July of 2017, I had the dealer replace the coolant control valve and water pump on my 08 Prius. Code T1121 was satisfied but summer-time mpg was still lower than normal (42-43 from 45-46). It's winter now and my mileage is between 37-38 mpg with no snow and temps between 30 and 45 degrees F. So I checked my radiator coolant temperature with a laser after a 20 minute drive, it's around 60 degrees F and cold to the touch. With 160,000 miles on it and figuring I should change the $10 thermostat for starters; I pulled the thermostat and tested it in a pan of heated water with the new thermostat. Surprise: they both opened up at about the same temperature (just before boiling point). Does anyone have an idea as to what could be causing this situation. There are still no codes and the heater seems to be working normally.
The thermostat should start to open at 82 degrees C or 180 degrees F. With ambient air temp at 30 to 45 degrees F it does not surprise me that the radiator is measuring 60 degrees F after a 20 minute drive especially if this drive is on city streets. It would have been good if you measured the temperature of the cylinder head at the same time. I would not be surprised to find the cylinder head temp to be at or near 180 degrees F hence the thermostat would only have barely opened.
If you have any kind of OBD-II reader, you can read the coolant temperature right from the engine's coolant temp sensor. Even a generic reader will work for that, because coolant temp is a standardized OBD-II parameter, nothing Toyota-specific. That would give you a number I would put more stock in than an IR gun pointed at the radiator.
The 20-minute drive was at an average of around 50 mph from my home in the country to town. I did take a block temp that was 108F and the radiator temp was taken of the actual fluid, not the radiator. I took the cap off to take the reading.
When I had a ScanGauge connected, and monitoring coolant temp, in winter it often peaked around 60~70 C, with easy highway driving. Third gen, but maybe similar.
Yes, since 70 degrees C is 158 degrees F, the thermostat should stay closed in that scenario. That is why it takes a while for Prius interior cabin heating to take effect and is supplemented by the PTC electric heating elements, the engine really doesn't produce tremendous surplus heat as it is relatively efficient.
Don't know if you saw my reply but it was not a "city" drive. And the measurement was after removing the radiator cap. The block temp was 108 F. (Could not see the head for the air cleaner, etc.) I stuck my finger into the coolant and it felt cold. That particular day the ambient temp was around 40 F.
Basically it’s using most of the heat your engine is producing to heat the inside of the car. It’s running thru the heater core mostly and that is why the radiator has such a low temp. There is nothing wrong with the car.
I can reliably shut the engine off at red lights, when the car's only partially warmed up, by turning the temp down, or shutting system right down. The computers are always considering cabin heat demand, when deciding whether or not to shut down engine.
So getting 10 mph less under the same circumstances but 2 years & or 20,000 miles later is "normal." I cannot believe that the cooling system is so much more efficient than a typical 4-cylinder engine that, in 40-degree weather the radiator temp after a 20 minute drive at an average speed of 50 mpg will not create a higher coolant temperature than 60 degrees, just because I have my heater on.
The 10 MPG could be many things. You could have different tires that could account for almost that much. Our advice here is worth exactly what you paid for it, nothing. You always have the option of taking your car to the dealer if no one here can help you. Best of luck.
