Idle and IAC, TBI

[AlexanderHL]

Hi

My 89 g20 was running very rich and i have got it to run better. It is a 305 that had 350 injectors and 350 fuel pressure regulator. All replaced now :)

However it still doesnt idle good at all, still running too rich.

I know the IAC works as i have tested using paperclip in aldl mode, and it can fully extend and retract, but it doesnt seem to want to retract in idle-


So my question is how does the ECM control idle with the IAC? Since the engine doesnt know if its running rich or lean on startup (O2 not up to temperature) i guess it resorts to the BLM tables?

So, is there a factory blm setting hard coded in the ECM that car resorts to and then improves upon? Or is the BLM always learned by driving the car? If so, does it "reset" if i unplug the battery for long periods of time?

Could i get the IAC to open up on idle by slowly lowering rpm and giving the ECU time to learn and adjust blm tables accordingly?

Thankfull for any answers :)

/Alex

[MO LS Noobie]

Your base fuel settings are "128"on fuel trims. Once the computer is cleared it resorts to the 128 setting, and then modifies it with the oxygen sensor for your BLM setting (below 128 the computer is removing fuel from the base setting, above 128 and it is adding fuel). Any setting within 10% is considered normal. Resetting the computer just takes it back to the base 128 setting. The 128 is vehicle specific and takes into account the VE, fuel injectors, etc. on a coldstart looks at BLM and modifies things according to coolant and intake air temperature, load, and BARO.

For your idle there is a base setting. That is set by the throttle blade screw, usually underneath the sealed port. If you extend the IAC all the way, during a hot idle event, the idle speed should be approximately 500 RPM, no AC and in park/neutral. When you give the computer control over the IAC it will retract allowing more air to bypass the throttle blades. This is also learned in the memory of the computer, and is also controlled by the coolant temperature so that you get a faster idle speed while the engine is warming up. It does not need the oxygen sensor input (rich or lean) because it is just trying to achieve an idle speed. The oxygen sensor will be used to adjust fuel trims.

[steveo]

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

[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.