p38 bmw 2.5 td

hi i would be most grateful for help here. my range rover engine is idling at just over 2000/2500rpm. how can i tuen this rpm down? it is a straight 6 diesel bmw engine.

engine has been out and new crank fitted etc,and retimed. however we cannot lover the high rpms??
any ideas would be gratefully received.

Thanks guys!