hope someone can help and maybe check their x5 but my 04 e53 3.0d sport, whilst probing around the engine bay I noticed the top radiator hose that goes to the expansion tank was warm and then got HOT. BUT the bottom hose was COLD, the car was idling for about 45 mins and neither the fan kicked in or the bottom hose heated up, so is my thermostat buggered?temp outside was 20C not too cold at all, should the fan have kicked in whilst idling for so long? or it being a diesel takes much longer?or is the rad in the diesel ultra efficient????

I drove it around all summer in sweltering heat and never noticed if the fan actually came on or not, but having said that wouldn't I have fried the engine by now if the coolant was not circulating/thermostat not opening?

any advice would be much appreciated