Our focus throws not heat in the cabin during idle. the heat shows the car is plenty warm but no heat inside the car. We have had the thermostat replaced (twice) and the coil. the system has been flushed repeatedly. Our mechanic is stumped and has suggested that we need head gasket work. Is this really neccessary?
I guess that his theory is that a gasket leak is forcing air into the cooling system and that is blocking flow to the heater core. Most decent sized shops should have a device to detect exhaust gas in the cooling system. Test for that, and if it’s there then head gasket failure would be the best bet.
If there’s air trapped in the cooling system it can cause this problem. Have your mechanic try this to remove any air from the cooling system.
Get the engine up to operting temperature. With the engine idling, slightly loosen the upper radiator hose clamp. Take a small flat bladed screwdriver and slip this between the upper radiator hose and the hose neck on the radiator. Allow the engine to idle until all that comes out of the upper radiator hose is coolant. Remove the screwdriver and retighten the hose clamp.