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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

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You're saying it alternates warm/cold? Or does it get warm, change to cold and then stay that way? Does it have an automatic temp control for the heat or a slider? –  Paulster2 Feb 19 at 0:52
    
No it does not stay cold. It warms back up. It has a knob that u turn for temp control –  user4528 Feb 19 at 1:11
    
Warm cold warm cold –  user4528 Feb 19 at 1:18
    
Both the hoses coming out of heater core are hot. –  user4528 Feb 19 at 1:59
    
Is it influenced by engine speed? As in cold at idle warm above idle? Does the temp gauge fluctuate with the heat output? –  mikes Feb 19 at 2:13

2 Answers 2

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.

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it seems to blow hotter air while driving. Now it does not have any heat until the temp gauge spikes. It will spike then come right back down and blow heat. What causes temp hand to spike like that then bliw heat? –  user4528 Feb 21 at 19:55

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.

Good luck

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it seems to blow hotter air while driving. Now it does not have any heat until the temp gauge spikes. It will spike then come right back down and blow heat. What causes temp hand to spike like that then bliw heat? –  user4528 Feb 21 at 20:06
    
First you need to make sure the gauge and temp sensor are working. If all the electrical stuff checks out, id start with making sure the coolant is full, then make sure there are no air bubbles in the system. When bleeding the system turn the heater on full blast to force any air out of the heater core. It sounds like it may be a water pump or thermostat issue. A stuck thermostat would be my first guess. Several things can cause the temp to spike though. Start with the easy stuff. Do the radiator hoses feel stiff once the car warms up? Any obvious leaks? Does the engine run smooth at idle? –  user4546 Feb 22 at 0:35

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