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Discussion Starter · #1 ·
Subject: 2012 Lexus CT200h Premium Navi
Mileage: 264140 mi
Symptoms: Engine coolant temperature light displays. Engine overheating if prolonged 70+ mph driving or faster if under higher load.

Not losing coolant.

Water pump / thermostat - replaced with Aisin aftermarket unit; original pump impeller had high resistance to turning by hand. Thermostat still functioned but was replaced anyways as PM with a Dorman unit Problem persisted.

Cooling system bled by loosening EHR temp sensor. Problem persisted.

Checked EHR temp sensor electrical resistance and temp sensor wiring reference voltage. Resistance was within spec range when cold and ~700 ohm at > 176 F. Did not find an issue, so did not test drive. Continued to EHR thermostatic valve, actuator, and exhaust valve. Checked baseline position at cold temp (~ 68 F).

Warmed up engine and rechecked actuator rod position. No position change even though the thermostatic valve was 195 F on the outside.

Disconnected thermostatic valve from exhaust valve pulley to test each independently for smooth range of motion. Confirmed thermostatic valve actuator rod could not move and exhaust valve could move smoothly with minimal effort.

Isolated issue to stuck actuator rod on thermostatic valve. Toyota part is currently on back order. Sourcing is uncertain and I’ve safety wired the exhaust valve open to bypass the EHR heat exchanger.

Torture test drive pending.
 

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Discussion Starter · #2 ·
Overheat condition persisted.

Upon return home, let exhaust system cool down. Confirmed safety wire was still holding valve in open position.

Next test is to replumb the coolant hoses to circumvent the exhaust heat exchanger.
 

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Discussion Starter · #4 ·
Thanks for taking a few stabs at it.

1. Correct, it does not overheat after idling in maintenance mode. The fan eventually turns on so maybe it’s curtailing the overheat. Need to monitor variable, trendable data rather than binary.

2. I don’t believe so either because -
Combustion chemical leak test completed. No significant combustion byproduct ground in coolant. Control test using exhaust did turn solution green vs the solution exposed to coolant reservoir which remained blue, similar to unexposed solution.

And, cooling system pressure test completed using Mishimoto MMTL-CPT-28 with warmed up engine. Pressurized coolant reservoir to 20 psi and it maybe lost 1 psi at the end of 5 mins.

3. There is one other supporting symptom. The coolant level is ballooning in the reservoir and is increasing the pressure in cooling system. Coolant level returns to same Full line on reservoir after cooling down.

4. Not sure. Need to figure out how to test it.

5. Heater functions and is the only way to lower the temp if I can’t move off the road or slow down too much. Coolant overtemp light never goes solid so it shouldn’t be detrimental.

6. Not sure how this could happen since there is only one elec connector on the water pump and only connects to pump one way. Issue could be on the pump side’s electronics but that would need another test to see if the impeller direction can be reversed.
 

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Discussion Starter · #10 · (Edited)
Mileage: 226100 mi
Status: Overheating persists under same conditions.

Latest tests and results:
1.) Monitored OBD2 data.
Temperature is typically 200 F idling or bleeding cooling system, and steadily increases to around 220 (city) to 230+ (hwy) on my 36 mi commute.
Coolant fan ON at ~208 F.
Cooling system temp warning light blinking ON at ~242 F.
Cooling system temp warning light solid ON at ???…Im guessing around 250 F +.

2.) Flushed cooling system with Prestone cleaner. No change. Issue persisted.

3.) Removed thermostat and flushed cooling system again. Issue persisted. It may even overheat faster?

Replies:

4) I would look around the radiator to make sure there arent any leaves / other debris blocking the radiator. And squeeze the hoses to get a rough feel for their health.
Nothing grossly out of the norm. The condenser fins exposed in the air dam are a little smooshed but nothing else to note. This is typical in many cars. The AC functions just fine.

5) Its interesting to me the heating system cools the motor. That could only happen if there is at least some coolant circulation going on. Adds more useful context.
This is still functioning the same, even after flushing by the cooling system twice and removing thermostat.

6) its possible they could have reversed the pin outs at the factory. Has happened before.
I will be replacing the water pump after replacing the radiator, if that doesn’t cure it.

New items:
7) I believe on our cars the heater works outside of the thermostat circuit. I would check to make sure the thermostat rating matches or is below factory, and just remove it and try to see if you can replicate the previous overheating.
Thermostat removed, cooling system flushed a second time in 2 weeks, and refilled with 50/50 coolant mix. Overheating still persists under same conditions.

8) did the car overheat before the aftermarket water pump was installed? Check to make sure its flow rate is similar to stock.
Yes, but not in Jun-2021, which was after the head gasket replacement in Mar-2021.

9) is it the original radiator? Radiators can corrode on the inside over time causing the small passages to gum up, especially if there has been water in it that wasnt distilled.
Yes. Replacing it next since the thermostat was ruled out.

10) has anyone ever put a head gasket or radiator leak "fixer" in it? These could also gum up some of the passages.
Not to my knowledge and I have not…yet…although this issue is testing my patience and I may still.

11) there is a chance the water pump gasket or gasket maker is blocking one of the coolant passages around it. I haven't changed one myself on these cars so I don’t know if such passages exist.
Not sure how I’ll test this yet.
 

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Discussion Starter · #12 ·
I wonder if part of the radiator is blocked. I don't suppose there's any real way you can tell if there are cold spots in it when it's operating. But sounds like replacing it is a good idea at that mileage anyway.
If I bought an IR camera and had full line of sight to one side of the radiator, I could see the flow based on temp profile. Not sure that’s worth the diagnostic effort since radiators are low cost.
 

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Discussion Starter · #14 · (Edited)
Mileage: 266249 mi

Radiator replaced.

Ex situ water pump visual check at commanded 3000 RPM was good. Pump impeller physically rotates, stops and starts on command without binding. Did this with the new scanner I purchased.

Thermostat opening at ~196 deg F confirmed via IR gun in stove pot.

40 mi test drive, without AC, completed at speeds > 70mph for several minutes at a time. Overheating indicator lamp did not illuminate. Wasn’t able to monitor coolant temp with scanner. Value was frozen at -40 F. Maybe be a suddenly dead ECT sensor and not just a connectivity issue with scanner?

Looks somewhat hopeful.

Pics will be uploaded.

Ben
 

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Discussion Starter · #15 ·
Combustion gas leak test








Cooling system 20psi leakdown test (5 min duration) using Mishimoto MMTL-CPT-28 ($87.95 from Amazon)







Thermostat functionality test - stove, pot, IR gun, zip tie, thermostat removed from thermostat housing (could also do the test with the entire housing in the pot but would need more water)






Electric water pump (Aisin WPT-190) visual inspection after 6k miles of use; old pump was binding


Radiator R&R








 

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Discussion Starter · #19 ·
Radiator was the root cause.

All other items were replaced prior, later confirmed to be functioning as expected for their individual role in managing coolant temp (e.g. thermostat confirmed to open at expected temp, water pump physically spins, etc.), and the engine was still overheating.

it was not until the radiator was replaced before it would no longer overheat.
 
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