Car is a 960 wagon with the B6304F motor, 106,000 miles. Idle RPM slowly drops from 750 RPM when the engine is cold to 500 - 550 RPM as the engine warms up. Is this normal? IAC is clean and appears to be functing correctly. Throttle Position Sensor indicates about 10 degrees when the throttle is closed(spec is 9.5 to 11.5). Putting the car into Drive causes a monentary drop in RPM. Turning the AC on does the same thing, RPM drops then returns to its initial value. Also have 233 diagnostic trouble code(long term idle air trim). Code can be cleared but will return after 10 to 15 starts. Throttle plate is clean and appears to close properly with little to no mechanical play. Car runs good except for the low idle RPM. The ECT(engine coolent sensor) has been replaced. Does anyone have any suggestions on how to fix these problems?

My 960 does the same thing (128K)as far as the idle rpms dropping from around 750 to around 550 to 600. I don't have or ever had a 233 code since I owned this car for the past 3 years. When I change from N to D the engine rpms don't change or when the AC unit is on the rpms remain the same. I will be interested to see what other posts have to say.