I've posted this on a couple other forums so if you are a cross-forumer, I apologize.

OK folks, need some help. I've posted within some other threads about this issue but felt like I needed to start a new one as I don't know what to do next. I had the motor rebuilt a few years ago due to oil leaks and just age. The guy that rebuilt it said the motor looked pretty good inside. The computer said it didn't even need boring, just honing. He did replace the water pump, as well as everything else suggested to be replace when having the engine apart. It came with a 12 month/12,000 mile warranty. Well I had other problems (mostly brakes) and didn't drive it much for the past 2 years. I've worked out the other problems now I just have this cooling issue.
My Sammy was pushing antifreeze into the expansion bottle during warmup but kept pushing until it would spit out the overflow. It used to run with engine temp just above the bottom Norm line - which I'll call 10%. It's now running in the 50-80% but still between the low and high norm lines. After the engine cools, the expansion bottle is empty and when looking into the radiator, I cannot see antifreeze.
Step 1: change the thermostat and radiator cap. No Change
Step 2: went to the shop and had them flush the system and replace t-stat again (thinking maybe the one I bought was bad). During flush, they did a pressure test. Held pressure for 30 min. No Change
Step 3: thinking maybe while the engine was out and the radiator was empty, the crap in the 30 year readiator dried, broke off the sides and clogged the pores so bought new aluminum radiator and another new cap. Installed and flushed again. No Change.
Step 4: removed the t-stat completely and ran for 30 minutes. While it did expand into the expansion tank, it did not overflow out. The temp gauge never got to normal temp. Sucked antifreeze back into radiator from expansion tank when cooled. Radiator had correct level when cooled.
Step 5: bought a head gasket check kit. The one that sucks air from the radiator and "sniffs" for CO2. It's blue but will turn
yellow in the presence of CO2. Sucked air thru for 2 minutes as the directions stated and liquid remained blue. To test, put device behind exhaust pipe and sucked air into the test kit. Liquid turned yellow as expected. Also, I still have no T-stat in the housing so when running the test with the radiator cap off, I see good flow as I can see it flowing across the top of the radiator.

Per the FSM, there are 2 T-stats. One begins to open at 179*F and is fully open at 203*F and the other at 190* and 212* respectively. It seems to push the antifreeze and overflow before the t-stat opens so it's expanding from the bottom, up and out. Maybe I have the higher temp t-stat? [edit] I do have the higher temp one. Someone showed me they are marked on the T-stat.

I am at a loss as to what to do next. Any and all suggestions are welcome.

You say the engine is pushing coolant out to the expansion bottle, as well as out of it - during "warmup" - for this to happen, the pressure in the cooling system would have to exceed the pressure valve in the radiator cap (usually around 13 psi) - if it is happening during "warmup" ie before the engine is up to temperature, there can only be one cause - combustion pressure leaking into the cooling system.

Thanks Fordem. I did the test for combustion gas - sucking air from the radiator thru a blue liquid with engine running. The liquid did not turn yellow. To verify it was a valid test, I sucked exhaust gas from the tailpipe and the liquid did turn yellow.

So I may have fixed my problem. Please let me know if you disagree.
I'm embarrassed to say that I may have been the cause. My initial problem was overheating. As my 1st step, I swapped the thermostat. Here's where I went wrong. I just went to a box store auto parts supplier and they looked it up and gave me a thermostat. I am guessing it was a 96*C. I was still overheating and went thru all the steps in my OP. When I went to the shop to flush and pressure test the system, I asked that they replace the thermostat. I'm guessing they either a) replaced it with what was already there or b) didn't replace it at all. When I pulled the thermostat out and ran without one, I had both the original one and the one I pulled. The original one looked really bad with a lot of corrosion and just looked in bad shape. When I went to the parts store to replace the combustion gas in radiator test kit, I talked to the guy about my problem. He looked the t-stat up and said there were 2 thermostats - one at 206*F and one at 160*F. I happen to have the old ones in the car and he showed me how to tell what they were (Stamped on the housing - who would have thought that?  ). So the old one was stamped with 82* (179*F) and the newer one 96* (206*F). I bought the 160*F knowing it was a lower temp, installed it and I no longer have overflow of the expansion bottle.

However it is now running slightly below warm. Sorry for the poor pic. The pointer is just below the warm line when warmed up (bottom red line) while it used to be just above the warm line (top red line) before my problem. So opening the t-stat at 160* has solved my overflow problem as I no longer overflow out of the expansion bottle but will this temperature be a problem? Should I buy and install at 179*F t-stat? They are cheap but if it's working, why change unless I will have problems long term. Any suggestions would be greatly appreciated.