Thermostat (3rd gen) answers !

Right in the repair manual, open between 80 and 84 C (176 to 183 F), lift by 10 to 14 mm at 95 C (203 F), should be fully closed below 77 C (171 F). More information than that might be harder to find.

They're nearly bulletproof. Expanding wax opens them, a big spring closes them. To get one to jam open, you'd need corrosion or grot stuck in it, neither of which should happen if the coolant is changed often enough to keep the corrosion inhibitors active. Only way to make one fail closed is to severely overheat the engine some other way until the wax blows out. Other than those failure modes, who ever needs to replace one?

 

Corrosion products of aluminum in a liquid can have that white crystally/flaky look too. Had it gone a long time without a coolant change?

 

Why would you want to change the thermostat, my experience with thermostats is they stick closed and engine gets hot very fast, if the temperature is maintaining around 190, then there is nothing wrong with the thermostat, mine is pushing 290 k

 

Even if a thermostat cost the price of a beer, your wasting your money when you could enjoy a cold beer instead

 

Revived from the dead, still an issue that might help others.

Well the thermostat has stuck closed on my Prius a few times, this happens only in the first couple of miles when it happens, luckily I have the Ultragauge 2 on all my cars, this morning it got to 217 and my wife pulled over on side of freeway before any damage, then after sitting a little it opens up and works fine.
I did not have the Ultragauge on my last engine I did not think I overheated it, but now I have my doubts.
This has happened to my wife a few times in the last year 3-5 times too many, Ultragauge is cheap insurance thanks to the alarm I hope to never do a head gasket.
As I stated before this is how a thermostat acts, once they open they work fine, your car doesn’t run good for sometime on a trip then all the sudden your thermostat goes bad, once they are open there will not be any thermostat issue.
Buy yourself an Ultragauge you will not regret it, I own 3 and have told some co-workers about them and never heard a complaint.
They will save your car as lost as the driver knows what to do when temperature goes high.

 

Thermostats are more reliable than they used to be, but they are far from "bulletproof." Better modern coolant chemistry probably helps but in years past with the old glycol type it was not unusual for a thermostat to fail within 10 years or so. I have seen them fail on newer, well maintained vehicles but not as often.

 

Which way?

Stant, prominent maker of thermostats, has literature explaining how a fail-closed situation can result from overheating that ruptures the structure and blows the wax out. They don't list other causes for that failure mode, that I've seen. If the wax hasn't been blown out, its expansion is a more-or-less irresistible force.

Fail-open can be more common, as it requires only that the spring hasn't strength to re-close the stat as the wax cools.

 

What's considered overheating for a gen 3 Pruis? Also at what temperature does the read overheat light go on?

 

Anyone have any answers?

 

Gen 3 has two coolant temperature sensors, one in the cylinder head and wired to the ECM, one in the hose returning from the exhaust heat recovery system and wired straight to the combination meter.

Either one can make the red-thermometer light come on:



The one in the hose will make the light come on at 120 ℃ (248 ℉).



I haven't checked at what temperature ECM will make the light come on, based on the sensor in the cylinder head. I wouldn't be surprised if that's the same threshold, 120 ℃, but I haven't seen anyone actually check that the way I did with the other sensor.

Note that if the ECM has given a code for a water pump problem, it has a lower alarm point, where it will flat-out stop the engine at 105 ℃. The car remains powered up and can do electric driving (for the usual short distance and limited speed). You don't see a temperature light then (because this temperature is lower than where that light comes on); you'll just see the warning triangle and wonder why you have only EV driving. (Ok, you won't really wonder, because you will have had the check-engine light ever since the pump problem was discovered, and if you had read the codes then you would have known it was a water pump problem, and you would have taken care of it.) Moral: don't procrastinate when you see a water pump code.

Because the thermostat opens at 80 to 84 ℃, and the fans come on around 95 ℃, and I have a ScanGauge on my dash at all times, I seldom see the temperature above 90 ℃, and scarcely ever see it even nearing 100 ℃. If I ever saw it that high, I would already be wondering what the explanation was, and not waiting for a light to come on.

 

Great thanks very much Chapman. I noticed yesterday while driving up a very steep hill that the coolant temp sensor (hose returning from the exhaust heat recovery system) reached about 100C (212F). Normal driving, the temp stayed around 87C (189F). I plan to replace the water pump with OEM Aisin this week and hopefully the temp won't get up that high after replacement. Will post update.

 

Great thanks very much Chapman. I noticed yesterday while driving up a very steep hill that the coolant temp sensor (hose returning from the exhaust heat recovery system) reached about 100C (212F). Normal driving, the temp stayed around 87C (189F). I plan to replace the water pump with OEM Aisin this week and hopefully the temp won't get up that high after replacement. Will post update.

 

Ok upate time: I replaced the water pump and thermostat and thank God no more overheating, even going up a long steep grade. Temp reached a max of about 194F. then stayed around 189-190 going 75mph on flat with AC on. Next project is to clean EGR and install a catch can!

 

How many miles on it? Info link in my signature.