ICE Temps in Winter, your experience?

Sounds like it might be a problem. It’s just always hard to tell because type of driving makes a huge difference.

I would test it by keeping it in a situation where engine is rotating but not producing power. That happens when you drive faster than 43MPH or 70km/h but don’t use too much throttle. Then if you keep your heater at some pretty low temp setting (but not MAX COLD) and blower speed 1 engine should warm up (very) slowly. Test ant engine temp less than 80c or 176F but more than 65c or 149F. If thermostat is even leaking engine temperature will get colder.

This works because:
Engine only produces small amount of heat when it’s spinning but not burning fuel. This heat is enough for small heater use and air cooling that engine gets. But since engine is spinning water pump is also pumping. And if thermostat is leaking then coolant goes through radiator and engine cools fast.

Without grill block this test probably works at least down to about -5c or 23F. With grill 80% blocked this works even at -30c or -22F.

 

Bad thermostat, or el-cheapo "will-fit" that is not working correctly.

I am actually shocked that you do not have a stored DTC for "Cooling system performance below OBD requirements" or similar.

I'd have the thermostat changed by a qualified shop using ONLY OE parts ASAP. An ICE should NEVER EVER operate colder than 190* if the thermostat is working correctly, once warmed up.

 

But.....When you're parked or drive EV, the heater pump will pump coolant trough the system so you keep getting warm air out of the vents.
When this is happening the coolant temperature will drop. And at some point the ICE will kick in when its too low.

 

After the engine is fully warmed up it’s normal for engine temp to drop down to about 40c or 100F if you just sit in a Prius with engine not running and heater cooling then engine down. You can even get the engine temp dropped down to 40c or 100F if you drive in EV mode (even without the EV-button) while heater cools the engine down. That 40c or 100F is the minimum that Prius lets the engine temp drop to. Depending on a lots of things it can (and normally does) start the engine before it gets that cold.

Thermostat only controls the engine temperature when engine is spinning. That’s why I would test it the way I described in my previous post.

 

Correct, under the conditions you describe, the ICE is NOT operating.

 

Water temperature references:
40C - Prius enters S2 warmup mode (so long as it has had time to complete the ~60 second S1 warmup idle). Below this temperature the engine will stay on, even at 0mph. S2 mode allows autostop, but only at 0mph.
70C (73C on some models, I think 2004-2006) - Prius enters S3 and (if allowed to autostop at 0mph) S4
85C - Thermostat begins to open
92C - Thermostat is fully open
94C - Prius activates cooling fans

If you have the heat on, it will keep the engine running until the water temperature is significantly warmer than the temperature set, something like 15-20C. I'm not sure of the exact behavior. So the behavior of the car you've observed is normal; you just have to decide whether it's worth the fuel to keep your interior warmer before the car is warmed up.

Until the entire cooling system gets warm at 85C, the amount of coolant circulating isn't very much, and you can cool it off quickly in the wintertime. I keep the fan on the minimum setting if on at all until the car can get to 70C to allow S3 and S4 modes and improve my fuel economy.

 

I don't have a FSM for a Prius handy, but from what I was able to find, the Start-to-Open temp of a Gen 2 thermostat is 180*F. This means that the engine coolant temp should be AT LEAST 180*F during "normal" operation at speed, regardless of ambient temperatures. Anything less indicates a failed thermostat allowing coolant to flow thru it when it should not.

 

Those do seem a bit low. So probably you would benefit from fitting new thermostat.

With Prius this “at speed” means speed constantly above 43MPH or 71km/h. Any speed lower than that and engine can stop running and then heater use will lower the engine temperature. Even at higher speeds than that in cold temperatures it’s possible to cool engine with high heater use.