How to tell if Thermostat is bad (stuck)

I monitor our temps (with ScanGauge), and especially in winter, it's rare that temps make it to the peak (190 deg C) zone. The only time I'll see peak temps is on a protracted uphill climb, like going up a local ski hill, for example.

Your temperatures seem normal to me, but I'n not an expert. I did experience a thermostat failure on a previous car, and the symptom was abnormally low temperatures after protracted freeway drive: the thermostat was stuck in the open position, allowing full cooling all the time.

 

Testing the temp during city driving with the heater on is not a good idea. Get on the highway and see what it does, preferably with the heater off (you can heat it up going to the freeway). Doing it in city likely won't tell you anything.

 

The OP's reported engine temps are normal. I would suggest to the OP that grill blocking should be removed when calling upon the Prius to climb mountainous terrain. 203 degrees F is above normal operating temps.

The thermostat opens at 82 degrees C which is 180 degrees F. However the Prius engine is so small and efficient that in the winter the engine temp may remain below 180 degrees F, especially if cabin heat is turned on (as engine heat will be dissipated via the cabin heater core) which of course means that the thermostat will stay closed and fluid will not circulate through the radiator.

The Prius heater assembly even includes two PTC electric heaters to supplement the heat produced by the engine, since the cabin heat produced by the engine can be marginal depending upon ambient temps and operating load on the engine. I believe the two electric heaters can produce around 600W which is 40% of the capacity of a typical hair dryer.

If the OP wants to replace the engine thermostat, please see my thread here:
How to replace engine coolant pump and thermostat | PriusChat

 

If the original poster has been a careful observer and the behavior has changed, perhaps the thermostat is failing to close fully.

After several years of suspecting that the thermostat on my 2002 Audi A4 was no longer closing fully I finally had it replaced the summer. Continued careful observation of my scan gauge reported coolant temperatures and the behavior of the cabin heating very clearly confirmed that the new thermostat allowed substantially faster coolant and cabin heating in all conditions but especially in colder weather. The hot weather stabilized running temperature average was unchanged, though the detailed "waveform" around that average changed appreciably.

I certainly don't know what the usual failure modes of automobile thermostats are, but from observation it appeared that the primary abnormality in the last few years of my faulty A4 thermostat was a substantial "leak" passing through the thermostat when it should have been fully closed.

On balance, it seems plausible to me that the original poster has a thermostat currently in the same general failure mode.

Is there an accepted user test which can detect excess off-state thermostat coolant "leakage"?

 

I think you have mistakenly put (190 deg C) instead of (190 deg F)

John (Britprius)

 

The test would be to remove the thermostat and see if it remains closed at a temp below 82 degrees C, then opens at that temp.

However if you are going to the trouble of removing the thermostat, you'd might as well replace it.

 

Your observations sound normal, not giving me any impression it's not working right.

I watch coolant temp closely. Dealing with conditions outside way, way below zero, monitoring when heat is actually available helps. No sense craving the heater when all you'll get us cool air. But if the coolant temp is, might as well take full advantage.

 

Oh woops. Yes, well: centigrade works out about the same, fully warmed being around 90~93 degrees Centigrade, but NOT 190~193, LOL.

 

Thought I’d add to an old post. After 25 minutes driving I’d not broken 130* with the heat off. It was 50* outside, but the ice never shut off a stops. The result was 31 mpg on LRR tires... even when being careful.

The water pump was weeping so it made sense to replace the thermostat at the same time. After swapping both for new, mpg has gone up to 38+ mpg. 7 mpg for replacing a $10 part seems worthwhile. I suspect it’s been bad for years, it has dramatically changed how the engine runs.

Next up... reconditioning the traction battery to restore lost capacity...