Have a 2007 5.9 that runs cool ,bought it used with 30000 kms and it was the same then changed thermostat no improvement will warm up when worked but soon as stop temp drops fast have the grill covered have tried cardboard over the rad. no change.
Thanks jim

 

They do cool down quickly (mine does it too)when stopped and idling.If you have an auto trans use cruise control to raise the idle if it's been enabled.Hit the ON button then SET and it will idle up if its working.Can also raise the idle with the increase button.