2014 Jaquar Ftype V6S 26000 miles. Car has had Injector seized in head. Coolant Pipes leaking. Charger Coupler rattle, Broken Reservoir Cap. All fixed. No codes. Run car for a while, all good. Its 95 degrees here, so car sits 217 to 226. Then all of a sudden overheat. But I have replaced coolant and it is building pressure. I used the big tub...fills up goes down, when it gets hot it is bubbling up. Shut off engine. Coolant rises. Then falls back down. start engine air blows up thru coolant. I have clean oil. About to do compression and leak down test. I think a head gasket. Car runs perfect. Its a cat. But this overheat issue. I replaced thermostat with aluminum one that has a sensor which is not plugged in to anything because it is not an option on my model. Should be working though. Not sure if a closed thermostat would cause this or a bad waterpump. At this time I had done a leak test when I filled. No leaks, pressure holds. I keep thinking its a head gasket to exhaust leak.

 

Good point about the after market Aluminum Thermostat Housing. They come with and without the coolant sensor. When I researched this it appears that the thermostat version is only used on certain versions of the V-6 and not at all on the V-8's.

I find these engines/cars hard to fill until I started using a vacuum filler. It really works well and it's one and done. No air trapped and the coolant level changes very little after wards. I think your symptoms still point to trapped air?

Easy to tell head gasket or not? The test kits are cheap and all over including Harbor Freight.
I mean for $32 you can for sure rule out head gaskets,
Combustion Gas Tester
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engine running too hot, should be running around 203-212 thermostat is fully open.

 

Have you drilled a small hole in the body of the thermostat? This will allow air to escape that is being trapped under the thermostat causing the wax pellet to be not in contact with the coolant, so it won't open. A 1/16th hole will not greatly affect the warm up time, but it will bleed out air including any developed by water pump cavitation or head gasket leak.
Next test, rev the engine, does the lower water pump hose suck in at all? If so, the radiator is blocked and the water can't flow through fast enough to supply the water pump demand .....

T1 Terry

 

If he has the correct thermostat the "jiggle" valve will already be there. Note it needs to be in the 12:00 o-clock position so air bubbles can pass thru without getting caught on the edge of the thermostat. Drilling holes was a very old trick and not needed for many years.
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Replace the thermostat you need the sensor , my coolant will run 188 degrees in dynamic mode