Idling will indeed cool down a diesel but , They will also warm up to a certain extent at idle also theirs a happy medium they hit and . id say the thremostat stays closed to the radiator for the majority of the time , I have ran rigs i have ran tractors . Owned diesels and have yet to ever see my pickup i own now , no matter how cold it is out side Ever have the temp gauge drop below 165-150 while idling,, Incomplete combustion well thats whene we first crank em up in the cold all that banging missfiring and carrying on that the do,, Havent seen a engine yet cool back down to 0 degrees while idling ,, This up the idle and Incomplete combustion stuff i swear was dreamed up by some nut who dont understand how a diesel engine works or had been around to many with the thermostats removed,,,, With out the thermostat Thene maybe it would cool way way down but ........ With a working stat . Nope
Plus Most fans to this day on most engines are either clutched or electronic so theirs not a constant pull of cold air thru the radiator... last VW Diesel i owned you could leave it Sit and idle all day on a 15 degree day and every now and then that electric radiator fan came on....... Yea sure was cooling off wasnt it , NOT.