Simply put. 1999 D1 only overheats in locked low. Drives without any issues for hours. Changed water pump, thermostat, belts, radiator, hoses all with upgrades. Could this be the transfer case Transmission or head gasket? Please need some good advise we are at a loss.

 

You might get more eyes on this if you post in the Discovery 1 sub-forum instead of this general one.

 

How about the viscous fan clutch?

 

My first thought too.