Does anyone know how to tell if the PCM is operating in open or closed loop mode? Is there a pin to measure on the diagnostic connector? This is a 95 Eclipse 2.0 DOHC non-turbo ODB1. I have an Idle surge problem but don't know if it is temperature related (coil pack maybe) or only in closed loop mode. Car idles perfect for about 15 minutes, then you can hear the idle stumble then begin surging from 1100 to 1600 rpms or 1400 to 1800rpms depending on whether you step on the gas and idle at 2000 or higher and release. If you open throttle, no surging. When it surges, you can hear the IAC port sucking a lot of air, and if you close the port opening, the idle comes down. Read most of the posts on this subject and have replaced IAC because pintle would not turn but collar did. New part, pintle and collar turn together. Still surging. Is it possible a contaminated O2 sensor as a result of a bad head gasket (#4 cylinder burning coolant) could cause this? The reading swings between .2 and .8 volts so it looks good. TPS, coolant sensor value within range. With my finger on IAC, cannot feel click when key turned on so I'm not ruling out PCM, but no codes except (43)misfire occassionally. Also, when not surging, turning steering wheel increases idle the way it should so I think IAC is working. Also, no vacuum leaks detected.

Originally posted by EclipseW_RS
Does anyone know how to tell if the PCM is operating in open or closed loop mode? .......... The reading swings between .2 and .8 volts so it looks good.

When the voltage changes like that closed loop mode is working.