After replacing valve seats on 2000 Mitsubishi Montero- when on open loop it runs fine but then on closed loop after its warm it has a horrible idle like barely runs when you unplug the mas its fin?

When you unplug the MAS, the ECM goes back to open loop mode, so this does not necessarily mean the MAS is bad.

Put a scan tool on the system and find out what sensors are giving erroneous data.

Its also possible that the ECM needs to relearn the engine, as it probably stored operational data for a poorly running engine, but the open/closed loop point seems more interesting.