Okay, I've got the spark and fuel curves set. I can't for the life of me get the MSII to produce a constant idle speed from one run cycle (start to shutoff) to the next. The engine will run hot at 800 RPM happily one cycle, and then something like 1500 the next. Starting is also suffering, being somewhat of a pain when cold and pretty difficult when hot. I know I have the cranking PWs correct because it WILL start quickly IF the IAC is in the right position.

What I'm wondering is how the GM IAC works? From what I understand, the system closes the IAC when the engine is shut off. And then, I would guess, the IAC is opened to a certain position to allow for starting before it moves to whatever position necessary to obtain the desired idle speed?

From what I've read on the MS forums, the MS doesn't close the IAC upon shutdown (it can't as it doesn't have a constant 12V power supply), and it relies on the pre-crank period when powered up to open the IAC wide before pushing it back closed to X position (as is set in the tuning software). I'm just curious as to how I can get a consistant cranking position out of the thing as it's driving me !

MSII 2.905 code running Tuner Studio 2.0.8. Might take me a bit to get the tune off of the other computer...

 

I believe Key on engine off GM extends the pindle maximum then retracts it to a position determined by coolant. Then when running it looks up idle RPM. There may be an adder for initial start(flare) as well. Are you certain the IAC is wired correctly?

here are IAC routines explained by ECM.

https://www.thirdgen.org/forums/diy-...uide-book.html