I don't know how an open thermostat would cause this problem. I can see it if the problem was the reverse: heat while idling, no heat while driving. For this situation an open thermostat doesn't fit.
Neotate, does your coolant temperature stay dead center when you are driving on the interstate? I mean DEAD center, not 2 degrees off.
Last edited by WDE46; 11-29-2012 at 11:15 AM.