[JohnJL]

I am at the end of my rope with trying to get this motor to settle into a ~900 rpm idle. I would love some help from some of the Megasquirters here. I did a live teamsession with a tuner but he got stumped too.

The motor:

bottom end is a 3.2 on a 3.0 crank with 10.5:1 compression pistons.
Twin plug with 2xMSD cdi boxes driving 2 vibration coils.
DR20 cams
Carrera aluminum intake
Stock carrera throttlebody, refurbished and functioning properly. Adapted for Mustang TPS.
Carrera idle valve. It passes a little bit of vacuum when fully closed, and opens fully properly. It closes on default when disconnected.
Throttle bypass screw cover removed.
Bursch headers, mk muffler.
Rising rate fuel pressure regulator baseline at 43psi.
JB racing 12-plug distributor

What is is doing:

The motor starts up no problem. It idles at 1500 steady and even. Temperatures settle in normally and megasquirt is responding and logging normally. AFRs are ~12-13 when idling.

When I try and get the idle down the RPMs go into an oscillating cycle between 800 and 2000 and eventually dies.

What I have tried;

I set the throttleplate to a small opening where it will idle steady, and adjusting the bypass screw directly effects the engine speed and MAP.

Open loop idle.
Set timing locked to 14 degrees. Have tried 10 to 16 with no difference in the results.

Disconnected the idle valve and checked it is closed (through it does draw a small vacuum. Not enough to effect the idle speed when blocking and unblocking it.

Looked all over for vacuum leaks. Double-checked torque on the intake. All the gaskets are new. Sprayed some brake cleaner around them while it was running and no change in idle. Sprayed around the injectors, throttlebody, idle valve connections, regulator and MAP sensor connection. No change in idle. No brake booster so thats not leaking.

It seems crazy to me that the idle oscillates and then falls on its face even with timing locked, no change in throttlposition and no change in idle valve. I dont think it is lean-surging, the AFRs are ~12-13.

Any helpful hints to diagnose why this is happening?