A diesel engine is Moving so much air internally, at idle ... it is Air cooled. Has little use for the liquid cooling system...
Blocking the air flow through the radiator Will help get the engine & oil warmed up... The engine needs No additional Cold air blowing across the outside of the engine...,
Gasoline engines will Heat at idle ... a diesel engine, Not somuch...
I have Always blocked air flow on diesel engines during the winter.... Either with factory air dams or the ones I have devised to do the same.... On tractors, mowers, trucks & heavy equipment...I have never had a diesel engine that had any over heating problem with blocked rad air flow, even when worked Very hard... KennyV