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?

IAC data.jpg
before tune 05-26-14.bin
log1 starting to warmup.xdl

[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.



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!