Challenges setting minimum idle air.

[scorp508]

I've been chasing a situation where my engine stalls when cold rather easily, as well as when the engine is warm if coming to a quick stop.

According to TunerPro RT my IAC steps were ~60 at idle once the car is warm. This seems a bit higher than others here have suggested it should be.

The first thing I did was remove my IAC and clean the pintle and bore.

Before/after. Yes, the O-Ring was put back even though it isn't in the after shot.
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I then attempted to go through the procedure to set minimum idle air and reset the IAC here: https://zr1.net/forum/showthread.php?t=27764

All was going well, but in step 6 my throttle stop screw was basically already turned in (clockwise) as far as it can go, so my understanding is I have no adjustment left to "lower" the minimum idle air. It was a hex head screw, 8mm I believe. Have any of you had to get a longer screw?

This was after messing around with it a bit trying to back it off. When I started the nut and screw head were both all the way in.

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Interestingly if I let the engine run the idle would start around 1200 (ECM's commanded desired idle was 850), then start climbing in small increments and slowly run away. In other words, it would start at 1200, then 1250, then 1300, etc. and keep climbing until I killed the engine. Vacuum leak? I don't understand how with the IAC unplugged and the throttle body blades not moving the engine was able to gain so much RPM. The TPS was at 0.53v the whole time. Wondering if I should smoke the intake and look for leaks.

The car does idle dead nuts where the ECM wants it, but was hoping to get IAC counts down and improve this stalling situation.

All ideas are welcome. :cheers:

[Mikey]

I am confused. You describe trouble keeping idle and high IAC counts followed by surging rpm that seems to be air leak related.

[scorp508]

Sorry I probably could have laid it out better.

Before doing anything.

1. Car holds ECM commanded idle speed great. IAC shows steadily around 60 steps at idle once warm.

2. When cold, car stalls when coming to a slow stop nearly 100% of the time.

3. When warm, car occasionally stalls only when coming to an unexpected hard stop.


While trying to set min air via stop screw and IAC disconnected.

1. Can't get idle low enough via stop screw. Screw is already turned inward as much as it can physically go.

2. After about 20 seconds of engine running the idle begins creeping upwards with no end in sight until I kill the engine.


Put it back together as I had to go pickup my son at daycare, and did the final IAC reset steps even though I wasn't happy with it yet.

1. Car holds commanded idle.
2. IAC steps now 80. My stop screw is not "as closed" as it was when I started, so this sort of makes sense.
3. No apparent runaway idle increase.

[XfireZ51]

Did u plug in ALDL, TURN IGNITION ON, then unplug IAC first? The. Unplug ALDL B4 turn off ignition? This closes off IAC port.
Also, throttle blade closes as u back out set screw. At 60 counts, most of the idle air is going thru the IAC.

[scorp508]

Did u plug in ALDL, TURN IGNITION ON, then unplug IAC first? This closes off IAC port.

Yes, I did. To confirm I did the right thing, I pinned the ALDL pins used to show the CCM/ECM/ABS/etc. codes on the dash. I did see the code reading process start before I got out and unplugged the IAC.

I could remove the IAC again and confirm the pintle is fully extended before moving to the next step, unless that runs some kind of risk.

Also, throttle blade closes as u back out set screw. At 60 counts, most of the idle air is going thru the IAC.

Now it is closer to 80 with the screw backed out more than it was before. Hmm. I ordered a smoker to see if there's a leak somewhere, should be here on Friday. When I have more time to do this I'll go through the entire procedure once more. I was sort of rushed by having to quit and pick up the kiddo yesterday.

I don't know if this matters, but at the point where I was going to adjust the stop screw with the engine running I did have TunerPro RT datalogging via the ALDL to get a better read on RPM than the dash itself.

[XfireZ51]

I don't know if this matters, but at the point where I was going to adjust the stop screw with the engine running I did have TunerPro RT datalogging via the ALDL to get a better read on RPM than the dash itself.

I unplug IAC w Ignition ON but not started. Then go back and turn OFF ignition. I do use my datalogger to check RPM. U should turn throttle set screw until idle is slightly lower than the desired idle speed. That way the IAC will intervene(when everything is plugged back in) to raise idle to desired. I usually do this after the fans kick in so as to have an additional load on motor and an additional margin of IAC counts before finalizing.

If u have IAC counts, that would indicate no leak especially w 60. The IAC is doing all the work of keeping the motor idling.

[scorp508]

I unplug IAC w Ignition ON but not started. Then go back and turn OFF ignition.

I did the same.

I do use my datalogger to check RPM. U should turn throttle set screw until idle is slightly lower than the desired idle speed.

That's the step I was on, but the idle started creeping without me making screw adjustments. I'll give it all a go again when I have more time on hand. :cheers:

[scorp508]

How far should a fully extended pintel be? I'm measuring 17.6mm from the tip to the black casing just before the o-ring. Closer to 27.5 if I measure tip to the flange that mates against the TB.



https://uploads.tapatalk-cdn.com/20240703/fb9bd8ec131859c135878104440c94e3.jpg
https://uploads.tapatalk-cdn.com/20240703/d712c6c8b057ffbd59858628d0ce2310.jpg


This looks like close to 28mm to the flange is accurate. I just tried again and even with the screw fully backed off I can't get it below 1200 rpm when commanded to 850, but if I button it all up it idles nice and low.

https://uploads.tapatalk-cdn.com/20240703/6b7f15854fbb6f6559d30686e52d043c.jpg

[XfireZ51]

When u connect ALDL, turn ignition ON, how many steps are showing for the IAC using ur datalogger?
One other thing to try is take IAC OUT and plug IAC port to seal it. Make sure the IAC IS UNPLUGGED b4 trying to start motor.

[-=Jeff=-]

To confirm the IAC is all the way in. I loosen it so I see it move away from the TB, then tighten after I unplug it.

I was fighting IAC stuff last year. Mind posting your IAC table settings in the PROM?

[scorp508]

I was fighting IAC stuff last year. Mind posting your IAC table settings in the PROM?

From another thread...


One other step I do is to modify the IAC Min Air table in the calibration to match what the actual IAC counts are in operation.

It would appear I don't have this table as the only IAC related thing in my XDF is "IAC Position vs. Coolant Temp". My understanding (possibly wrong) is the IAC vs. CTS is only used during Open Loop. However, open loop does seem to be where most though not all of my stall issues take place.

Looks like it is in my best interests to go through the IAC procedure anyway and see where I land.

Currently it is mostly the same as the stock BMCB with a couple changes.


Deg C / Deg F / Steps
-40 / -40 / 95
-16 / 3 / 78 (Stock BCMB value is 90)
8 / 46 / 51 (Stock BCMB value is 32)
32 / 90 / 24 (Stock BCMB value is 12)
56 / 133 / 10
80 / 176 / 10
104 / 219 / 13
128 / 262 / 25
152 / 306 / 45


Is this table read as a base value, or in other words, "no less than this many steps" when the table is being utilized?

[-=Jeff=-]

While I don't know how the car utilizes the table completely, my car had some modified IAC table and I had a stalling issue, changing the IAC all back to stock fixed the stalling issues I had

[scorp508]

So, I may have found part of my problem.

I purchased a Kent Moore J 37027-A Idle Air Control Motor Analyzer Tool. With the IAC motor removed and the tool connected I can successfully drive the pintle back and forth.

I opened the pintle (shortened it) with the tool, measured the distance from the pintle tip to the housing, then plugged the car harness back to the IAC motor. I then paperclipped ALDL pins 4 & 12 to show codes on the dashboard and turned the key to the "on" position. I could not visually see any IAC movement. I left the key on for a couple minutes and the position did not change as the measurement was the same.

I then repeated the steps but this time paperclipped ALDL pins 3 & 4 to make the SRC light blink for codes instead of the digi dash, but had the same results.

For whatever reason the car is not fully extending the pintle when I have the ALDL paperclipped as the procedure suggests it should.

I suppose that leaves me with at least a few options. Either I'm not using the right combo of ALDL pins, or this doesn't work in 1994 models, or perhaps the car's harness is messed up and unable to extend the IAC on its own, or something else which hasn't come to mind yet.

The 1994 FSM procedure is absent of the steps to extend the IAC.

Idle Air Control (IAC) Reset Procedure

Reset IAC Valve Position:
1. Depress the accelerator pedal slightly.
2. Start and run the engine for five seconds.
3. Turn the ignition OFF for ten seconds.
4. Restart the engine and check for proper idle operation.


Off to do a little more investigation. I may check if the wires from the ECM to IAC for continuity.

[scorp508]

I tested the 4 wires from the ECM (Red connector A, terminals 4, 5, 9, 10) to the IAC and all passed continuity.

[rufcar]

My 90 has been doing the exact same thing. I did install a NOS IAC and it seemed to help but does stall when cold. Today its stalled 3 times from my garage to my home which is 3 stops and about 800lf. Once home I loaded up drove 30 mi to a show and 30 mi back and never stalled. I have not reset anything. I am pretty certain my problem is more common in 90 cars and 93 up seem to not have that problem. The PO of my car was a GM wrench and everything is new. I spoke to Hibeck and everything he mentioned was done. I thought the same as you that if I can get that bottom idle up say 100 rpms it might go away but I have not attempted that. I will continue following to see if you ever solve it. One thing Mark suggested is the soft line at the rear of the engine as it can suck in oil causing that problem but mine was replaced with the new hard line. Jim

[scorp508]

I found my Vetronix Mastertech scan tool (came after the Tech-1A) does have an IAC test where you can increase/decrease the desired idle speed. Moving the idle up/down via the tool did work, so at least I know the ECM is capable of moving the IAC.

I think what I'll attempt next is just manually move the pintle in via the Kent Moore tool instead of trying the ALDL part as that seems to not work in 94, then do the rest of the process.

[scorp508]

One thing Mark suggested is the soft line at the rear of the engine as it can suck in oil causing that problem but mine was replaced with the new hard line. Jim

I checked my MAP hose as well, and it is the correct hard line with thankfully no oil in it at the moment. :)

[XfireZ51]

If u datalog, I use ALDLdroid, connect everything up and then scan w ignition ON but no engine start, see what the IAC STEPS are. Should be 155 if fully extended. Before datalogging, I usually go thru this step just to check the nominal values in various sensors, ie O2, MAP, TPS etc.

[scorp508]

Finally a GOOD update. :D

Xfire, I realized I overlooked something when paperclipping my ALDL before and trying to extend the IAC. The one combination I entirely overlooked was pins 4 & 6 (for a 94) to flash the SES light for codes. I had only done the CCM and Ride Control pin combos. Once I used pins 4&6 to flash SES, then yes the IAC motor fully extended itself as you said it should and I could carry on from there. :cheers: Perhaps a small tweak to the steps helps anyone in the future.

Anyway, I also got some good information from Marc Haibeck including my now knowing a '94 parks itself at 135 steps at engine off instead of 150 like earlier years.

With all of the above I'm currently here which is way better than where I was. The car is no longer stalls while coming to a stop while warm and the engine transitions much smoother between gears. I need to let it cool off fully to try it in open loop which the originally problem of stalling was. :)

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I couldn't get it to idle on its own lower than ~750-775 with the IAC fully extended and unplugged. This is a ported/cammed LT5, so maybe this is about as good as I'll get.

[XfireZ51]

Mine is ported and cammed as well. U want to set the idle rpm just below where u want it to idle normally so that the IAC will intervene and keep it from stalling out. My motor idles somewhere around 900 rpm +/- 25 or so rpm. U should also work on timing and fuel tables to smooth things out and avoid any choppy transitions of both. Also, the throttle follower tables should reflect the IAC steps u r actually at for idle.

[scorp508]

Mine is ported and cammed as well. U want to set the idle rpm just below where u want it to idle normally so that the IAC will intervene and keep it from stalling out.

It idles smooth at 850 once warm and I went 75 below that with the throttle stop screw. Should I bump that up to say 800-825? I wasn't sure just how quick the IAC can react.

U should also work on timing and fuel tables to smooth things out and avoid any choppy transitions of both.

I'll check them again and see what it's like down there.

Also, the throttle follower tables should reflect the IAC steps u r actually at for idle.

I don't believe these follower tables in the XDF I'm using. :cheers:

[XfireZ51]

The way to set the IAC is after u have it idling just below ur desired idle, say ALMOST stalling, IOW once u have set ur idle just below where u actually want it, connect everything, restart w diag tool and now see hiw many IAC steps u have for ur desired idle. I shoot for something like 10-15steps. If its above that, then open the TB, below that close TB. The point is u want to achieve as little ECM intervention necessary for the motor to operate. IAC reaction is also something that can be modified slowing it or speeding it up.
I like to set my IAC when fans are running due to the extra load so as to give me an extra margin.

[scorp508]

I gave it another try yesterday while prepping the car for Carlisle and was able to get the IAC steps to 16 at warm idle with fans on. :cheers: