Learning more about the engine idle and control systems on the Miata engine. Had stalling on cold start up, where throttle would need to be held down until the engine was warm.
Got the engine up to operating temp, connected the “TEN” and “GND” on the diagnostic port (sending a constant signal to the ISCV), then adjusted the throttle body bypass “idle air screw” to 850 rpm. Then checked that the throttle damper was adjusted to make contact with the throttle body at around 3000rpm.
Found the issue to be a reversed check valve in the air tube that feeds the ISC valve, which basically stopped the ISCV and air valve from doing what they should. Switched the check valve and now the car idles during cold start fine. Also when at operating temp, pulled the wire connected to the ISCV, heard a sound and the idle jumped up to 1500 rpm. Workshop manual says you should hear a click and rpm should go up to 1200 rpm. The discrepancy with my car is likely due to my idle screw being poorly adjusted.
Good site discussing issues with the idle control system: https://www.miata.net/garage/isc.html