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Hunting idle, even on open loop

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If got a idle that just won’t settle down.

The car used to idle just fine

Idle target is 1000 rpm, an it’s isolating between ~800 to ~1200

I’ve put it into closed loop with a fixed duty cycle, ive turned off idle ignition control, I’ve placed all surrounding ignition cells on the same timing and I still have a bad surge.

The setup: turbo 1.8l Miata

target idle AFR: 14.0

idle ignition timing: 14°

Injector duty cycle at idle: .6%

idle air solenoid duty cycle: 26%

any suggestions?

Either you have an intake manifold vacuum leak (check all vac hoses connected correctly, and the devices they are connected to aren't leaking).

Make sure Idle Air Solenoid isn't stickly / blocked. If you open the throttle slightly (adjust throttle stop?) and this stops the surge, then I would look at the IAC either mechanically, or adjust it's duty cycle to stabilize the engine speed.

Great I’ll check that out! But wouldn’t a leak just act like the IAC is open a little more?

The problem with some leaks is they are "variable" depending on the manifold pressure. That can encourage the surge.

I have no vacuum leaks. I’ve pressurized the entire intake, and it holds pressure with no leaks. And I’ve eliminated the pcv.

it’s not the IAC solenoid, as I have unplugged it and am running purely off the idle air screw.

is there any other factors that would cause idle to hunt?

if it's not air, then it could be fuel delivery -- do you see fluctuating Lambda values? Fuel Pressure, Fuel Filter, Clogged Injector.

Does your car have a fast idle thermo valve? A failing one of those would result in this and it’s a common item to go bad.

So, it was active lambda control. I had the proportional lambda control min rpm set to 750 rpm. It was becoming unstable AFR, and causing the rpm to surge.

I bumped the min rpm to 1500 and it idles like a dream again

Is it factory cam?

If you want a stable, solid idle without any idle strategy I would suggest AFR closer to 13, more air and less timing. Make a timing "hole" at target speed with timing increasing at engine speeds below the target to generate more torque to catch it if it falls low or loads are applied.

It should sound "soft" at target with this strategy.

I’ll try richening up at idle and taking some timing out, thanks!

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