Is there a way to test and see if it is the water pump that causes the car heater to blow hot air one time and then, next time it blows cold air? We flushed the heater core and did other tests, so we do not believe it is the heater core. We left the radiator cap off and ran the car (ford thunderbird..'94) for 20 minutes and watched the antifreeze level come up, but doesn't go down, not circulating. We cannot afford to go to another garage. None of them seem to know what to do anywyas and we keep paying people to fix the heater and it is not fixed. We have put two new thremostats in car, too, figuring that maybe the first one may have been faulty, but should we try that again?
Note that the main purpose of a water pump in your car is to keep the engine cool. The heat that you get through the heater comes off the smaller hoses on the water pump. If your car isn't overheating chances are the pump is ok. Check your engine temp after the car has ran for awhile. The temp guage should go up when the car is warming up, then the guage will drop when the thermostat opens. It won't drop alot but you should be able to see it. Another thing you can try is to follow the heater hoses to where they go into the firewall and touch them after the car has ran for awhile. If your heater is on full they should both feel about the same temp. There is a control valve on the heater hoses that regulate the flow of water to the heater core and this valve may not be working properly.
The radiator actually holds the water/coolant and the pump circulates the mixture throughout the engine. Upon returning to the radiator, the water is cooled from air passing through the fins of the radiator. The pumps job is to circulate the mixture in order for the hot water to cool hence, the engine is then cooled.
Remove the thermostat totally from the car and then start it with the radiator cap removed.Verify that there are no vacume controlled valves in the hoses or intake manifold.If the water isn`t moving in the radiator,the impellers are broke in the water pump most likely shaft is broken.Dumb question here(is the pump belt driven?On the front of the engine or internal pump?)
Check heater control valve ( on one of the heater hoses going into the firewall), make sure there is no vacuum leak to it or the heat door. Does air blow on the floor when your heater is on? When you slide or turn the temp selector on your control panel you either move a cable or supply vacuum to a door that shifts air flow from vent to floor, is this happening?
On many cars you cannot see the coolant circulating especially at idle. Just because you do not have heat does not mean the water pump, and if your car is not overheating it is probably - I say probably - doing its job. So stop putting thermostats on, probably assume the water pump is okay. So, put a thermometer into the radiator to see how hot the coolant is getting - should be ~180 degrees or so. Hmmmm - Does this car have an electronic climate control system? Perhaps that is the reason you have an intermittant problem. Assuming manual climate control - It could be a vacuum leak to one of the manual vents that is allowing cool air to mix with the hot. Or, maybe the slider from hot to cold is not working. What troubles me is the intermittant problem here. So, I will go out on a limb and say it could possibly be a bad head gasket for reasons I will not get into here. In any case, Good Luck!
