RAY: Here's the answer: What was wrong was that the thermostat was opening up too soon. It was in fact broken, it was never closing. The engine was never reaching operating temperature. Instead of the water getting up to 190 degrees, which it was supposed to, it was probably going up to about 150. You wouldn't notice this in September and October, when you have limited use of the heat, but you will notice this in December and the frost is on the pumpkin. With the operating temperature way down like that you would get poorer mileage, and you would notice it in the cold weather because it will never get up to 180. But in July it would get up there and you wouldn't know anything is wrong.
[ Car Talk Puzzler ]