1995 mercury mystique idle problem. the idle will not increase when placed in gear(drive) causing slow idle .?

car does not stall but idles too slowly when in gear, when radiator fan comes on, etc. car has had a recent complete tuneup and runs very well other than this. i am not getting any codes back, check engine light is not on. i have replaced throttle body, iac, throttle position sensor, and egr sensors with used parts. no change. why is pcm not increasing my idle? thanks