A faulty thermostat is to blame for your freezing car.
Dear Tom and Ray:
I'm having a problem that just started recently. When I start the car, it takes a while for the engine to warm up. If I sit there, eventually the temperature gauge will go up to normal. But once I start driving, it goes back down to "cold," and cold air comes out of the heater. Turning the heat on causes the engine temperature to drop even faster. It is freezing in the car! I took out the thermostat and tested it, and it is working properly. I changed the heater core, too, and the water pump is new. Can you tell me what else I can look for or test? -- Joe
TOM: Well, look for that thermostat that you tested and then reinstalled. Because that's what's causing your problem.
RAY: I agree. You can test a thermostat by putting it in a pan of water and heating it up. You use a thermometer to see at what temperature the thermostat closes and opens up, and then you match that to the thermostat's ratings. And I take it yours passed the test.
TOM: But thermostats are finicky. They can work intermittently. And they can be "lazy," and open or close only some of the time. Besides, since the average thermostat costs eight bucks, we ALWAYS replace the thermostat first if the symptoms point in that direction.
RAY: Your symptoms are pointing in that direction, and jumping up and down with a neon arrow, Joe. They're textbook. Your thermostat is stuck in the open position, and it's never letting the engine get up to operating temperature.
TOM: So your coolant is always circulating through the radiator. And when you drive, cold air blows across the radiator and cools it off even more.
RAY: Running the heat cools off the engine, too, since the heater core is really another small radiator.
TOM: So, pop a new thermostat in there, Joe, and your problems should be solved. And chalk up the cost of the new heater core you installed to an expensive lesson in how to work upside down while lying under a dashboard.