My Camry 92 engine temperature start to drop when speeding up above 50 mph and less heat, it drops bottom at speed above 75 mph. If let the engine runs on parking, heat comes as normal, if the engine gets hot, the fan turns on as usual. What possibly cause this problem? I noticed that after replacing the timing belt and water pump this problem happened.

Your advice is appreciated.

Tom

Probably your thermostat.

The faster your car goes the more air is pushing through the engine compartment. Maybe the more air is just cooling your engine off.

Probably your thermostat.

Same thing happened to me. In the winter, at any speed above 40, my temp would drop. At 60mph, it would drop all the way down and only cool air would come from my heater. I changed the thermostat myself (10 minute job) and the heat that came from my vents felt beautiful after having a cold car for 2 months. Summary: As Brian R. said, it's likely your thermostat.

The thermostat reduces coolant flow to your radiator when the engine temperature is below normal. When the thermostat sticks open, your cooling system is running full blast through the radiator. On the highway in cold weather, huge amounts of cold air are being forced through the radiator which prevents your engine from warming up. These radiators are designed to handle 100+ degree weather (or else Texas would not have any Camrys). Feed them freezing weather and they need to have their cooling ability limited.

Thermostat !

JJ