Thermostat Question

The thermostat operates on a termanally sensitive spring. It softens when it warms up; allowing fluid pressure on the other side to pass as it gets up to the designated temperature. Any contaminants or rust can impair it's operations. (ie. get it stuck closed, open, or partially open)

Hope this help

 

If you're talking about the one in the hose elbow down by the water pump, I think it functions like all the others I know of. A spring holds it closed, and a pellet of wax trapped in a cylinder expands with heat and pushes it open. It's a slowish effect, nothing sudden about it. The thermostat starts to lift a bit above 80 ℃, definitely opening by 85 ℃. It should be fully open (10 to 14 mm lift) by 95 ℃.




House thermostats on the wall, now those frequently are just on / off (though even that has been changing in these modern days of variable-rate furnaces, A/Cs, and heat pumps),

 

That's a weird part of your story. What reader did you buy? The car's ECM will answer the pump-RPM question any time the reader asks.

The easy test for that is to take the stat out and suspend it in a pot of water on a stove with a suitable thermometer, and heat the water up. You are looking for noticeable opening between 80 and 84 ℃, fully open (10 to 14 mm lift) by 95 ℃, just as in the picture.



If it doesn't do that, replace it. If it does do that, your problem is something else. (This is a better approach than just saying "I'll replace the stat and see if that fixes it", because then if it doesn't recur right away (say, the conditions are somehow different on your next climb) you can go around for a while with a false sense of having fixed it, and then later see that you still have some other problem.)

Other problems could be restricted coolant flow, restricted airflow through the radiator (lots of bent-over fins or trapped debris). Check the operation of both radiator fans; they shouldn't be needed much when the car's in motion, but they should definitely both be running at HI speed well before that light comes on.

If the coolant ever went unchanged a really long time, there could be corrosion products in the passageways reducing flow. If a stop-leak product was ever put in, passages could be gunked up with that.

It would definitely be nice to get a continuous scan-tool display of temperature and pump RPM.

 

Yep; at a minimum I'd install a continuous ECT monitor, to keep you out of the danger zone and avoid blowing a head-gasket; while trying to figure this out.

 

It's common for third gen water pumps to fail around 150k miles. It's prudent to change the pump and thermostat, say every 100k.

Ever cleaned the EGR system?

 

Did you save the old water pump? Did you inspect the impeller on the pump and did it spin easily?

The new pump that you installed. Was it Toyota OEM, or was it an aftermarket pump?

 

Was the new water pump oem Toyota? Lots of aftermarket units are suspect.

No head gasket sealer in your ownership? Radiators do get compromised with sealer.

Use your Car Scanner to display both coolant temps so you know if both agree or one is high causing the alert.



The 7C0 thermistor is monitoring the output of the exhaust heat system, which is only in full use during warmup. It also has a mechanical thermostat which allows exhaust to bypass the exhaust heat system's heat exchanger. I have heard of one case where that mechanism failed causing too much heat to 7C0. It's not clear to me that would actually overheat the engine since it's a small flow which is always picking up some heat from the exhaust no matter what.



It does not take a mountain climb to raise the temperature as a test. Just find a half mile steep hill (the kind with the high grade warning sign), usually in a rural neighborhood, and drive it aggressively with max ac. It should not exceed 205f with a good cooling system.

The most common problem is an aftermarket water pump replacing a failing oem pump.

 

In Car Scanner that value is for the inverter water pump.

 

Now that rjparker mentions it, I think I remember some discussion around here about whether Car Scanner even has the PIDs for the engine water pump target and actual RPM, or only those for the inverter water pump.

 

Both of the temperature sensors that you are now monitoring can turn on the high temperature warning light on the dashboard.

 

The light is built into the combination meter. The 7C0 exhaust heat recovery system sensor is wired straight to the combination meter, which directly turns the light on at 248 ℉ (120 ℃).

The 7E2 cylinder head outlet sensor is wired to the ECM. The ECM and the combination meter are connected over a communication network, and messages pass over the network to turn the same light on when this sensor reads too hot. I assume "too hot" is probably also 248 ℉ (120 ℃) for this one also, but I have never gone out and tested that like I did for the EHRS sensor, so for all I really know it could come on at some other temperature for that one.