'91 Corolla - heater works well in town, but on long trips gets cool

I own a 1991 Toyota Corolla. It’s winter in Kansas, and I have a heater question. When driving around town it takes almost 10 minutes for the heater to warm up, but when it gets going the car stays nice and toasty. I recently took a 2 hour trip at about 75 mph in cold weather and noticed that after about 45 minutes or so the heater had quit producing hot air. The air was lukewarm at best, and I was unable to get it to heat up again for the duration of the trip. I had noticed this same thing happening last winter on 2 hour highway drives between college and home, but had ignored it and toughed it out. This year I am married, and my wife would rather have a warm car! Any suggestions as to the cause?

A flaky thermostat is a possible cause and it is a good place to start because it is an inexpensive part that’s easy to replace. You may even choose to do it yourself. If you go to a mechanic, mention the actual problem and he may have other ideas. Accept a radiator flush as well if he suggests it but be wary of additional costly repairs.

I agree with Steve. The T-stat opens and closes to regulate the temperature of the coolant and one that sticks open could cause your symptoms. Lots of air passes through the radiator on the highway and unless the T-stat’s regulating properly the air will take all the heat out of the coolant, especially with a small engine. Small engines don’t generate much heat.

Does the temperature gage indicate normal operating temp. when driving at highway speed in cold weather? If not, the thermostat is definitely at fault. At least the radiator is not clogged. At highway speed, small engines need to have the fan on medium to get enough warm air out of the heater. You could try to block off the airflow with cardboard and see if the temp. goes up.

At highway speeds in cold weather the temp gage reads cold - at the very bottom of the dial. Sounds like it’s the thermostat. Thank you all very much!

Definitely replace the thermostat, running that cold is also not good for the engine or the gas mileage either.