Idle and IAC, TBI

[AlexanderHL]

these statements apply to any ecm not yours specifically

the iac doesn't have anything directly to do with idle mixture. it uses it to control idle speed instead of moving the throttle plate like a carb or fly by wire engine. the computer meters its fuel to adjust the AFR, it doesn't add airflow to meter its AFR via the IAC. in other words the IAC adds airflow and the computer takes that airflow and calculates the fuel

your idle 'speed' is controlled with most modern ecms by a combination of IAC and timing advance

your idle 'mixture' is controlled by a combination of your VE table (or maf table if maf equipped) and the startup enrichment tables or whatever that takes the coolant temp and/or intake air temp and/or a 'time since start' timer and adds enrichment fuel, this is what replaces the 'choke' in a carb

Yes, but how do i get the iac to "learn" to be in correct position at idle? since for the ECM to adjust tables it needs to have O2 sensor up to temp. Right now my engine surges at idle.
I know i can put it in aldl mode and fully extend it and manually adjust the set screw for the trottle blades, but seems that the whole point of having an IAC is so that this shouldnt be needed?

[MO LS Noobie]

You are correct in saying that you should not have to train the IAC valve. There are baseline settings that should work out well, but you're dealing with a 35 year old vehicle with unknown repair history. You set the base idle speed when it is fully warmed with all loads turned off. This sets how much base air the engine needs and then the IAC just adds a small amount to bring it up to the correct idle speed, depending on engine temperature. Then when the engine is cold the PCM opens it more to act like the fast idle speed on a carburetor, and there is a slight delay function programmed into the PCM that mimics the dash pot that slows the return to idle speed. It constantly monitors idle speed and adjust the IAC accordingly.

I am not as experienced with the OBD I systems as I am the later model OBD II, but if it is surging at idle the IAC valve is overshooting the target idle speed. If your vehicle is stock then there is a problem with the engine, IAC valve sticking, vacuum leak, EGR flow, spark timing, or fuel. If the truck has a "tune" in it, posting the tune file here we could look at it and see if there's something wrong.

If it is rich as you stated originally, might clear all the adaptive values by disconnecting the battery cables from the battery and shorting them together with a 10 amp fused lead for 10 minutes with the key in the "on" position. Then disconnect your oxygen sensors and turn the key off. Re-connect your battery and start the vehicle. This will put it in the baseline settings and prevented from changing the fuel mixture. It should run pretty good if the "tune " is correct. It should not be rich or surge.