I have a 1997 ford f150. New water pump, new thermostat 195°, heater core has been flushed out. Problem is the heat will blow warm for few mins then blow cold air for few mins. What else should we do? We need heat! Thanks
There is a blend door actuator in the heater box; that should open when you turn the knob to "hot", allowing warm air to come out. It is probably stuck, or it's not operating correctly. Could be a bad actuator, or a problem with the circuit.
Fords often have a heater core bypass that allows coolant to bypass the heater core, kinda like a pressure relief valve. To test it, warm up the engine and pinch the bypass closed (wrap a towel around the hose if you use locking pliers to avoid cutting it) which forces coolant through the heater core. Check to see if you have heat...if so then put a valve in the bypass hose and close it off during the winter.
If still no luck then... -check the coolant level- -Check radiator cap- -Bleed the system- -Pressure check the system- -change thermostat- -check blend door operation- -Check heater control switch-
If you have heat while revving the engine but no heat at idle, check your water pump.