heater blows cold air at idle
 

i have a 08 silverado 5.3 v8 and when you stop the heat turns cold.when i give it gas it warms right back up.thought it may be water pump but a friend told me it could be a vacum line.any suggestions?

Check your coolant level.

Allan

Test the temperature of the engine coolant by using thermometer and make sure you have enough coolant. Next see if both heater hoses are equal temperature. If one is hot and the other is cold, most likely the heater core is clogged and needs to be back flushed with a water hose. In rare cases the heat won't work because the water is not circulating with enough flow rate due to a rusted off water pump impeller, so be mindful of this should you not get your heat back after you flush the heater core. You may also need a water pump as well, especially if there is a lot of rusty coolant in the cooling system.