IAC not responding like I think it should 7747

[mmigacz]

When my engine is fully warmed up, I have the IAC steps set at 0 to get the most amount of air flow over the throttle blades. Because this TBI is installed in a boat I only need IAC adjustments for cold starts and warm up. When I start a cold engine the IAC steps start 144. When they decay, I would expect them to decay down to the step value in the IAC - position vs temp table. Below is a snap shot of a cold start data log. Notice how the IAC decays down to 33 before it stabilizes. If the temperature is 16 degrees C shouldn't it decay to 75 steps?

7149
7150
7151

[lionelhutz]

I believe it follows the target idle rpm vs coolant temperature table once running. The IAC will go to whatever steps are necessary to maintain the designed idle rpm.

[mmigacz]

The way I thought this works.

At start-up the IAC starts in the park position (144 steps)
It then decays to the IAC - position vs temp
Once the temperature reaches 55 degrees C the IAC - desired C/L idle speed gives the IAC a target RPM to adjust to based on coolant temperature.

The example I showed above was below 55 degree's C, so I assumed the IAC would adjust to 75 steps (@16 degrees C). There are no target idle RPM's at 16 degree's C on the IAC - desired C/L idle speed.

What am I missing?

[steveo]

im not sure why it's doing that, it should target the 100rpm idle and just bottom out the IAC?

[jim_in_dorris]

C/L is closed loop. I looked at the log, you never are closed loop, that table isn't used.

[steveo]

perhaps they're speaking of closed loop idle speed control rather than closed loop fueling metering... big difference..

[mmigacz]

im not sure why it's doing that, it should target the 100rpm idle and just bottom out the IAC?

Why would it even target 100rpm when the temperature is less than 55 C. The lowest temperature in the IAC - desired C/L idle speed table is 55 C. The example was at 16 C. I could understand it wanting to target 100rpm once the engine reached 55 C or higher. :mad1:



The way I understand it, IAC logic doesn't care whether you are in closed or open loop fueling.

[jim_in_dorris]

Well, your understanding is incorrect. I just walked thru the assembly source code for the real time service routine 7 (where IAC is handled) in the $42 HAC and if the closed loop bit is not set, it skips loading that table!

[mmigacz]

Well, your understanding is incorrect. I just walked thru the assembly source code for the real time service routine 7 (where IAC is handled) in the $42 HAC and if the closed loop bit is not set, it skips loading that table!

I appreciate you looking into this.

When you say the table is skipped are you only talking about the IAC - desired C/L idle speed table? Shouldn't the IAC - position vs temp table be active after startup (open loop fueling)?

[jim_in_dorris]

You are welcome. If I understood better what you are trying to do here, perhaps I could point you in the right direction. I assume you are trying to control the idle speed. what idle speed are you trying to achieve? Remember that with that cam, you may need to idle a little higher. My guess is somewhere between 750 and 850. Anything below 500 automatically will invoke stall saver and move idle to 500 minimum, and I am not sure if the cam you are using will like idling that low. Also, there are a lot of conditions that may or may not adjust the IAC postion (like the park/Neutral switch, or A/C which I assume you don't have)

[mmigacz]

You are welcome. If I understood better what you are trying to do here, perhaps I could point you in the right direction. I assume you are trying to control the idle speed. what idle speed are you trying to achieve? Remember that with that cam, you may need to idle a little higher. My guess is somewhere between 750 and 850. Anything below 500 automatically will invoke stall saver and move idle to 500 minimum, and I am not sure if the cam you are using will like idling that low. Also, there are a lot of conditions that may or may not adjust the IAC postion (like the park/Neutral switch, or A/C which I assume you don't have)


Actually, my idle is great at operating temperature. It stays consistant at 800 RPM (0 IAC steps). No I don't have AC and I don't use the Park/Neutral switch.

The part I'm trying to adjust is the idle when the engine is cold. I want it to idle higher.

[EagleMark]

Actually, my idle is great at operating temperature. It stays consistant at 800 RPM (0 IAC steps). No I don't have AC and I don't use the Park/Neutral switch.

The part I'm trying to adjust is the idle when the engine is cold. I want it to idle higher.You have no use for P/N... but the ECM does. Hopefully it is in Drive at all times or this is all a waste of time.

Second remember we changed other IAC tables! So don't expect all table to be in sync with bin! Besides the IAC logic of this ECM/OS is known to be dismle at best but can be twaeaked to work! This is how programmers have gone through code with others and came up with this conclusion.

Third Your out of IAC steps so your dot going to get any more idle RPM! 0 is good when it's the hottest day after a full throttle run and checking IAC counts. Being an in depth tuner with your conversion you should know this is usually maybe 8? or 7? But now we hit a hotter day and a WOT run, pulled hard, worked harder then normal and we hit 0.

See the problem is your hitting 0 already! Way to soon. Going to have to give it a little more room to breath so most of the time you have say... 10? Now you have that extra 10 for IAC to play with to adjust Idle!

HTH! :rockon:

[mmigacz]

You have no use for P/N... but the ECM does. Hopefully it is in Drive at all times or this is all a waste of time.

Second remember we changed other IAC tables! So don't expect all table to be in sync with bin! Besides the IAC logic of this ECM/OS is known to be dismle at best but can be twaeaked to work! This is how programmers have gone through code with others and came up with this conclusion.

Third Your out of IAC steps so your dot going to get any more idle RPM! 0 is good when it's the hottest day after a full throttle run and checking IAC counts. Being an in depth tuner with your conversion you should know this is usually maybe 8? or 7? But now we hit a hotter day and a WOT run, pulled hard, worked harder then normal and we hit 0.

See the problem is your hitting 0 already! Way to soon. Going to have to give it a little more room to breath so most of the time you have say... 10? Now you have that extra 10 for IAC to play with to adjust Idle!

HTH! :rockon:

Glad your feeling better!

Right now the Park/neutral wire is open so the ECM thinks its in gear all the time (like a manual transmission setup). I do have a neutral safety switch, so I could use it give the ECM feedback when it is in drive and neutral. I was always under the impression it was not necessary with this application being in a boat (velvet drive transmission). Do you see any benefit in using it?

What changed in the IAC tables? Is there a link that describes the changes?

I'm aware that I should have 7 or 8 steps for idle adjustment. However, someone suggested that this is not necessary in a boat. I was trying to maximize the air flow over the throttle blades to make the smoothest idle with this cam. However, if the right thing to do is adjust for 7 t0 10 steps (fully warmed up), I will do it.