Coolant High Temperature Warning

With those miles, and mention of head gasket replacement, could be previous owner also used a stop-leak product, say during the head gasket failure period.

With at least some of these stop-leak products a 50/50 mix of white vinegar and water is recommended (by stop-leak product manufacturer) to dissolve and flush the product.

Perhaps drain a coolant sample (from radiator drain) and check it’s condition, look for “globules”. If they’re found, mix with aforementioned 50/50 mix and see what happens.

If this is happening, and you go through with a flush, then follow up with straight water flushed, you will end up with the system with trapped water, no coolant. There’s a link in my signature about strategies for this.

It’s my belief that carbon-clogged EGR system is the cause of virtually all third gen head gasket failures. Right or wrong, it is worth checking and cleaning, both the EGR components and the intake manifold. See link in my signature.

Note: if on a phone turn it to landscape to see signatures.

 

Do you have a temp gun that you can shoot before and after the thermostat housing the water coming out of the head pipe going into the big rubber pipe to drop into the radiator to drop down and be cooled maybe you can isolate where the high temperature is or is beginning and continuing and then drops just enough to get barely pass the red light and that's pretty dangerous sitting in the 205 to 240 range That's up there even with pressure in the system that's still on up there when you turn that off you should almost hear gurgling in the jug under the hood on the passenger side where all the coolant generally goes and the two skinny hoses on top of that jug at that point will be untouchable and at this point at those temperatures whoever's reporting them I would expect the top radiator rubber hose it's about an inch and a quarter inch and a half the big one and the bottom one to be very hot and usually when I see a bunch of this problem right after the thermostat housing will be cool I can hold that hose but go up to the top hose the metal part is untouchable and even the rubber portion when it turns to rubber will be untouchable to the human hand extremely hot so it seems like water is not circulating is what I'm generally thinking The water pump is usually messing up or turning too slow but that all has its own codes. Maybe it's time to look at the thermostat It can be tested in its housing It looks kind of cheaply made this spring and bracket and all this mess but it is what it is I wonder what would happen like in other cars if I took it out or put in a 5/8 restrictor plate in place of the thermostat I need to try that in one of these two Z's.

 

Per the repair manual, the thermostat should be fully closed at 77 ℃, starting to open at 80 to 84 ℃, fully open (10 to 14 mm valve lift) by 95 ℃. 95 ℃ is also where the cooling fans turn on (if there isn't some other reason they're on anyway, like the A/C operating). I normally see around 89 to 93 ℃ on the highway.

If you're getting up to 120 ℃ where the warning light comes on, that coolant is plenty hot. If it were flowing well through your heater core, you'd be blasting really hot air. If it were flowing well through the radiator, it shouldn't be staying that hot.

Mendel might be on to something with the possibility that someone could have poured glop into the cooling system.

 

Even 215f is too hot especially at a low rpm operation such as maintenance mode. I have a digital gauge I have watched over the last three years and it takes 70 mph on a 105f day with ac blasting to break 200f. The highest I have ever seen once was 208f on a similar day when climbing a steep private road after freeway driving. Normally it is 195f under summer highway driving which quickly drops to 180f at stop lights.

So your situation is showing excessive temperatures in all engine operational modes. I would not experiment with diy solutions. Your engine is at high risk.

After highway driving on a hot day with ac