Challenges setting minimum idle air.

[scorp508]

Originally Posted by -=Jeff=-

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

From another thread...

Originally Posted by XfireZ51

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]

Originally Posted by rufcar

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.

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. 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.

IAC Results.jpg

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.