Coolant Temperatures

Knowing the temperature of the engine coolant is empowering information. In the Winter, it tells you when warmth is available from the heater. There's no point in turning the fan up to high if the coolant temperature isn't hot yet. The reverse is also true; there's no reason to turn down the fan if there's plenty of heat still available. The other need for heat is to cleanse emissions; too cold won't allow the chemical conversion to occur.

The engine will shut off remain off as until until the temperature of the coolant drops below a particular threshold. Having an aftermarket gauge connected to the Prius offers an insight to how it operates.

145°F is the standard threshold. When coolant is at least that warm, the engine will shut off and remain that way until additional heat is needed for cabin warming or battery-pack draw exceeds the "green" level as indicated by the Eco-Meter in the Gen-III Prius. (Neither the Classic nor Iconic models of Prius provide draw information.)

114°F is the ECO threshold. This lower temperature tolerance is a feature introduced with the Gen-III Prius. It allows the heater to extract warmth from coolant much longer than the previous models, which results in the engine remaining off longer.

157°F is the EV threshold for the Gen-III Prius. EV mode (up to 24 mph) is available as long as battery capacity is at least 3 bars on the display and coolant temperature stays at least that hot.

130°F is the engine warmup threshold for Prius PHV. While driving in EV mode, the automatic switch to HV mode will not end until coolant temperature warms up to this point. As soon as it does, if power demand is within the EV range, the engine will immediately shut off.

Knowing about the 130°F threshold is quite empowering with PHV. If already close to that or above, you know that engine shutoff after using it for hard acceleration will happen surprisingly quick.
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I forgot one...

103°F is the ECO warm-up threshold for Gen-III when the heater is not used and the Prius is not in motion. This feature is nice if you have to wait for a stoplight after only driving a very short distance.

Both my commute to work & home in the 2010 took advantage of that one. With the PHV, there's no need for it. :eyebrows:
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