I’m guessing there is an air bubble somewhere in the system I’ve taken the over flow cap Off many times to try and let air out of the system I can’t seem to get enough air out is there a bleeder valve for the coolant system on the 5.4 can anybody help I need heat I got a little girl. this truck is like brand new even though it’s a 2003
Your coolant is probably low in your radiator. Do not check the level in the overflow jug - check the radiator when it is cool. When your engine slows to idle, there is not enough hot coolant flow (due to a low level) to keep the heater core and HVAC discharge air hot.
You could have a bad radiator cap, and/or an coolant reservoir jug HOSE that could not be tightly fastened or have pin holes in it. When the engine cools, it draws coolant from the reservoir back into the radiator. If the cap is bad or the hose has a leak, it will draw air into the radiator.
Is your upper coolant hose collapsing (even partially) when the engine cools? If so - it is a classic sign of a bad radiator pressure cap.
If your radiator is low, you probably have a slow leak somewhere.