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I am having issues with surging at lower cruise speeds and we finally figured it out it is being caused as my IAC is turned on or off depending on the rpm. The tune file given to me has the IAC set to turn off at 1700 rpm and this apparently is too high as it turns off and on as the car is cruising at lower speeds which may vary between 1500-2000 rpm. I can't say why the file has this 1700 rpm setting.
The engine will idle smoothly at 900 rpm and is set for a high idle point on a cold start at 1200 rpm.
How do you determine what is the proper point to turn off the IAC? Looking at other tune files, I see the IAC turning off at various lower levels but no explanation as to what the best setting is or how to determine it.
I am just beginning to learn about all this and the first issue I run into is about 40 levels above my knowledge and paygrade ;-(
Its pretty rare to close the IAC or "turn it off" in any conditions, typically outside of idle conditions the valve is positioned at the correct opening so it is ready to catch the engine as it approaches idle next time (this "correct position" would usually come from a base position or feed foward table). Some turbo toyota's are the only common application I have seen where the valve is closed under boost conditions - they do this as they arent plumbed as a conventional throttle bypass type setup.
Turning it off was probably a poor choice of words on my part. When it exceeds a certain rpm, the pintle is moved to a park position, waiting for it to drop below that rpm point.
My question is how to determine what that rpm point should be.
I would say anything above your high-idle value. I would only increase it from there if it affected holding the high idle, or I was experiencing stalling problems returning to idle (and then you might just need to increase the "park" value).
There should be more than just an RPM condition, usually you would have at least TPS or APS, vehicle speed & RPM conditions to meet before idle control attempts to take control of engine speed. In most cases an RPM activation condition about 400-500RPM above normal idle speed works well for most ECU's I play with. But all have slightly different strategies in how the "ramp down" towards idle target happens so these lockout conditions will vary a bit depending on ecu.