I have not seen anyone comment yet specifically on the hose routing to the top of the expansion tank. Look at the photo. The lines cross. The photo in a previous post is correct. That is a very common root cause of fluid overflow.

 

You need to use an OBD II reader that allows you to read coolant temperature in real time while you drive. Should start opening the TST at 185F and fully open at 195F; the fans should turn on around 205 or so (or if the AC is running); at 212F you have a problem. Do not trust the gauge on the dash -- it holds at mid until you overheat. I would not worry about what goes into the overflow tank; worry about blowing coolant after a hard drive, although it sounds like a stuck TST to me (best case), it could be a blown head gasket (far worse case). Google blown head gasket and learn the signatures: sweat those, not tank levels. Every time I changed the TST and water pump, I only drained about a US gallon of coolant from the radiator; even so, as it fills, you need to remove air bubbles (follow RJ237 above or massage the hose after the TST opens, or just let it idle for a while and then top off). Cooling systems are pretty routine (until you get to the octopus hose to the heater and throttle body) but overheating is a disaster in almost any modern engine.

And pay strict attention to the point Spark noted above: misroute the plastic hoses you will get blowing coolant! Jim Lombardi has posted pictures of the correct alignment many times.

 

Post #6 shows the pipes -- they are correct

 

Maybe you should get one of these :

Combustion Leak Detector at National Tool Warehouse

 

All the coolant that was in the fender overflow was sucked back into the system overnight. The level in the expansion tank looks like where I left it.

With the car off, just squeezing the return radiator hose and I can hear coolant/air transferring to and then returning from the fender overflow bottle. Don't think that should be possible with a good cap.

Keeping positive that's the problem and holding off on the hydrocarbon detector for now.

Every combustion/coolant head gasket issue I have seen before has presented with bulging, over pressure radiator hoses and that's not happening here. Hopefully I'm not just in denial.

 

Quote from E-Swift - " Temp appeared to hold stead at or below "half", and the heater seemed to work fine. Ambient temp was about 40F. The last thing I did was pull up a constant, nearly mile long 10% grade to my house. No sign during the pull of any issues. As I rounded the driveway and pulled in front of the garage, I saw the needle jump straight up and the red coolant light turned on."

The temperature gauge is really an idiot light that stays firmly in the middle until the temp reaches something like 230F, then it jumps to the top. By this time, the engine may have been running hot for a long time.

You may think about installing a "Real Gauge" kit sold by JagWranger, a member of this forum. It turns the temp gauge into a accurate, functional unit that will give you the real story on your engine temp and adds am audible high temp alarm.

I installed one on my XJR and feel a lot better knowing I will have some warning should my engine overheat. Nothing kills these aluminum engines better thanrunning hot for too long.
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