What is the IAC doing while not at idle? I was driving around today watching the IAC counts and a few other things and they are moving all over when well over idle speed and on the throttle.
86 MT. I know Ford, only because Forscan tells me, had different modes for a couple different states but I had assumed that when not controlling the idle speed the IAC
on one of these would be closed. The ECU seems to be using it for something. Do the MT cars programing have some idle delay or such for between shifts?

 

The idle air control (IAC) valve primarily focuses on controlling the amount of air bypassing the throttle plate when the engine is at idle. Its main job is to ensure the engine doesn't stall when the accelerator pedal is released and the throttle plate closes. However, the IAC also plays a role when the engine is not at idle:

• During Acceleration: The IAC valve works in conjunction with the engine's main computer (ECU) to maintain smooth acceleration from idle. Although the throttle plate opens to allow more air during acceleration, the IAC valve may fine-tune the airflow to prevent hesitation or misfiring, according to GreatWater 360 Auto Care.
• During Deceleration: When the accelerator pedal is released, the throttle plate closes, and the IAC valve becomes essential to prevent the engine from stalling. It allows a controlled amount of air to bypass the closed throttle plate, ensuring the engine continues to run smoothly until it reaches a stable idle speed.

In essence, while the IAC valve's most prominent function is during idling, it actively assists in transitions between idling and other engine operating conditions to ensure a smooth and responsive driving experience.

 

The IAC follows the throttle off idle to provide some dash-pot function and smooth out the airflow as the throttle closes. Here is some data from a 5 speed tpi car to illustrate:

https://datazap.me/u/tequilaboy/feli...9479-8504-7441

Same data with additional signals displayed: https://datazap.me/u/tequilaboy/feli...9479-8504-7441

 

That's what I was looking for, thank you. Curiosity satisfied.

Related question, what is the actual voltage of the TPS that the ECU considers to be idle?

 

Tps load axis < 1.17% (or 3 raw counts). This should be about 0.7* volts.

Tps load axis example data: https://datazap.me/u/tequilaboy/aldl...1-180-70-38-31

In the above log, the tps load axis was still zero as high as 0.69 volts and was equal to 1.56% (4 counts) at 0.73 volts. Sorry there is no data between 0.69 and 0.73 volts.

In the previous log, you can see the IAC already reacting to 0.65-0.67 volts, so some minor variation is possible from car to car or due to learning.

 

Another observation/question, my 86 4+3 acts like it will not idle down till the car comes to a stop. You can clutch in and go to neutral at 75mph and it will stay at 10 or 1100 till you stop rolling when it almost immediately idles down
to 750ish, and I can see the ECU is commanding this on the scan tool. I assume that's intended behavior? I know the TPS is reading right. For downshifting maybe?

 

Prevent stalling, most likely. My (box stock) '92 LT1 does the exact same thing. As does my '95 putt-putt Subaru. I don't appreciate it, but that's what they do.

 

I'm gonna have to pay you to keep answering my random questions.
Pretty sure I know the answer to this but does an (early) 86 ecu alter the fuel map to account for the EGR being activated?

 

There is no direct impact of EGR on fueling. However, the exhaust gas should displace some air in the intake and cylinder and influence the load slightly.

I've reviewed a few logs from a few different cars, and I can see no noticeable change in the mass air flow, but I would expect the mass air flow to drop slightly with EGR active for a given manifold pressure and rpm or throttle position (difficult to isolate due to nature of design).

Here are a couple of examples: https://datazap.me/u/tequilaboy/89-n...1333-1336-1343

https://datazap.me/u/tequilaboy/feli...40-11164-11290

The examples seem to show a slight increase in the mass airflow/load for a given throttle position, so its possible that some intake air is really flowing into the exhaust. Unfortunately, I don't know if the EGR is actually functioning on both of these cars, but the diagnostic switch seems to indicate so at least in once case. The influence seems to be so minor that it is negligible.

 

I don't know about TPI cars, but I know the later LS1 idle speed is increased while the vehicle is moving. I don't know all the reasons why but I think one could assume it helps prevent accidental stalling of engine and improved throttle response off idle. IAC valve adjusts air flow to raise or lower idle speed.

It is one of those driving refinements that exists in OEM tunes but generally not seen in aftermarket EFI systems.

 

Fixed it.