problem will be in heat control switch in dashboard
Check the thermostat or the radiator. The thermostat could be sticking or the radiator could be plugged.
First thing you should check is the antifreeze level. If that is fine, replace the thermostat. If it still over heats after the thermostat, then most likely you need to replace the waterpump. If your antifreeze is low, check for leaks, the radiator, hoses, heater core, and water pump. If the water pump is leaking, that is your problem, replace that and thermostat all in one.
The temperature of the coolant entering the heater core is controlled by the engine thermostat. The engine coolant thermostat is generally located at the engine end of the upper radiator hose.
The 1978 Chevy El Camino heater core is located behind the passenger side kick plate. Remove the heater core water hoses. Remove the heater core retaining bolts. Reverse the process to install the new heater core.
The heater hoses connect the block, radiator, heater core, and waterpump. To replace, drain the radiator and catch the anti-freeze for recycling. Remove the hoses and put on new hoses. It is kind of nasty but not very difficult.
check coolant check for leaks.......radiator cap must be free of rust and tight....plugged radiator, heater core. change the thermostat its probably stuck shut
It could be the thermostat. Open up the radiator cap when it is hot and see if the water is flowing in the radiator. If it is not then it may be a thermostat.
It will take a little longer for the cooling system thermostat to open up if the heater fan is on. The heater fan and heater core act like a small radiator and disperses heat energy when in operation.
Could be: Low coolant in the radiator - or - thermostat stuck open - or - heater core plugged.
Your heater core is plugged and you need a thermostat.
the heater hoses usually lead away from the thermostat and go into the fire wall. On the other side of the firewall is the heater core. the top hose on the radiator goes into the thermostat most of the time
follow the upper radiator hose, that should lead you to it.