same problem with my f250. when the truck is driving the heat is great but when i sit idle for 20 min it starts to cool down. the dealer told me the heater core is partially blocked. is this true? why cant they try to unblock it instead of installing a new one? i dont know if they checked the water pump but the engine does not over heat. i kind of hear a high whistle for a few seconds when i give it gas or let off.could that be some kind of vaccum leak? and could that be causing the bad heat at idle??? Please help!!! they want $350 to replace heater core and i am not sure that is the problem. they are known for guessing at fixes.they replaced brake pads and calipers for pull to the right when hard braking only to find when it need inspection the drag link was bad. but didnt admit they were wrong about the pads and calipers!!!!!!! now the after replacing that the truck no longer pulls! PLEASE HELP