Most likely heater core or line running into heater core plugged at Bypass. The reason it blows cool air while idling is fan blowing through core cools off the antifreeze in the core..meaning fluid is not flowing through much. Remove heater hose from waterpump that flows into heater core. Use a piece of hard plastic tubing with end cut at angle(get at hardware store..get largest diameter to fit inside heater hose)...and insert into heater hose all the way up to blocked area. Rotate round and round...reaming out the blockage with the hard plastic tubing angle cut end. Shove through as far as you can past the blockage. Take out. Pour some antifreeze into heater core hose then hold it lower than heater core and let drain out so debris flows out. Remove heater hose from other side of heater core( best where it flows back into engine.) Use garden hose and insert into heater hose. Give it a blast and back flush all fluid out of heater core till it runs clear. Put hoses back on and antifreeze in reservoir...give it a try. You can have shop do same thing. Many have a high pressure back flush setup to do this power flush. A power flush will not do the trick good enough.. You must ream out that blockage! If you haven\'t changed your thermostat for 3 yrs, might as well replace at same time.
Could be low on antifreeze. The heater core is the highest spot in the cooling system and it may develop an air pocket. Another possibility is the thermostat is stuck partially open and doesn't let the engine warm correctly, watch your temp gauge on the dash and see if it comes up to the middle like it should while idling. If it doesn't, it may need replacement.
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