Ok, so call me a little confused on the whole IAC thing. I recently had a no start condition that I believed to be caused by a sticking IAC. I took it all apart, cleaned it all up, and the car runs great. I just have one little quirk that I was hoping somebody could help me with. Now when I start the car up it fires up and instantly jumps to ~1700 RPM and then it takes a few seconds for it to step down and idle where it is supposed to.

I have a datamaster data log of this.

Key On, engine not running:
IAC Pos 160
Eng Temp 11.8*C
Idle RPM 3188
RPM 25

As soon as I start it I have the following:
IAC Pos 160
Eng Temp 11.8*C
Idle RPM 888
RPM 1700

Now over the period of 15 seconds, the IAC steps down in a nice linear fashion until I have the following:
IAC Pos 49
Eng Temp 12.5*C
Idle RPM 888
RPM 900

I can pull up my program in tunercat and take a look and I have the following:
Target Idle RPM vs. Coolant Temp:
8*C - 900 RPM
20*C - 850 RPM

IAC Warm Park Position vs. Coolant Temp:
8*C - 100
20*C - 90

I know the IAC Position values are way higher than stock. Many years ago I had to play with them to get it to behave right with my aftermarket TB. I have no problem tweaking them if that's what needs to happen, but I'm confused. What is the relation between the IAC steps in Tunercat compared to the IAC steps in Datamaster? Why is it starting out at 160 if I've got it programmed to start at somewhere between 100 and 90 for this temperature range? Is there somekind of offset between the two values or are they not linear with each other?

In the ECM Constants table I have the following:
IAC Default Keep Alive Min Position - 32 steps
IAC Minimum Keep Alive Min Position - 10 steps
IAC Maximum Keep Alive Min Position - 70 steps

These appear to be stock values. Right after startup, when the car finally settles down to it's idle, the IAC position is 49, so this would appear to be within the proper range of the keep alive values. Once the engine is fully warmed up I actually have an IAC position of 0 at idle, according to datamaster. Meanwhile the commanded idle is 688 and it's actually idling at 775.

This tells me that the minimum throttle stop on the TB might be a bit too far open and I might need to try and adjust it back a hair. If I remember right this is tricky on my POS holley TB. Anyway, that still doesn't explain how the steps I see in datamaster don't seem to line up with the steps in Tunercat. Ideas?

 

I backed off the minimum throttle screw one full turn and when I started it up, it only went to about 1200 RPM's or so. I've had trouble with this TB, if you back it off too far it'll want to stick in the closed position. Holley's QC isn't the best.

I would still like to know if anybody knows about the IAC counts in the two programs.

 

Could it be something to do with learned vs commanded IAC settings?