1991 NX 1.6l AT will start, then drop to 500-600 rpm when cold. It idles and runs rough and weak up to 1500-2000. Above that it then runs smoothly. Idles and runs well when warm.
Check engine light turns off after start. Air regulator was cleaned and had correct resistance. Could this be a problem with the air regulator? Are they different for the automatic and manual transmission cars? Which part numbers for 1.6L?

Maybe test your coolant temp sensor. Check for vacuum leaks as well.

Thanks, I will check the coolant temp sensor and vacuum. The temp gauge seems to be working, but no numbers.
I replaced the vacuum hoses and tested some diaphragm and solenoid valves with a vacuum tester and manual.
I cleaned the throttle and used a fuel rail injector cleaner. I replaced several parts including fuel filter and hoses, spark plugs, pcv valve,
and IAC valve. Tachometer was intermittent. I checked the distributor connector and timing. Then I cleaned the ECU connector, reconnected it, and applied corrugated tubing to wiring harness on left behind plastic cover. The tach works. Odometer does not work, but the rest of the instruments work.

On all the old ones there are two temp sensors, one for the gauge and another for the ECM. The gauge sender is usually 1-wire with a block ground, the ECT is usually 2-wire with ground back through the ECM.

Thanks, I will check. I found the sensor specs.

I found 2 coolant temperature sensors on the motor. There is a 2 blade plug coolant temperature sensor on a coolant passage and a 2-wire connector thermo-switch on the thermostat housing. Found in 2 different chapters in Chilton's. The thermo-switch is only found on US auto trans cars. I will look into this.

I cleaned the coolant temp sensor plug. The sensor had 2k ohms cold. The thermo-switch seemed to be open cold. I found an ecu connector pin diagram. Probably need to do more testing. What does the thermo-switch do?