Have a 2003 f150 that I am going through the first winter with. When I leave it idle for awhile to try to get it warm the engine temp does not seem to come up and does not throw much heat. When I start driving the temp comes up fine and throws heat. Just does not heat up at idle. Any thoughts?

 

Low coolant level comes to mind at first. Next would be a thermostat that's stuck open or semi-open...