I did a search, it didn't find a good result on this.

Today I wanted to see what an IAC does on a 2.8L on a cold (65F) engine from first ignition on and start. Just curious.

I unplugged the cable from the IAC and plugged in into my spare IAC that I let rest on the engine so I can watch it.

I started the engine, it idled OK considering the IAC on the Throttle Body was disabled.

The IAC being looked at didn't move. I expected it to do some kind of homing sequence, but nothing.

I know the IAC works because I ran an H-Bridge and some Arduino code on the bench and it goes in and out just fine.... its a simple stepper motor and I happen to know how to control them.

I waked away and let it idle for 5 minutes. When I came back, the Pintle seemed to be moved out a bit. This makes sense as the ECU at that point was probably trying to close the valve since the engine was warm. I figured it moves so slowly it's hard to see... ok, good to know.

I shut off the engine for a minute or two, then restarted. The engine all of sudden now will not idle (stalls) and I cannot rev it AT ALL. I tried restarting a few times, no improvement. So I plugged the cable back into the original IAC still on the Throttle Body. I restarted the engine but it is still malfunctioning. I tried resrating a couple of times.
No improvement.

Why would unplugging the IAC, warm up the engine, and then replugging in the IAC cause havoc with an engine that ran fine a few days before this simple experiment?

Who is that on-line guru that has all sorts of tips on Fieros. He is very popular but I forget his name. I remember a long time ago reading he had a procedure to reset the IAC.

What is his name?

I am letting it sit over night, I will disconnect the battery for 5 minutes to reset the ECU, put it in DMDIAG Mode for a few minutes, and see it that gets it running again.

Any advice?

Thanks


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Paul Romsky

[This message has been edited by Romsk (edited 11-04-2024).]

quote Originally posted by Romsk: Who is that on-line guru that has all sorts of tips on Fieros. He is very popular...

Definitely no one here.

quote Originally posted by Romsk: I unplugged the cable from the IAC and plugged in into my spare IAC that I let rest on the engine so I can watch it. I started the engine, it idled OK considering the IAC on the Throttle Body was disabled. The IAC being looked at didn't move. I expected it to do some kind of homing sequence, but nothing.

Consider yourself lucky!

quote Originally posted by Patrick Here: When I "tested" the IAC valve while it was removed from my duke years ago... the pintle shot off into the grass. It was difficult to find. However, I at least knew the IAC valve was functioning.

Patrick thanks for the reply.

The guru is not on this forum anymore... he may have past on but his web articles are still floating around.

He is mentioned all the time on this forum and many people leave links to his pages. He doesn't use a web alias/avitar, he goes by his given name... I just can't recall it.


Before I made my little H-Bridge tester for my IAC (using my spare IAC on the bench), I was pushing and pulling the pintle in and out by hand. It's hard to do because of the gearing inside. I don't recommend doing it, but it is possible with just the right force. I tried to pull it all the out to measure the total throw of the pintle (someone asked me about that length). In the past couple of times I did that, it had an internal stop that didn't let me pull it all the way out. But when I tried it the other day, I must have pulled on it too hard and broke the stop, it came right out. So I said that's it, I need a little circuit to work the pintle electronically. I was able to screw the pintle back it and locked the spring in its proper place. I have a video on how I can control the IAC on the bench.

https://paul.romsky.com/index_fiero.htm
Near the bottom of the page are two links for the brief video.


With my stepping code I was able to determine that 725 steps from full open will cause the pintle to fly out (just like yours). I changed the code to limit the teavel to about 600 steps so it's nowhere near that point.

There is no feedback from the IAC back to the ECU, it is what we call a dumb actuator. So I was expecting the ECU to perform some kind of homing procedure at ignition on. I was prepared to catch the pintle and spring. The experiment was partially successful, but now, that simple unplugging of the IAC from the car (I didn't want to remove it), my Fiero can't Idle and I can't rev the engine at all... something has confused the ECU somehow. The car was fine before that.

Paul, check your PMs.

I am having a Brain Fart... what does PM' stand for on Fieros?

Private Messages. Check your Inbox.

quote Romsk: I remember a long time ago reading he had a procedure to reset the IAC. What is his name?

Perhaps you saw this IAC procedure:

quote Originally posted by Chicken McNizzle: DO NOT tamper with the idle adjustment screw! If you have then you have wasted time of messing with the IAC. The whole purpose of the IAC is to control incoming air that is bypassing the butterfly to effectively control the idle. If you open the butterfly with the set screw ( which was actually capped from the factory ) your not allowing the IAC to perform it's function AND the TPS will read even more voltage which throws even more off. Simply back out the throttle set screw until the butterflow just barely catches in the bore, and has a smooth open and close ( no catching ), clean THE HELL OUT OF the IAC and IAC passage, ensure that the TPS is reading .45mv at idle ( key on engine off - can be tweaked by manipulating the TPS throw arm with pliers ). The IAC will self adjust in the bore once installed, simply jumper A and B together before plugging the 4 pin connector back on to the IAC ( MAKE SURE IAC IS INSTALLED INTO THROTTLEBODY BEFORE PLUGGING IN ). The 1 1/8" is the extent of the pintle travel inside the bore, however can travel further when it is removed. If the pintlle is further out than 1 1/8" when installed, you will simply crush the stepper motor and related parts inside the IAC casing. So to sum it up.. 1. Remove IAC, clean the PISS out of everything 2. Ensure set screw has not been tampered with, if it has, reset it as stated above 3. Make sure TPS reads .45mv at Key ON Engine OFF 4. adjust pintle length to 1 1/8" -measured from mounting surface of IAC casing to tip of pintle 5. Install 6. Jumper A and B in the ALDL to put into diagnostic mode 7. Plug 4 pin connector into IAC, you will hear the stepper motor bottoming out the pintle at the end of the bore inside the Throttlebody 8. Remove jumper 9. Start Car - there may be some initial hesitation, and even dying from the motor the first 2-3 starts, the IAC will self adjust 10. Check for vacuum leaks by covering the IAC inlet passage inside the bore of the TB with your finger, if the engine continues to run at ANY RPM, you do indeed have a vacuum leak

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Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

Also, that "homing procedure" happens when the car is going about 40mph, which makes sense because you wouldn't want it to happen upon key on because done people start the car immediately, and that could happen when it is extended (no idle air) making it hard to start.

Vintage-Nut:
Thanks. I never saw the video. Who made the video?
I have no intention on touching any capped idle screw. I think the ECU just got confused in my experiment.
Thanks for the procedure.
One thing, the TPS should be 0.5 to 0.6V in the idle position (but below 1V is the absolute max for Idle). You said .45mV.... 0.45 milli Volts = 450 micro Volts (that is almost 0V). I think you were thinking about the O2 Sensor that outputs 0.45V at the Stoichiometric 14.7:1 Air/Fuel Ratio. 0.45V = 45mV.

1985 Fiero GT:
Ok, the IAC is not Homed at Ignition On because normally there is not enough time, so the ECU Homing it at 40 MPH makes sense. Thanks!

It looks like putting the ECU in Diag mode will Home the IAC. My GUI makes this easy.

https://paul.romsky.com/ind..._gt_aldl_monitor.htm

Me? If I designed the system, the Homing would begin as soon as the Ignition goes On toward the fully In (open) position. Within a few cranks the engine should start, then the ECU would finish Homing it (less than a second later), only then control of its position would be updated as needed. I pulsed my spare IAC on the bench as fast as possible. From the fully Out position, to the fully In position it takes about 2.5 seconds. Or, I would have homed it at Ignition Off where normally it has more time... but I was not at GM when IAC systems were developed... hey, I was grade school at the time [smile].

[This message has been edited by Romsk (edited 11-05-2024).]

quote I never saw the video. Who made the video?

This IAC procedure is from a post on PFF without a video....

quote One thing, the TPS should be 0.5 to 0.6V in the idle position / You said .45mV

You're correct; Chicken McNizzle made an error when he said .45mv on his procedure / the Throttle Position Sensor voltage at idle position should ~0.50 volts

quote It looks like putting the ECU in Diag mode will Home the IAC

Yes, that is correct:

quote Originally posted by Hudini: Setting the IAC is easy. Place a paper clip in the ALDL terminals A&B to put it in diagnostic mode just like you would do for setting timing. Turn the key to RUN and wait about 30 seconds. As said above, you will hear it making noise as the ECM is commanding the IAC fully closed. After 30 seconds pull the IAC power connector while the key is still in RUN. Now you turn the key to OFF, remove the paper clip and reconnect the IAC. Now drive the car over 35mph to relearn idle.

Vintage-Nut:

Thanks, I will try this... but one thing: my Fiero is not registered. I start it now and then but I haven't driven it in a few years.
So all I can do is idle. My local garage mechanic does not have repair plates... I have to find someone with repair plates that can drive it for me to re-learn the idle and build new A/F tables after resetting the ECU.

Maybe I could make a cheesy dynometer in my garage (sounds like a fun project). This way I can simulate taking it for a drive with hills and such.

We can close this thread. My question has been answered. How do I close a thread?

quote Originally posted by Romsk: We can close this thread. My question has been answered. How do I close a thread?

You don't, the thread stays here basically forever, for other people who have the same question, and search up the answer, if people don't continue posting, it will fall down the list and you won't see it any more without searching for it, great way to accumulate information on this forum (search is a good tool, but new or common questions are always welcome, as Cliff mentioned in general Fiero chat).