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You've taken a good approach so far. I'm just going to vomit ideas here incase anything helps spark something for you:
1) it happens at 70mph so we know its not related to fans.
2) nothing is leaking and coolant levels aren't decreasing so its not a head gasket etc
3) Is the only sign of it running hot that indicator? any other evidence?
4) Could be the case that either air is not getting to the radiator at 70mph, or coolant from the radiator isn't getting to the engine. Check for blockage around radiator and squeeze coolant hoses to make sure they aren't collapsing.
5) Does your heat still work? (this could indirectly hint at the amount of coolant flow going on)
6) Is the water pump wired the correct way? Some electric water pumps will flow one way or another based on what polarity they were wired in. If you're sending coolant the wrong way through the system it will not work right, or not at all.
 

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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.
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.

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.

6) its possible they could have reversed the pin outs at the factory. Has happened before.

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.

8) did the car overheat before the aftermarket water pump was installed? Check to make sure its flow rate is similar to stock.

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.

10) has anyone ever put a head gasket or radiator leak "fixer" in it? These could also gum up some of the passages.

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.

closing statement: The cooling system holds pressure, so in your case there can really only be two issues: lack of coolant flow and poor heat exchange.
suspected coolant flow issues are the thermostat, collapsed hoses, insufficient water pump.
suspected heat exchange issues are a gummed up radiator, radiator not getting enough air flow, or improper coolant to water ratio.
Disclaimer: There may very well be special coolant system considerations I don’t know about on CTs. Im diagnosing it like a standard motor.
 

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Ok I went through your profile. After seeing that great head gasket write up Im going to switch gears here. I definitely still think there is value going through the list I posted. BUT there may be more at play. There is a chance the cooling system is working fine but the engine is running very hot. In this case the cooling system wouldnt be able to overcome the engine.

If thats the case I would suspect something to do with exhaust being backed up. Possibly the "gravy" clogged up the CAT or something in the EGR.

although I wonder too (scatter brained) if these cars have an engine temp sensor that activates the water pump (not just the fan like other cars). May be worth looking into if such a device exists and anything on the associated circuit.
 
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