Weird Overheating Issue

Can you please give a short history of the car.
1. When did you buy it?
2. Any problems with the head gasket or stop leak added to the coolant?
3. Was the engine coolant replaced at the recommended 100,000 miles and every 50,0000 miles after the initial change?
4. Etc.

The high temperature warning light can be triggered by two separate sensors. One is the sensor that you can see with the OBD2 reader. @ChapmanF has described this in other threads on this forum. Do a search to see if you can find the thread, or maybe he will see this thread and explain it here.

A thermostat that is stuck open will not cause an engine to overheat, but a thermostat that is stuck closed will cause overheating.

Like you said, maybe there is air trapped in the coolant system.

Posted via the PriusChat mobile app.

 

If your problem has not read a code, almost positive that your engine water pump is on its way out.

In my case the high temp came on for a quick flicker then turned off. Happened again. Took it to my mechanic and read the code for the pump, which was starting to fail.

you see the temp light come on again, get it read and you can be sure.

 

@Brian1954

Actually a thermostat that is stuck open will eventually allow an engine to overheat.

As the coolant just circulates and continues to heat up depending on the use, it will overheat.

The point of the thermostat is to keep coolant in the radiator long enough to cool down and then get released to keep optimum temperature.

Free flowing coolant will always get to temp no matter what temp that is and have no chance to cool down.

 

I hate those pesky coolant ants.

Seriously, do your radiator fans run? Generally they will run with the AC but a bidirectional scanner test is better.

Has anyone put a sealer in the coolant? They are known to clog radiators and thermostats and are not that easy to flush out.

Verify your inverter pump is good as well. Should see movement in its reservoir. Perhaps you are now getting a hybrid temperature warning, which usually produces codes and lights the hybrid system car icon.

 

I was under the impression that when a thermostat is stuck open it will still maintain pretty much operating temperature because it's still passing through the calibrated plate which is the hole that's exposed when the silicone or wax pellet is heated and the spring lifts the opening portion of the thermostat up to its maximum opening that opening is a certain size and with that sucker open all day long it should still maintain operating temperature now if you remove the thermostat and you have that 2-in hole wide open then yes the coolant could move so fast that it could not dwell in the radiator long enough to lose the heat but with the thermostat in place and it stuck open it should maintain its normal 186 to 205 or whatever it is no problem at least that's what I've noticed in all other cars that I've removed thermostats in the old days put restrictor plates in which is basically a good and thermostat but using the plate so nothing will open and close anymore just water will flow through that plate that's what NASCAR cars use or used to use.

 

What ???????

I think you need to do a little searching on the internet to learn the function of a thermostat in an internal combustion engine. It is not used to limit the coolant flow through the radiator. It is used to limit the coolant flow through the engine so that the engine heats up to optimum operating temperature.

I agree with you that coolant can flow too fast through a radiator so that there is not enough time for the heat to be transferred to the copper or aluminum radiator fins and then to the air that is flowing through the radiator, but I am sure that the Toyota engineers designed the pump to create the correct coolant flow rate through a Prius size radiator. I learned this in thermodynamic classes in my first and second years in college.

Posted via the PriusChat mobile app.

 

The thermostat assembly is sized to the amount of coolant flow that's going to need to run through the pipe which is going to feed the radiator which with the amount of flow passing through the pipe passing through the radiator and then to the other pipe going back to the engine all that is calculated to provide a certain amount of dwell time of the fluid in the radiator part of fluid dynamics and all this kind of nonsense No I don't study it. But I do know about restrictor plates that were put in the waterways of NASCAR automobiles I'm in the middle of NASCAR country and have NASCAR garages all around me. Back in the early days of Hilux trucks I had a Hilux truck with a 3-in thick aluminum radiator called the desert cooler and the thermostat that was removed and a plate was put in place of the thermostat I believe it was a 3/4-in hole so that that hole that used to house the thermostat now had a plate in it with no wax pellet no device lifting up or down closing or opening the hole and in that plate was drilled I believe it was a 3/4-in hole thermostat housing and hose and all that go back on now with that setup no matter what went on that engine ran about 180° 183 even in the winter it warmed up reasonably quick and had heat in 5 minutes something like that. How all that was calculated I have no idea doesn't matter I also had removed the fan clutch and added an electric fan this is like 1984 something like that We had an engine in the truck that wasn't available in the United States. And it was being used out in Baja this is in the days when TRD was just being founded they were in a little hole in the wall shop in Torrance California and I'm talking about a little place.

 

When the full Physics According to Azusa comes out, it's gonna be a sensation.

If thermodynamics worked like that, I'm not sure how any existing thermostat would be smart enough to cope. All they are is a pellet of wax that expands and permits more flow whenever the temperature is higher.

Teaching that wax pellet how it really needs to keep the coolant in the radiator to cool down might be even harder than teaching an old dog new tricks.

 

When the aging hippy writes the definitive Prius book I will gladly buy the audiobook version.

 

@JimJitsu , can you please answer my questions about the history of the car that I asked in post #2.

Posted via the PriusChat mobile app.

 

And post 5 (or we will just ramble about thermostats and audiobooks)

 

I don't know those sensors by those 7C0 / 7C4 addresses, but one of them is the usual engine coolant temperature sensor, screwed into the cylinder head, and the other is for the coolant loop through the exhaust heat recovery system (that sensor screws into a fitting in a bend of hose that comes up behind the engine from the exhaust system below).

I wrote in some earlier thread that there's "only one" thermostat. I meant the one in the engine, but down by the exhaust heat exchanger on the exhaust pipe there is a second thermostat-like thing. Its job, when the wax expands, is to push a rod that closes off that heat exchanger path, once the coolant is warm enough already to not need any extra heat from the exhaust.

If that guy isn't doing his job, I could see that reading going up, up, up when the driving conditions make a lot of hot exhaust.

 

Not sure anyone knows since Techstream doesn’t label them that way.

A guy could monitor both with his scanner that shows $7C4 and $7C0 and then pull off the connector to the one on top that is easier to get. Then report back.

 

I had very similar issues. If there's no coolant leaking, it's going into the engine via the head gasket. That's the only way to lose coolant. This doesn't mean you need a new engine today - but it means you need to watch the fluid level. My coolant got low, that's I'm sure what caused the sensor to start acting up. Sometimes it worked, others not, others bad readings.

I have a small head leak, but it's gone 30k miles without fault (except a little extra added coolant).

Just my take - your mileage may be better (since I'm running a 150k battery pack).

 

When you replaced the engine coolant pump and thermostat, did you use new coolant when you filled it up, or did you refill it with the coolant that you drained out? I know this is not related to your overheating problem, I am just curious.

Posted via the PriusChat mobile app.