I bought a 1990 Spec Miata in March. I hope to do my first race Memorial weekend.
When I bought the car it idled around 1800-2000 rpms. The seller said it always did that. Now I am new to Miatas, but not these engines. I have done Rally Cross with 1.8L Escort GTs for years and years. I also have tuned fuel injected cars myself so I'm very familiar with ISC strategies. However this one has me stumped. After a lot of work, I now have it idling at 1500 rpms...
After much testing, there is only one way I have found to get the ISC to go into closed loop, and that is to start the car from completely cold and don't touch anything. It will fast idle as is normal on start up, then 15 seconds later come down to ~900rpm. As soon as you touch the throttle the RPMs will climb to around 1500 and never go down again.
What I checked:
Searched for vacuum leaks, blocked hoses, etc. Did everything except remove the manifold. No changes except when i block the ISC tube.
Checked air bypass screw (was turned all the way in), this doesn't make a difference because car never goes into closed loop
Coolant temp sensor - matches voltage and ohms range as car warms up
Neutral/clutch switch - measured from the ECU connector and is correct (clutch switch bypassed, but ECU still get 12v when in gear, 0v neutral)
Throttle position sensor - throttle blade closed the TPS is at exactly 5v, and the IDL is at 0v, changes to 12 with slightly open thottle
Removed and cleaned the ISC, valve is clean and moves with key on
Removed and checked the thermostatic air valve, also blocked it off completely with no impact
Removed and cleaned throttle body, even reset all the stop screws so the throttle blade comes back to perfectly closed
Set Airflow Meter back to stock (I think, hard to tell), appears previous owner adjusted the airflow meter quite a lot. This did help a bit, but only because the AFM is restricting air flow
Base ignition timing is ~14 to ~15
Basically checked everything here https://www.miata.net/garage/isc.html and here https://www.miata.net/garage/ECU%20Pin-Out%2090-93%20MT.pdf .
Sorry for the long post, but this is driving me crazy. At this point I wish I could just hook a tuner up to it so I could know what the ISC duty cycle is at. Is it worth it to just replace the ISC? They are so expensive.