This gremlin is driving me crazy, so I am hoping someone may have experienced a similar issue and can help me figure this out!

1988 Pulsar NX CA18DE engine has run wonderfully for the past two years after I did a rebuild. Took it for SMOG earlier this month and 5 minutes before I arrived at the testing station, it started to feel as if it were missing/stumbling. Sure enough, the issue persisted during the test so it failed. I managed to limp it home, but have been testing/troubleshooting for the past couple of weeks and have discovered only one issue thus far: voltage on ECU pin #113 (A.A.C. valve) is lower than the service manual states it should be; 2-3 volts instead of 8-10 volts when idling.

I have performed all of the tests in the service manual for the TPS, CPS, WTS, MAF sensor, EGT sensor, both EFI relays, and the related wiring harnesses for each without finding anything deviating from spec. I have also tested the EGR, A.I.V. and power solenoids controlling air flow, replaced the exhaust gas sensor (it was due), replaced the fuel pump, and performed the mode 3 ECU self-test multiple times (no codes except for 55 indicating a healthy system).

Vacuum is stable at about 22 inches and I've sprayed everything under the hood to reveal any potential vacuum leaks - nothing. There are no cracked hoses or damaged wiring harnesses apparent. The engine starts fine but idles a bit up and down between 700-850 RPM when cold, then really stumbles after warming up and shudders between 450-600 RPM, sometimes stalling. If I try to apply some throttle, it will eventually rev up, but is blatting and stumbling all the way up.

If I unplug that A.A.C. valve solenoid electrical connector, the engine dies immediately, which *I think* should not happen, since it is only an air bypass for increasing the idle speed slightly, but hoping one of you can confirm that for me. If I pull the electrical connector from the water temperature sensor (that's how it's ID'd in the service manual - not coolant temp sensor), then the engine runs beautifully again, so this issue is only happening after warm up in closed loop mode.

Since the only issue I've found is that low voltage to the A.A.C. solenoid, I tested that solenoid outside of the vehicle and it seems to be strong and functioning correctly. You can hear that it's not getting enough voltage as it has a slight hum when the ignition is turned on, so I'm sure it's not fully extending, but again, should not make this much of an issue methinks.

At this point, I'm considering looking for another ECU on eBay to try unless you have any other suggestions. I'd replace the injectors and coils if the engine had any issues in open loop, but like I said, it runs perfectly as soon as I pull that temp sensor. I did test the temp sensor and it outputs the correct resistance values from cold to hot per the service manual, so I'm stumped. Appreciate any insight - thanks in advance!