My tank keeps constant at 1” above the max cold line. Rising only slightly when engine is hot.
I still think you have an air pocket. Fill it as mine. Go for a drive. if it drops again and there are no signs of leaks…. there was/is a pocket of air.
A level that is dropping implies a void is being filled.

 

When it was overflowing, it was at the top cold line as it has always been. When I refilled it, I did just below the low cold fill line. I recently brought it to the middle, between the two. And now at your suggestion, I brought it to the top.

Yesterday the ambient temp was about 20 degrees lower than normal and ECT mostly ran high at 226F sometimes at 223 and occasionally dipping to 213 only to return to 226. After 120 miles of driving, the upper radiator hose was firm as normal. Then 30 minutes later, it was completely soft and the coolant level had dropped to the mid point. Now I have never had that hose go after that quickly usually it would take a day or two to lose pressure.

Today, I topped it off again to the top line and went for a drive. Hose firmed up within 15 minutes but coolant level had risen to nearly the cap and the upper hose became soft again. Let it sit for an hour and drove home. Hose became firm and coolant level was at the top cold line. With the engine still running, about 5 minutes later the coolant level had risen and the upper hose was soft again. ECT A & B at 199ish.

 

Just went in the garage and found coolant all over the floor. Not certain what happened, ECT was low and upper hose was soft when I left it.

 

Topped off with about a half pint of coolant to bring the level to the low cold mark (earlier I had filled the tank all the way as @guy had suggested). Ran engine for about 15 minutes and alternated between HI heat and LO cold AC. On HI, coolant remained at low cold mark and upper hose softish, on LO cold, coolant level dropped an inch and upper hose became hard. Alternated climate back and forth and coolant level continued to follow this climate based pattern. Ambient temp still low today by about 20 degrees.

 

Went for a 30 minute drive. Upper hose became hard as it's supposed to. ECT 210-223 most of the time, going up 226 occasionally. Returned home and upper hose remained hard. Going to let it sit for a while and check it again.

The only thing I can think of, topping up coolant to high cold mark somehow unbalanced the circuit, affecting pressurisation and causing it to overflow. I know, it makes no sense seeing as how I've always kept coolant level at the high cold mark. It also makes no sense changing spark plugs, coils and injectors would do the same thing.

 

I don’t understand why yours behaves any different than mine. I’ll take pics this morning for you.
I too have had all hoses replaced within the last year.
Is the source of the overflowing coolant the vent/bleed hose?
Or the coolant supply/expansion hose? Or the cap?
Didn’t I see a tsb about replacement thermostats being below qa standards?
Is the bleed screw tight…. Clearly, I am tracing the coolant system diagram…

As the expansion tank “continually separates air from coolant”. From the “cooling circuit flow” section, it seems expansion is strictly handled by the “compliancy of the hoses”. Still thinking there is an air lock as you seem to be overpressurized…

And I hate mentioning (especially to you) any process on how to suck eggs…
But, for laughs and giggles…. engine cold. open expansion tank. open bleed screw. start engine. allow to warm (not hot). top up tank until overflowing bleed. tighten bleed. Affix cap.

Section marked “engine coolant draining and vacuum filling. States when engine warm, level is 10mm above max line. Mine is always there when cold, rises another 10 when hot.

 

Remember, this all started when I replaced the injectors, coils and plugs. Nothing in the coolant circuit was touched.

Hoses, seals, gaskets, water pumps, expansion tank, intercooler pump, pipes, thermostat all replaced, sometimes multiples, over the last few years, most within the past year. I always use the airlift for refilling and it's always worked. The t-stat a few months ago. They were on global back order with no eta for nearly a year, the one I got from the main dealer was the latest version and it appears to function correctly based on ECT B.

Coolant comes and goes from the bottom, not the overflow.

When it overflows, I see the coolant rising from the bottom, then hit the cap and then out the cap.

Bleed screws tight and new.

I know it doesn't make any sense, but at the moment it seems this failure's dependent on the cold level of the coolant

 

I do know that. Why I am as perplexed as you…
Incidentally, my top hoses are always soft when the engine is cold.
Humor us both and ensure there is fluid leaking when you crack all the bleed screws (one at a time of course)??

 

Yes, usually the top radiator hose is soft after sitting, sometimes a couple of hours, sometimes over night. This morning the hose was hard and the tank was completely empty. But when I opened the cap, the coolant refilled and rose to the level I filled it to last night.

Yes, multiple times I've opened the heater and tank screws and only coolant comes out, no air. Front and rear heat work correctly. I haven't opened the intercooler bleeder in while as I've not replaced it yet.