what is the DESIRED idle rpm set in the ECM?
the ECM can only command the IAC if it knows how much air mass is supposed to be at idle
Right now, the MAF meter is reading 7-10 grams of air at idle. I do not know the air mass that the ECM is looking for at idle. I can command any idle RPM that I wish, but if there is a way to command air mass directly (as opposed to calculating the requirement based on bore, stroke, number of cylinders, and the idle RPM I've commanded) I am unaware of it.
throttle plate airmass plus IAC airmass = idle airmass
Sure, that's pretty easy to understand.
therefore idle airmass at your rpm that is measured, it knows how much the TB flows (because you told it how much) then can conclude how much DC to command the IAC via the IAC transfer function.
I'm not sure that I follow you when you say that I told the ECM how much the throttle body flows at rest. I did not input any kind of data that directly represents throttle body flow. I did unplug the IAC and adjust the throttle stop screw until the engine idled well - I guess the ECM could assume that during that procedure the IAC was at full close and come up with the air mass that the throttle plate allows to pass by while at rest - but since the Ford IAC doesn't completely close, I'm not sure how the ECM could come up with throttle plate air mass based on that since various throttle bodies and aftermarket IAC components would undoubtedly yield different amount of air mass when at full close - that is assuming that unplugging the IAC motor moves the unit to full close in the first place.
therefore idle air mass and throttle body airflow needs to be accurate.
if not then decay and pre position will not be very helpful...the things that bring the idle down slowly into closed loop idle.
Metered idle air mass is accurate, as accurate as the Pro-M MAF meter can be anyway. Throttle body air flow, I'm not sure how the ECM comes up with that - or if the Pro-M system used that information at all.
IAC decay and pre-position are not parameters that I am familiar with, nor am I familiar with whether or not they can be adjusted in the Pro-M software.
It doesn't seem that others with the Pro-M EFI system have this issue - this is what leads me to think I have a hardware failure, as opposed to a software / calibration / tuning problem.
My question was more geared towards the IAC, specifically is it likely or even common that an IAC would not operate correctly. Do they get lazy, do they have problems not returning to closed position, etc... I have taken the IAC apart and cleaned it, verified that it is operating in some fashion (it makes noise, and I can feel the unit engage in some manner or another by resting my hand on the case), but I still don't know if it's working properly.
The second part of my question was whether or not the BBK throttle body on the car is likely a culprit.
The problem seems air flow dependent, because I can cure the issue by tightening the throttle stop screw. However, by the time I get the screw to the point that the engine doesn't die on return to idle, the computer is actively pulling timing to bring the idle RPM down to the commanded 1000 RPM. At that point, can I assume that the IAC is being commanded to full close? Or as closed as it gets on a Ford set up?
If that is the case, then my first thought is that the IAC can't respond quickly enough to keep the engine from stalling. Since no one else with the Pro-M system seems to have this problem, that makes me think the IAC is lazy or otherwise not working. Again though, I'm not sure.
Maybe a better question is:
Is there a good test procedure for the IAC valve? Something that will verify both operation and response time/rate/frequency?