The last few days I will warm up my car (1998 Toyota Corolla) before heading to work (I live in West Michigan - the windchill has been around 10 degrees). When I get in my car the engine temp is half way between H and C and the temp in the car is nice and warm. After driving about 10 minutes or so I notice the engine temp is actually below C and the heater has stopped blowing warm air. Any advice?
Most likely a thermostat, next most likely low coolant in the system, not just the jug.
I agree with Waterboy and recommend addressing these items first, but if that doesn’t solve the problem, and the problem has existed as long as you’ve owned the car, it is possible that the engine wimply is dissipating heat faster than it’s generating it, especially if that 10 minutes is a highway drive.
When I lived in North Dakota we used to put corrogated cardboard in front of the radiator to allow the engine to stay warm on the highway. At extreme temperatures the engine would behave exactly as yours is.
As waterboy noted, most likely a thermostat. I had a similar issue. While driving, my heater would have low output, but if I was stopped for a few minutes such as at a stoplight my heater output would improve.
Turned out that the thermostat was stuck open. While moving, the airflow through the radiator made the engine run cooler than when stopped.
When your engine is cold, try starting it and feel the upper radiator hose. The hose should stay cool until the engine is warm and then suddenly warm up. If the hose slowly warms up as the engine is also warming up, then the thermostat is stuck open.
Make sure the coolant level is full. Open the radiator cap only while the engine is COLD.