see if it may be the canister, while i'm checking the purge solenoid?

at the moment I have a half decent vacuum guage but not a hand pump, im not sure if there are any vacuum tests I can do to diagnose it,,, I haven.t had any time to read up on it yet.

cheers,
jim

 

try emailing 1956MarkII he is the one who pointed out the cannister problem to me and could advise you how to test it.