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Has anyone built a custom idle controller.
My idle has never been stable and i think it might be better if i setup something that can bypass the computer. Something with relays maybe that can be triggered with the idle switch and ac.

that sounds very adventurous. I'm guessing it will be cheaper and less of a headache just to fix it instead of bypassing it. random guesses would be adjust the idle screw, the idle air screw that's on the throttle body, unplug your ecu for a little while and let it reset it self and maybe get a new o2 sensor. don't know if any of that would help just random guesses

x2.

Fix the problem without creating new ones.

02 is new
idle screw has been adjusted many many times and it runs for a few days but then it does whatever it wants.
it also has a tendancy to be stable at one light and then the next it idle hunts or is just too low or too high
then at the next light it is fine
then the ac seems to make it drop even though the input wire to the ecu has the input from the ac relay

This would be an interesting project. This is what you'd have to do:

Tap the engine speed sensor from the distributor. I'm not sure the format, but I assume it's a pulse.

Convert that signal to a steady voltage, and use that to modulate the IAC. Doing this in pure analog would be possible, but difficult because there are a variety of latencies involved. If you're good with microcontrollers, you could write some code to sort it out nicely.

Then add a bypass (relay probably too aggressive) and tap the TPS. Smoothly engage/disengage the IAC when the throtte rises above 0%.

You might have trouble compensating for sudden engine loads (ie. alternator/AC). You need to respond quickly enough to the RPM drop to prevent it from stalling, but moderately enough so as not to overshoot the desired engine speed.

Probably a multiplied overshoot would work the best.. ie. your table/calculation says 50% IAC will get you to 850rpm. You're at 600 at 40% throttle, and RPMs dropping, so apply 50%, plus half your distance to it (so 55% IAC), and re-evaluate next cycle. Same thing for reducing engine speed. Then adjust your table accordingly.

This would be a fun project.

*edit* You know, if you're going that far, you should go with a fully computer mapped throttle system. I guarentee you'd be the only MX-3 that has one. lol.

I did a BP engine swap 4 years ago. Bad idle was a common topic back then too. While doing my swap, I cleaned all of the ground lugs, some with sandpaper, and I added a few strategic ground wires while I was at it. My idle was rock solid up until recently. It's still pretty good.

I'm pulling out the clutch, and I might take the grounds out and clean them up again. It would be very interesting to see if idle gets solid again.

Regarding nightfire's post: you probably would want to use a PID control algorithm for your control loop, but I suspect it would be hard to find the right parameters for it.

i have added many grnd wires but i might have some other original wires that could be shorted.
I have replaced the wires from my 02 and Distributor completely although the originals are still in there and i need to pull them out.
I also had to bypass my temp fan wire to fan relay.